张春来
, 宋长青
Zhang Chunlai, Song Changqing
中图分类号: P934;K903
文献标识码: A
文章编号: 1001-8166(2018)01-0027-15
收稿日期: 2017-10-21
修回日期: 2017-11-30
网络出版日期: 2018-01-10
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Zhang Chunlai(1972-), male, Haiyuan County, Ningxia Hui Autonomous Region, Professor. Research areas include soil wind erosion and aeolian geomorphology.E-mail:clzhang@bnu.edu.cn
作者简介:张春来(1972-),男,宁夏海原人,教授,主要从事土壤风蚀与风沙地貌研究.E-mail:clzhang@bnu.edu.cn
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摘要
土壤风蚀过程包含风蚀动力过程和风蚀因子及风蚀速率在风蚀过程中的动态变化过程。前者实验和理论研究成果丰富,较清晰地刻画了颗粒起动、搬运和沉降过程;后者重点研究风蚀过程中各类风蚀因子的动态变化规律及其互馈机制,以及土壤风蚀速率对风蚀因子动态变化的响应机制,尚缺乏系统的研究成果。土壤风蚀过程研究的难点在于:①风蚀因子的动力学参数化;②风蚀因子动态变化过程的观测和定量表达;③风蚀过程的尺度问题;④模型的预报能力。现阶段土壤风蚀研究迫切需要开展以下工作:建立标准风蚀观测场,获取连续、完整的大田风蚀资料;研究风沙流饱和路径,解决尺度问题;构建具有扎实理论基础、基于动力过程和动态变化过程的风蚀模型。
关键词：
Abstract
Soil wind erosion processes include mechanical process and dynamic changes of the factors affecting soil wind erosion, as well as the corresponding changes of wind erosion rate. The former is rich in experimental and theoretical researches that have clearly defined the process of particle starting, transporting and settling. The latter focuses on the dynamic changes of various wind erosion factors and the response of soil wind erosion rate to the change of the factors, of which systematic research of which is very limited. The difficulties in research of soil wind erosion process include: ①Dynamic parameterization of wind erosion factors; ②Observation and quantitative expression of the dynamic changes of wind erosion factors; ③Scaling problem of wind erosion process; ④Prediction ability of wind erosion models. At present, it is urgent to carry out the following work on soil wind erosion. The first is to establish standard wind erosion observation field in typical regions to obtain continuous and complete data of wind erosion in the field; the second is to study the saturation path of wind sand flow to solve scale problem; and the third is to construct a wind erosion model with solid theoretical foundation and fully consider both mechanical process of soil wind erosion and dynamic changes of the factors.
Keywords：
土壤风蚀是土壤侵蚀的重要类型,主要发生在年降水量小于400 mm的干旱、半干旱以及部分半湿润地区。其定义基本承袭侵蚀概念,即“风蚀是指沉积物被风分离、搬运、沉积,它是松散、干旱和裸露土壤被强风传输的一个动力学和物理学过程”[1,2]。另一类解释认为,风蚀仅限于地表物质发生位移,导致岩石圈(或土壤圈)的破坏和损失[3]。邹学勇等[4]也从地球表层物质循环过程的角度,认为侵蚀只是该过程中的首要环节,不应包括被侵蚀物质的搬运和沉积过程,据此提出风蚀的定义应是“风力作用导致表土物质脱离原空间位置的过程”。与之相关的“土壤风蚀流失量”则是一个具有时间和空间尺度的概念,通常使用单位时间和单位面积上的土壤风蚀量,即风蚀模数,它不仅体现了风蚀的结果,还体现了被蚀土壤颗粒的搬运和沉降结果。
土壤风蚀本质上是一种具备独特气流—土壤界面相互作用属性的连续动力学过程。因而,土壤风蚀过程至少包含2个方面的含义:一是动力过程,包括颗粒的起动、搬运(蠕移、跃移、悬移)、沉降3个子过程;二是状态变化过程,即风蚀因子和风蚀速率在风蚀过程中的变化过程。土壤风蚀的系统研究始于20世纪30~40年代,董治宝等[5,6]、李小雁等[7]、杨秀春等[8]、邹学勇等[4]和南岭等[9]从不同角度回顾了土壤风蚀研究历史,阐述了土壤可蚀性与风蚀模型研究的重要进展。本文不再对此重复评述,仅从“过程”的角度,分析土壤风蚀研究已取得的重要成就和局限性,并提出土壤风蚀过程研究的关键科学问题和应该努力的方向。
土壤风蚀过程研究的核心内容是土壤颗粒分离、搬运和沉积的发生发展过程,各种影响因子动态变化过程,以及二者的互馈关系;核心目标是建立包含风动力过程和土壤状态变化过程的多维模型。
(1) 分离
分离是土壤颗粒在气流直接或间接作用下脱离地表,并产生明显空间位移的过程,是土壤风蚀动力过程的首要环节。一般认为,气流吹蚀、运动颗粒的冲击和磨蚀以及土壤团聚体在运动中的破碎是颗粒起动、分离的3种基本方式[10,11]。
当风速大于临界起动风速时,气流的直接吹蚀可导致沙尘颗粒脱离地表。这种现象被称为颗粒的流体起动。临界摩阻风速可以简单地表示为[12]:
式中:d为颗粒粒径,Ad和Bd-1分别代表重力与颗粒内聚力2种阻碍起动的作用力和流体拖曳力的相对大小。对大颗粒而言,Ad项代表的重力起主要作用,临界摩阻风速近似为:
式中:ρs和ρa分别为颗粒和空气的密度,g为重力加速度。该公式最早由Bagnold[13]根据力矩平衡方程推导而来,常用于确定颗粒流体起动的临界风速。此后的研究表明,“常数”C其实是颗粒Reynolds数的函数[14]。对小颗粒而言,Bd-1项代表的内聚力起主要作用。水分分布引起的毛细力和净电荷间的Coulomb力都可能对颗粒间内聚力有所贡献[15]。对于松散土壤,可以不考虑内聚力的影响。
跃移颗粒在运动过程中不断从气流中吸收能量,所以当它们冲击地面时,往往引起土壤颗粒的分离。当土壤颗粒以沙物质为主时,跃移颗粒击溅起来的颗粒数为2~6,速度仅为入射颗粒的7%~9%,起跳角与入射角的比值为0.5~0.6[16]。击溅颗粒数、起跳角、起跳速度通常假设为随机变量,根据实验结果确定各自的概率密度。当土壤颗粒以粉尘为主时,跃移冲击导致垂直方向的起尘通量可从理论上表示为[17]:
式中:u*为摩阻速度,i=3~4。松散沙床表面发生的击溅现象可能对风沙流发展过程具有显著影响,因而在数值模拟工作中得到了高度重视[18,19]。最近的风洞实验表明[20],沙床表层的运动颗粒可导致细颗粒供应量增大,有利于气流吹蚀和跃移冲击起尘过程的发生。
土壤颗粒分离的另外一个重要过程是磨蚀。早期的实验表明[21],冲击物的动能和目标体的胶结强度是磨蚀过程的控制因素。此后大量实验证实了这个结论。磨蚀敏感性Sa(有时也称为“磨蚀强度”)定义为磨蚀损失和来流碰撞颗粒的质量之比[14]。由实验得到的一个经验关系式为:
式中:v为来流碰撞颗粒的速度。为了克服此关系式没有考虑目标体力学性质的明显缺陷,可以定义磨蚀数An如下[22]:
式中:σs和E分别为目标体的屈服应力和Young氏模量。由此得到一个更加合理的理论关系为:
式中:V为目标体在每次碰撞中损失的体积。虽然磨蚀研究的目标体多为风棱石或雅丹,但是相关结果对跃移颗粒与低含水率土壤之间碰撞过程研究具有极为重要的参考价值。例如,脆性破损理论就被用来分析碰撞起尘的粒径分布[23];由磨蚀和雨滴溅蚀实验得到的经验关系式已被用于构建经验性风蚀模型[24]。
(2) 搬运
搬运是不同粒级土壤颗粒在风力作用下以不同方式产生定向的空间运移过程。在Bagnold的开创工作中,风力作用下沙尘运动被划分为悬移、跃移和蠕移3种基本形式[13]。后人在此基础上又引入了溅移概念[25]。各种运动形式的定义如下[26]:悬移是颗粒在远离床面的高空所进行的不与床面接触的长距离输运模式;能够击溅起床面上其他颗粒或自身可以反弹的颗粒的运动形式定义为跃移;溅移指一些被击溅起来的颗粒的运动形式,这些颗粒自身不能反弹或击溅起床面上其他颗粒;那些位移不直接受气流影响的颗粒的运动方式定义为蠕移。本文沿用悬移、跃移和蠕移这3种经典名称。因为溅移颗粒在轨迹上接近于跃移,在能量上接近于蠕移,所以跃移轨迹参数研究中实际包含了溅移部分,溅移对输沙率的贡献则往往被纳入蠕移。风沙运动受多种因素控制,3种运动形式之间是逐渐过渡的。通常情况下,颗粒粒径70 μm和500 μm可作为悬移—跃移和跃移—蠕移的大致分界点[27]。
跃移运动为蠕移和悬移提供动力,是近地表颗粒的主要运动形式。跃移轨迹具有抛物线形态特征。跃移过程中,颗粒在水平方向一直处于加速运动状态。因此,跃移长度在上升段要小于下降段。高速摄影与频闪摄影实验显示[1,28~30],起跳角、跃移长度和高度等特征参数的变化范围很大,而降落角变化范围不大。起跳角以30°~50°居多,占39.5%;其次为60°~80°,占27.7%;极个别颗粒起跳角超过90°[28]。降落角通常小于30°[1]。典型的起跳角和降落角分别为55°和10°[10]。跃移高度和长度一般不超过30 cm和50 cm。在坚硬的戈壁地表上,最大跃移高度可超过3 m[31]。已有大量实验证实跃移颗粒存在旋转运动,但报道的旋转方向和速度差别很大。这很可能与早期的实验设备和技术相对落后有关。近期的二维高速摄影实验显示[32],如果风洞来流方向从左向右,那么绝大多数跃移颗粒做顺时针旋转,瞬时转速可达300 rad/s。
跃移运动可以通过力学模型进行精确描述。Owen[33]较早建立了均匀跃移模型,其主要假设为:①所有跃移颗粒的轨迹、速度、加速度等动力学特征完全相同,但颗粒运动具有一定的随机性;②跃移层风沙流的自平衡性,即输沙率的大小与气流方向上的距离无关,且竖直方向上的净输沙率在床面处为0;③跃移层对气流的作用相当于粗糙元,其粗糙度正比于跃移高度;④床面处气流剪切力与流体起动时相同。在此基础上,后人发展出很多更接近实际情况的跃移模型[34~36]。时至今日,仍有更精确的实验或理论工作在检验Owen假设的有效性[11,37]。
颗粒的受力分析是建立跃移模型的关键。早期的研究者仅考虑了重力和拖曳力[13]。后来,升力、Magnus力、Saffman力、静电力等[10,19,38,39]被陆续引入。这些力可以分为气动力和场力2类。前者包括拖曳力、升力、Magnus力和Saffman力。后者包括颗粒处于重力场和电磁场中而受到的重力和静电力。气动力的精确表达应该是流场应力在颗粒表面的面积分。通过求解强非线性的Navier-Stokes方程获得流场应力的方法往往面临很大困难。因此,各种气动力表达式都是一定简化假设前提条件下的近似解。因为解的叠加原理不适用于强非线性偏微分方程,所以在对跃移颗粒受力进行分析时,将不同气动力进行线性叠加的做法[10,19]是不合适的。哪种气动力表达更适合于描述跃移运动是研究课题之一[40,41]。根据跃移轨迹的高速摄影实验[32],重力和拖曳力起主要作用,受颗粒旋转速度控制并与拖曳力垂直的升力有时不容忽视。
受实验技术所限,蠕移运动一直是风沙物理学研究的难点。虽然很早就尝试测量蠕移[13],但传统的测量手段很难将其与跃移区分开来。最近研究表明,根据蠕移和跃移运动特征,调整集沙仪的入口尺寸或内部结构,可以将二者区分开来[42,43]。无接触的摄影方法也被用来测量沙床表层的蠕移运动[44]。风洞实验给出的蠕移颗粒水平运动速度范围为0~0.14 m/s[42]或0.11~0.37 m/s[44]。可见,蠕移颗粒运动特征仍需要深入研究。
悬移运动又可以细分为20~70 μm颗粒的短距离悬移和小于20 μm颗粒的长距离悬移[10,27]。前者悬浮的时间尺度为数小时,输运距离一般为几十到数百公里。后者则可悬浮数天至几周,输运距离可达数千公里。根据参考文献[45]及其附注,von Karman最早从流体力学角度估算了粉尘离开地表在空气中的持续时间和输运距离。40多年后,地学工作者重复了同样工作[46]。在阐述von Karman工作的原始文献中[45],沉降速度系数存在笔误,给出的结果偏小。一些经典中文专著[3,47]在论述悬移时引用了该文的错误公式。其实,粉尘在空中的停留时间和输运距离理论表达式中的系数不是40而是648。据此划分的长短距离悬移与绝大多数观测相符合,故沿用至今。粒径大于20 μm颗粒的长距离悬移是粉尘研究的未解之谜[48,49]。考虑到湍流扩散假设引入了较大的误差,我们可根据运动学原理重新估算输运距离[50]。不同于跃移和蠕移颗粒,细小的黏土颗粒能以团聚体形式做悬移运动[51~54],其团聚和运动的物理机制有待揭示。
以上着重介绍了气流对单颗粒的输运。实际情况是很多颗粒同时被输运,在一定时间和空间内形成风沙流。为了定量描述风沙流,定义在单位时间内通过与来流方向垂直的单位宽度的全部颗粒的总质量为输沙率[13]。输沙率沿风向变化可分为3个阶段[10],即初始迅速上升阶段、中间短暂高峰阶段和后期下降并逐渐趋近于稳定状态。一般都将稳定状态的数值作为输沙率。风沙流从发生到稳定的距离被称为饱和距离。风洞内的饱和距离为7 m[13],野外观测结果约为600 m[55,56]。
根据颗粒运动形式,输沙率由蠕移、跃移和悬移3部分组成。悬移输沙率可以忽略不计。蠕移输沙率所占比例约为25%[13]。虽然被工程实践广泛采用,但进一步的理论和实验研究均显示蠕移比例随着风速增大而降低[42,57,58]。跃移输沙率q可写为摩阻速度u*的函数。最常见的关系为:
前一个关系式由Bagnold从动量定理导出,后一个关系式的修正项也有其他形式,其目的在于体现只有当风速大于起沙风速时才存在风沙流[13]。跃移输沙率公式数量众多[10,11],在定量上的差别不是很大,其共同特点在于经验常数必须通过实验确定。
风沙活动主要发生在近地表几十厘米内,对高度变化敏感。实验结果表明可用指数函数、幂函数或分段函数等多种形式拟合,大多数结果表现为指数分布,贴近地面处的偏离可能既与集沙仪效率有关又受颗粒运动形式的影响[59,60]。
(3) 沉降
沉降是空中的土壤颗粒在风力、下垫面条件改变的情况下重新回落陆地表面的过程。沉降方式可以归纳为3种[13,45]:①当风速低于起动风速时,缓慢气流中的沙尘颗粒发生沉降。在这种情况下,因风速变低,地表不再有沙尘起动,亦无明显的击溅过程。②当平坦地表上方的风速垂向梯度消失时,跃移和蠕移颗粒发生沉降。③当地形变化(尤其是坡度突然变化)或风沙流遇到沙障、植被、建筑物等阻碍时,流场结构改变,沙颗粒在部分地表区域沉降,粉尘也可附着于障碍物表面。
近地表蠕移和跃移颗粒在土壤风蚀区较近的下风向处形成平沙地、砾浪、沙丘等风积地貌形态,高空粉尘经远距离输运后可在特定区域日积月累成数百米厚的黄土层。注意到风积地貌类型多样,颗粒的沉降过程只能根据具体情况来分析。沙颗粒在背风坡的沉降是沙丘动力学中具有共性的物理过程。以脊线为起点,沉降速率在水平方向上的变化趋势为先上升到最大值,再以距离的指数形式下降[61]。粉尘颗粒的沉降可分为干沉降和湿沉降2种形式,在陆地表面以前者为主。目前,已有一些流体力学模型定量描述粉尘在平坦地表或植被附近的干沉降过程[62]。粒径不同,控制干沉降速率的物理过程是不同的[63]。当d≤0.1 μm时,Brown运动为主,0.001 μm颗粒的沉降速度高达1 cm/s。当d≥5 μm时,重力起主要作用,沉降速率为数厘米每秒。当0.1 μm≤d≤5 μm时,湍流过程为主,沉降速率低于前2种情况。这3种物理过程都已建立了成熟理论[64]。然而,当前的干沉降研究[10]似乎没有充分利用这些物理学结果。湿沉降泛指与降水有关的云、雨、雪、雾等气象现象所引起的粉尘沉降,常在远离粉尘源区处发生。云层中的粉尘颗粒直接充当凝结核,云下粉尘则被雨滴冲涮。目前,对后一种情况了解较多[63],能够计算降雨对悬浮粉尘颗粒的冲刷速率[65]。
风蚀因子是控制土壤风蚀过程的重要环境因子,科学确定风蚀因子是正确理解土壤风蚀过程和风蚀强度的重要前提。
(1) 风蚀因子分类
影响土壤风蚀过程的各种自然和人为干扰因素,包含大气、土壤、植被、土地利用方式和管理措施等要素,称为土壤风蚀影响因素。Chepil[66]最早基于风蚀动力将其认为最重要的影响因素划分为3类,即空气因子、地表因子和土壤因子。其中,空气因子包括风速、湍流、空气密度、黏滞系数等,地表因子包括粗糙度、地表覆盖、障碍物、温度、地形等,土壤因子包括土壤结构(取决于有机质含量、碳酸钙含量和机械组成)、容重、含水率等。此项工作对风蚀因子分类及各类因子动力学研究具有开创意义,为此后的风蚀模型研究奠定了基础。从最早诞生的风蚀方程(Wind Erosion Equation, WEQ)[67],到后来的Bocharov风蚀模型[68]、德克萨斯侵蚀分析模型(Texas Erosion Analysis Model,TEAM)[69]、风蚀评价模型(Wind Erosion Analysis Model,WEAM)[70]、修正风蚀方程(Revised Wind Erosion Equation,RWEQ)[71]和风蚀预报系统(Wind Erosion Prediction System,WEPS)[72],无一不借鉴了Chepil因子分类的思想和成果(表1),其中,WEQ平行考虑了可蚀性因子、气候因子、坡长和坡度因子、地表粗糙度因子和作物残茬因子。RWEQ在修订风蚀算法的同时,风蚀因子重新划分为可蚀性因子、气候因子、土壤结皮因子、地表粗糙度因子和结合残茬因子。WEPS将风蚀因子进一步细化为气候因子、水文因子、土壤因子、作物因子、分解因子和管理因子。Bocharov模型将25个影响因素归纳为风况因子、表层土壤因子、气象因子和农业活动因子4类,虽考虑全面,但只是一个理论的框架模型,没有给出风蚀量与各因子之间的定量关系式,因而无法得到实际应用。TEAM模型中包含的风蚀因子有风力因子(摩阻风速)、土壤可蚀性因子、地表覆盖因子和顺风向裸露地表长度。它和WEAM的共同特点是以摩阻风速和临界摩阻风速的形式在一定程度上体现了风蚀过程的动力学特征,但所考虑的影响因素过于单一,也没有体现其他因子的动力影响。上述模型并没有在风蚀因子动力分类的基础上更进一步建立基于风蚀动力的预报模型,而是采用统计方法在风蚀量与几个主要因子之间建立经验关系式。WEQ,RWEQ和WEPS均属于纯粹的经验统计模型。显然,这偏离了Chepil风蚀因子分类的初衷。
表1 土壤风蚀影响因子分类
Table 1 Classification of wind soil erosion factors
| 分类与出处 | 因子分类 | 要素 | 参考文献 |
|---|---|---|---|
| Chepil因子分类 | 空气因子 地表因子 土壤因子 | 空气因子包括风速、湍流、空气密度、黏滞系数,地表因子包括粗糙度、地表覆盖、障碍物、温度、地形,土壤因子包括土壤结构(取决于有机质含量、碳酸钙含量和机械组成)、容重、含水率 | [66] |
| 风蚀方程 (WEQ) | 可蚀性因子 气候因子 坡长和坡度因子 地表粗糙度因子 作物残茬因子 | 迎风坡长度小于152.4 m时的山丘可蚀性、年均风速、降水量、蒸发量、土垄高度、土垄间距、田块总长度、风障保护长度、作物残余物重量、残余物截面指数、植被粗糙变量 | [67] |
| Bocharov模型 | 风况因子 表层土壤因子 气象因子 农业活动因子 | 4类因子中,共有25个风蚀影响要素。风况因子包括瞬时、日平均、年平均、最大风速、风向、气流湍流程度和风速的频率分布;表层土壤因子包括土壤机械组成、土壤湿度、土壤团块结构(不可蚀成分含量)、结皮、土壤结构的水稳性等;气象因子包括气温、土壤温度(土壤冻结)、降雨强度与降雨量、空气相对湿度等;农业活动因子包括农业活动引起的田块内表土微地形起伏和沟垄尺寸、上年风蚀性质、顺风向田块长度、邻近田块的性质、田块周围防护措施类型和结构、植被覆盖度和作物残留物覆盖状况、耕作方法 | [68] |
| 德克萨斯侵蚀分析模型(TEAM) | 风力因子 土壤可蚀性因子 地表覆盖因子 顺风向裸露地表长度 | 风力因子(摩阻速度) 土壤可蚀性因子 地表覆盖因子 顺风向裸露地表长度 | [69] |
| 风蚀预报系统 (WEPS) | 气候因子 水文因子 土壤因子 作物因子 分解因子 管理因子 | 气候子模型;水文子模型包括融雪、地表径流、土壤贮水量、潜在蒸散量、潜在土壤蒸发和植物蒸腾量;管理子模型包括地表处理方式、土壤处理方式、植物体处理方式和土壤改良;土壤子模型包括垄沟高度、自由糙度、结皮、结皮厚度、结皮覆盖度、结皮稳定性、结皮的松散可蚀性物、干团聚体稳定性、团聚体粒径分布;作物子模型包括物候、作物生长期日长、作物萌发、生物量产出、地上和地下生物量分量、叶茎面积生长过程、衰亡期叶面积指数下降过程、植株高度;分解子模型包括作物的直立秸秆分解、地表残留秸秆分解、埋入土壤秸秆分解、根系分解 | [72] |
| 风蚀评价模型 (WEAM) | 风力因子(摩阻速度) 土壤可蚀性因子(粒度和水分) 土壤表面覆盖因子 | [70] | |
| 修正风蚀方程 (RWEQ) | 可蚀性因子 气候因子 土壤结皮因子 地表粗糙度因子 结合残茬因子 | 田块长度、由正坡向负坡转折点的长度;易蚀碎屑含量;土壤湿度;土壤粗糙度;气候因子包括风速、风向、太阳辐射、降雨量、雪覆盖;土壤结皮因子取决于黏粒、有机质含量;植被因子包括平铺残留物、直立作物残留物、植被冠层、作物残茬分解等 | [71] |
| 土壤风蚀动力模型 (DMSWE) | 风力侵蚀因子 粗糙干扰因子 土壤抗蚀因子 | 风力侵蚀因子包括风速、风向、湍流、空气密度、空气黏度;粗糙干扰因子包括植被/留茬覆盖、植被/留茬高度、平铺残余物覆盖、平铺残余物质量、土垄高度和间距、地形起伏度、砾石覆盖、土块覆盖、土块尺寸、空气动力学粗糙度等;土壤抗蚀因子包括土壤比重、土壤颗粒尺寸分布、盐分含量、有机质含量、土壤水分含量、土块密度、植物根系密度、pH值、结皮覆盖等 | [4] |
随着土壤风蚀研究的深入,Chepil风蚀因子分类主要针对耕作土壤的局限性逐渐显现。鉴于此,邹学勇等[4]认为有必要重新思考风蚀影响因子的定义和分类,进一步明确各类因子对土壤风蚀过程的影响方式和作用机制。按照有利于或者不利于土壤风蚀的属性原则、三维空间上的连续性原则、土壤风蚀过程中表现出的同类力学特性原则,以及实际应用中的可测度原则,邹学勇等[4]将土壤风蚀影响因子重新划分为风力侵蚀因子、粗糙干扰因子和土壤抗蚀因子3类(表1),分别对应风力侵蚀力、粗糙干扰力和土壤抗蚀力。这一分类方法的优点在于,从风蚀动力学角度,将各风蚀影响要素按照所处空间位置和力学属性合理分开,在风蚀影响因子参数化过程中,各因子及其次级风蚀影响要素成为独立的变量和自变量,避免交叉出现相同风蚀影响要素的情况,在构建基于风蚀动力学的土壤风蚀模型时,能够满足各自变量相对独立的建模原则。邹学勇等[4]据此构建的土壤风蚀动力模型(Dynamic Model of Soil Wind Erosion,DMSWE)框架,指出了土壤风蚀模型研究的努力方向,已得到了国内同行的认同。本文的相关论述即围绕风蚀因子的DMSWE分类方法展开。
(2) 风力侵蚀因子
在DMSWE因子分类体系中,风是土壤风蚀发生和发展的基本驱动力,也是一次风蚀事件中变化最直观的因子。在较小的时间尺度上,气流脉动是普遍存在的现象,而气流脉动与剪切力直接相关,气流对地表剪应力的最大值控制着颗粒的起动[73],脉动风速对平均风速和沙粒起动风速的差值的覆盖则控制着风速的间歇水平[74]。风的脉动强度足以引起沙粒输移的变化[75~77],李振山等[78]的风洞实验测量结果表明脉动幅度可达2.5 m/s,这是基于平均风速的输沙方程计算结果往往与实际情况不相吻合的一个重要原因。就平均状态而言,风沙运动研究中常用的流场描述是针对充分发展的壁面流而建立的半经验混合长度湍流边界层理论。风力因子具体表示为对数廓线中的摩阻速度(或相应的壁面剪切力),实际应用中有时也表示为某一高度处的平均风速。当风蚀发生尤其是形成明显的颗粒输移时,风速的垂直梯度减小[33]。跃移层的存在相当于增大了地表粗糙度,从而间接改变风速。基于这种认识的平均流场描述仍处于不断发展中[10,11]。目前,风沙运动中的瞬时流场细节正在逐步被揭示,湍流结构成为近年来的热点和难点问题之一[79]。最近,风洞条件下的PIV实验[80]表明,跃移层内存在着顺风向的快慢条带;高分辨率的野外观测[81]显示,在距地面30 m高度范围内存在着更大尺度的相干结构。这些工作证实风蚀过程中风力因子存在瞬态变化,且影响因素复杂。尽管尚未讨论风力因子瞬态变化对风蚀过程的影响,但已为风力因子的精细研究提供了良好的借鉴作用。例如,已有学者开始尝试从涡的产生和消亡角度定性探讨地面与气流的相互作用[82]。
单次风蚀事件中,风速一般存在上升、(相对)稳定和下降3个变化阶段。由于风蚀速率与风速存在幂函数正相关关系,如果不考虑风蚀过程中地表性质的变化,仅风速的变化即会导致风蚀速率的显著变化;即便风速不发生变化,风蚀速率也会因地表可蚀性的动态变化而随吹蚀时间的延长而发生显著变化,一般表现为风蚀速率的迅速减小[83]。因而现有研究大多采用风蚀事件中的平均风速及风蚀持续时间计算风蚀量的方法,必然导致较大误差。基于这一认识,Liu 等[84]论述了采用风蚀事件中的最大风速估计风蚀量的重要意义,Yang等[85]通过分析单次风蚀事件的最大风速分别与平均风速、持续时间和输沙率的关系,探讨了最大风速的作用。单次风蚀事件时间跨度既有可能数小时,也可能数天,更长的时间跨度则较为罕见。
在日、月、年时间尺度上,土壤风蚀的发生具有不同时间尺度的准周期规律。以年为周期的分布规律表现为冬春强烈、夏秋较弱,这与降水和风的年内分配规律相一致。而在更长的时间尺度上,陈渭南[86]利用历史资料统计认为黄土高原的北部、鄂尔多斯地区风蚀强度的长时期周期还具有3年、7年、23年、50年、145年等不同时间尺度的准周期性规律。
(3) 粗糙干扰因子
粗糙干扰因子直接改变气流场结构,对气流与地表间相互作用力的大小和分布产生显著影响,因而在颗粒风蚀起动和搬运等物理过程中发挥重要作用。本文将风力作用下或风蚀过程中,具有明显变化特征的粗糙元,如植株、流沙地表的沙波纹、农田地表翻耕造成的垄状起伏、地表不均匀蚀积形成的微地形等,称为动态粗糙元;将具有较大杨氏模量和屈服应力,风力作用下仅产生微小变形和位移的粗糙元,如砾石等,称为静态粗糙元。气流剪应力是研究粗糙元空气动力学效应的切入点,这个剪应力被分解为地表和粗糙元2部分[87]。对于静态粗糙元,考虑单个有效影响区域和粗糙元动力效应的随机叠加,可以建立剪应力分解模型并给出风速表达式[73,88]。动态粗糙元既能像砾石粗糙元那样提供一定的防护区域,也可以通过自身运动有效耗散气流动能。其中,弯曲和振动是植物粗糙元最主要的响应形式[89],植株主干或茎的振动模态已有较成熟的振型函数来表达[90],冠层的高阶振动在能量耗散中起主要作用[91],这些运动特性都会导致平均速度廓线的变形失稳进而产生较大尺度的涡[92]。不仅如此,气流和植物振动之间还存在强烈的耦合作用[93],使得植物粗糙元的动力效应更趋复杂,因而静态粗糙元的剪应力分解模型远不适用于植物粗糙元。微地形对近地表风力的作用方式和强度尽管显著不同于植物粗糙元,但在缺乏植物保护的裸露地表,微地形是控制地表空气动力学粗糙度和剪应力分布的重要因素[94],对土壤风蚀的影响不容忽视,尤其是农田地表翻耕造成的垄状起伏微地形[72]。风蚀过程中凸起部位的磨蚀和局部凹坑的堆积,使得微地形始终处于变化状态,从而导致地表空气动力学粗糙度和剪应力分布具备动态特征。微地形及其动力效应的这种动态属性,同样不能采用静态粗糙元的剪应力分解模型来表达。迄今为止,针对动态粗糙元情形下的地表剪应力分解及其对土壤风蚀影响的研究尚未开展。
(4) 土壤抗蚀因子
土壤抗蚀因子取决于土壤质地、有机质、碳酸钙含量、土壤含水率、土壤结皮等土壤理化性质。其中,土壤质地、土壤含水率、土壤结皮结构稳定性等既影响土壤风蚀过程,同时也受风蚀的影响而成为可变的暂时性质[95]。湿润表土在风力作用下逐渐变干,这一过程对风蚀速率的影响研究仅有初步涉及。风力分选和跃移颗粒冲击打磨,则使得表层颗粒组成/可蚀性颗粒含量处于变化状态。地表风蚀粗化是我国北方广阔的荒漠草原区草地和翻耕地普遍发生的现象[96]。跃移颗粒对地表的打磨和撞击可以产生更多的可蚀性颗粒,导致风蚀量往往数倍于初始地表可蚀性颗粒总量[97],因而跃移颗粒的打磨作用使得地表颗粒组成和可蚀性颗粒含量变化和风蚀过程趋于复杂化。在有物理结皮和生物结皮的地表,风力作用下结皮表面上堆积的松散颗粒易被吹蚀并与地表发生碰撞,碰撞过程破坏结皮形态和结构稳定性,并加剧地表粉尘释放[98,99]。上述3类土壤抗蚀性因子(表土含水率、土壤质地、土壤结皮)均显著影响土壤临界侵蚀风速和风蚀速率[100]。研究人员建立了多个临界剪切风速与各因子之间的关系式[66,101,102]和理论模型[13],但风力作用下以及风蚀过程中各因子的动态变化规律及其对风蚀临界风速的动态影响未得到重视。
模型是科学表达土壤侵蚀过程、土壤侵蚀评价和土壤侵蚀预报的重要手段。近一个多世纪以来,风蚀模型研究一直是土壤风蚀研究的核心问题。迄今较有影响的风蚀模型包括WEQ、RWEQ、Pasak模型[103]、Bocharov模型、TEAM、WEAM、WEPS等。邹学勇等[4]对上述模型进行了较详细的评述,本文不再重复介绍,仅从风蚀过程的角度,简述主要模型的特点。
基于过程、体现土壤风蚀“动态”特征的风蚀模型主要有TEAM,WEAM和WEPS。其中,TEAM基本方程中,磨蚀调整系数和剪切风速2个变量使得该模型具备“过程”特征,但磨蚀调整系数并非时间变量,而是与土壤可蚀性、地块长度相关的空间变量,其他因子则采用平均状态。WEAM是基于物理过程的风蚀预报模型,包括4个主要变量,即剪切速度、土壤粒度分布、土壤水分含量及地表覆盖因子。对于后3个变量,Shao等[70]意识到风蚀过程本身以及天气过程都会导致其发生变化,但由于问题的复杂性,模型中依然设定为常数。可见,TEAM和WEAM更多的是基于动力学过程的简易模型,没有涉及风蚀因子随吹蚀时间的变化及其导致的风蚀速率的变化等问题,因而无法成为真正意义上的基于过程的风蚀模型[104]。真正引入时间尺度且考虑风蚀过程变化因素,较为成熟的模型,只有WEPS。该模型将地表状态作为随时间变化的指标,每次模拟开始前,首先对地块进行初始化,模拟完成后,根据物质守恒原理更新地表状态(包括土壤颗粒组成、砾石覆盖比例、结皮、地表粗糙度、残余物盖度和地表湿度等),作为下一轮土壤风蚀模拟的初始状态。上述变量以天或风蚀事件为时间步长,即在同一天的风蚀过程或一次风蚀事件中,各状态因子仍按平均量处理,意味着该系统无法定量表述一天内或一次风蚀事件中各种风蚀影响因素连续、动态的变化及其对风蚀速率的影响。即便如此,WEPS仍是目前最为先进的土壤风蚀过程模型。
土壤风蚀研究方法主要有野外调查观测、风洞模拟实验、元素示踪法、遥感和GIS方法等[105,106]。野外调查观测是了解风蚀状况、实地测定风蚀量最基本和可靠的方法,一般通过地表状况(颗粒粗化程度、风蚀痕迹等)和土壤剖面的观察,并结合周围自然地理环境和土地利用情况,综合分析土壤风蚀程度。野外测量方法有测扦法[107,108]、风蚀痕迹法[109,110]、集沙仪测量法[111]、粒度对比法[112],以及很少使用的风蚀盘法、风蚀圈法、陷阱诱捕法、集沙盘法等[113,114]。其中,测扦法是风蚀量或风蚀厚度的直接测量方法,结果最为准确、可靠,不足之处是只能反映较长时间尺度的总风蚀量(极易风蚀的流沙等类地表除外),单次风蚀事件或更短时间尺度内的风蚀量则很难测定。集沙仪是风沙流量测定设备。理论上,地块上、下风向风沙流量同步测定结果之差即为集沙仪之间长度范围内的蚀/积量。受地表不均一性和风沙流时间、空间上的间歇性等不确定因素的影响,测量结果也具有很大的不确定性。其他方法普遍存在原理上的局限性,实用性较差。总之,现有的野外观测方法尚不能为掌握风蚀过程动态变化规律提供足够可靠的实地观测资料。
鉴于野外测定的不确定性和观测条件的限制,风洞模拟实验因条件可控和便于操作而成为土壤风蚀影响因子作用机制、风蚀因子与风蚀速率之间定量关系研究最重要的手段。几乎所有的土壤风蚀经验模型都离不开不同条件风洞实验获取的大量数据。风洞模拟实验应用于土壤风蚀研究可追溯至20世纪30~40年代,Bagnold[13]专门设计建造了风沙研究领域的第一座风洞,并利用该风洞观察了不同风速下跃移沙粒的运动轨迹,测定了沙面以上的风速廓线。此后,风洞成为风沙物理和土壤风蚀研究不可替代的重要工具。Chepil[115~125]早期关于风蚀影响因素的一系列重要成果都藉由风洞模拟实验获得。风洞模拟实验尽管存在相似准则方面的掣肘,但可通过一定的技术手段克服这一不足,例如通过实验段前方风洞底板铺设并调整粗糙元布局的方法,模拟野外条件下类似地表的风速廓线,实现动力相似[126]。核素示踪技术在近几十年被广泛应用于土壤侵蚀过程研究,其中核爆产生的放射性同位素Cs-137示踪应用最为成熟[127~130],并成为定量研究20世纪60年代以来的总风蚀量或年平均风蚀速率的重要工具[131~134]。然而Cs-137示踪技术难以提供短期内的土壤侵蚀信息,而且由于核禁试和30.17年的半衰期,使得土壤中的Cs-137含量呈减少趋势,最终将难以准确指示土壤侵蚀,其应用也会随时间的推移受到限制。在此背景下,Be-7作为宇宙射线轰击大气氮、氧原子靶核产生于平流层和对流层上部的天然放射性核素[135],具有与Cs-137示踪技术相似的原理,成为近年来土壤侵蚀示踪研究的新方法[136]。杨明义等[137]、张风宝[138]、Liu等[139]和Zhang等[140]证实了Be-7示踪技术在土壤水蚀研究中的可靠性。Yang等[141]将其拓展到土壤风蚀研究中,建立了Be-7示踪的土壤风蚀速率估算模型。除了核素,风成沉积物示踪材料还有矿物组成、稀有元素、生物标志物、磁性、粒度、颜色等诸多物理、化学和生物学特征。复合指纹识别技术即综合了上述方法的优点,近年来成为探明土壤水蚀过程中运移分布、沉积规律及泥沙来源的重要手段[142],但对于以风蚀事件为对象的风蚀过程研究而言,该技术还很不成熟。
20世纪90年代以来,随着计算机和信息技术的飞速发展,以 GIS 为核心的“3S”技术逐步成为地学定量化研究的科学方法之一,为土壤风蚀影响因子的数据库管理、空间分析与制图提供了平台。张国平等[143]借助 GIS 的空间分析功能,得到风速、土壤干燥度、地表植被指数、土壤质地和坡度等风蚀影响因子的空间分布图,并建立了全国1∶ 10万土壤风力侵蚀数据库;师华定等[144]借助GIS和RS技术,得到了内蒙古高原土壤风蚀环境因子的空间分布图,包括干燥度、风场强度、植被指数、地形起伏度。高尚玉等[96]利用 GIS 和 RS 技术完成“京津风沙源区土壤风蚀分布图”,该工作充分利用RS的高时间分辨率实现了土壤风蚀的动态监测。“3S”技术在土壤风蚀研究中应用的局限性:一是严重依赖于其他方法的成果和对风蚀过程的理解,其准确性需要实际调查或测量验证;二是由点到面的尺度扩展尚缺乏依据。
在过去几十年的时间里,土壤风蚀研究取得了巨大成就。但是在针对风蚀过程的野外定位观测、风蚀因子参数化、局地研究向区域空间的拓展、模型的预报能力的加强等方面,目前尚存在大量难点和需要解决的关键科学问题。
风蚀过程的观测即土壤颗粒分离、搬运和沉积过程,以及影响因子和风蚀量动态变化过程的观测。对于风力因子的观测,随着人们对近地面风的理解逐步加深,传统测量仪器已不能满足需要[145]。例如风杯和螺旋桨式风速计,不能有效测量二维或者三维湍流风速;洁净环境下常用的热膜风速计在风沙流中极易损坏。三维超声风速仪不仅能有效测量三维风速,同时具有高频率和高稳定性等特点,近年来已被广泛应用于野外风速的观测[146]。对于粗糙干扰因子,动态粗糙元在风蚀过程中既存在复杂的瞬态变化,也存在趋势性变化,且不同类型的动态粗糙元具有截然不同的运动特征,而瞬时变化就可能对风蚀过程产生较大影响,传统观测手段和仪器很难获取此类资料。对于土壤抗蚀因子,尽管Chepil[1,124]早在20世纪50年代便研究了土壤颗粒组成和水分含量对风蚀的影响,但类似研究迄今仍至少以天为时间步长,而一天当中的土壤风蚀足以显著改变表土含水率、颗粒组成等抗蚀性因子,因此不应忽视土壤抗蚀因子在更小时间尺度的动态变化。困难同样在于缺乏有效的表土抗蚀性因子动态监测手段。对于土壤颗粒流(风蚀物)的观测,从传统的集沙仪到更高采样频率的粒子计数系统如Sensit和Safire[147,148],再到具有连续称重功能的集沙仪[149],风沙流跃移颗粒动态变化过程逐渐清晰,但风蚀量的连续观测仍是一大难题。无论风蚀因子还是风蚀量的动态测量,都亟待创新思维和技术手段,提高对土壤风蚀过程的测量和分析能力。
数值模拟是风沙运动研究的重要手段,尤其在跃移颗粒运移轨迹[18,34]、风沙流中的沙粒浓度分布[150,151]等方面获得了较好的模拟结果,但由于存在床面形态变化和表土颗粒内聚力等因素考虑不足、模拟尺度太小等缺陷,对土壤风蚀完整过程的数值模拟尚处于空白。
风蚀影响因子参数化是按照一定的分类原则,将影响土壤风蚀的各要素分类归纳为风力侵蚀力、地表粗糙干扰力、土壤抗侵蚀力3类影响因子,在此基础上建立这3类因子的力学表达式[4]。邹学勇等[4]较为详细地阐述了各个因子参数化的思路和方法,但在实际操作中,尚存在诸多困难。对于风力因子,稳定风场条件下近地表风的性质已基本清晰[152~155],风力因子的参数化方法早已成熟。但在不稳定的非均匀风场条件下,风力因子的力学表达仍存在困难。对于粗糙干扰因子,形状规则的刚性粗糙元(静态粗糙元)情形下,粗糙干扰力可采用较为成熟的剪应力分解模型进行参数化,而动态粗糙元条件下,特别是具有复杂运动和变形特征的柔性粗糙元,剪应力分解机制尤为复杂。对于土壤抗蚀因子的参数化,邹学勇等建议利用表土抗剪强度来定量描述土壤抗风蚀能力[4],但表土抗剪强度的测量仍存在技术困难,而且土体抗剪强度与表层颗粒抗剪力之间的力学转换关系存在理论瓶颈。
广义地讲,尺度是指在研究某一物体或现象时所采用的空间或时间单位,同时又可以指某一现象或过程在空间和时间上所涉及的范围和发生的频率[156]。由于风沙流饱和路径问题的复杂性,以及地表属性的空间异质性,甚至包括学者们对土壤风蚀概念的不同理解,导致空间尺度转换成为土壤风蚀研究中悬而未决的科学难题,这不但给不同空间尺度土壤风蚀的定量评价造成很大困难,也成为风蚀模型研究必须攻克的关键问题之一。
风蚀因子和风蚀速率动态变化现象已为国内外同行所了解,但各类因子以及风蚀速率的变化并非孤立,而是相互影响和制约的。风力因子—粗糙干扰因子相互耦合表现在,风力因子的变化改变地表动态粗糙元的变形和运动规律,后者反过来通过改变风速脉动、湍流、风向而影响风力作用于地表的方式和强度。风力变化是改变风蚀量的动力原因,风蚀物的运动反过来影响风力因子的一系列属性,甚至改变地表粗糙干扰因子,例如空气动力学粗糙度[26,33,157~160]。风力侵蚀因子—土壤抗蚀因子之间的互馈主要表现在:风蚀过程中表土颗粒组成的变化(一般表现为地表粗化)既是导致风蚀减弱的重要因素,也影响微地形的发育和演变,进而影响地表粗糙状况和近地面风力。这些互馈现象普遍存在于风蚀过程,但相关研究仅限于风沙流和近地面风速、地表空气动力学粗糙度之间的相互影响。问题的复杂性在于,目前仍没有能够精确测量各个因子之间反馈过程的仪器或者方法,而且各因子之间的相互作用都是连续动态变化的,增大了互馈研究的难度。
土壤风蚀预报模型是风蚀研究的重要目标之一。由于上述问题的存在,基于过程的土壤风蚀预报模型的构建,仍属难题。
针对国际土壤风蚀的研究现状,迫切需要开展如下几方面的工作:
(1) 建立标准风蚀观测场,开展风蚀过程野外定位观测,获取连续、完整的现场资料。野外观测一直是土壤风蚀研究的重要途径,但国内外同行观测结果的可比性和普适性很差,很难据此获取大田条件下风蚀量与风蚀因子的规律性结论。这也是风蚀模型普遍难以得到大田验证的一个重要原因。对土壤风蚀过程研究而言,野外风蚀观测既需要在地表不受其他扰动条件下开展长期定位观测,以揭示不同时间尺度下3类风蚀因子和风蚀速率的动态变化规律,也需要具备尽可能理想的空间条件(地势开阔、地形平坦、地表均一,且各方位具有不小于常见风速条件下风沙流饱和路径的较长距离),以揭示风蚀因子和风蚀速率在田块尺度内沿风向的变化规律。这一优势是风洞实验所无法比拟的,也是风洞实验结果验证所必需的,但长期以来没有得到应有的重视,反而成为风蚀研究方法的短板。本文作者等近期分别在河北坝上地区和内蒙古高原中部地区筹建的农田标准风蚀观测场和荒漠草原标准风蚀观测场,就是为提高土壤风蚀野外定位观测能力和水平,进而提高风蚀过程研究和模型研究水平而采取的重要步骤。为此,需要开展风蚀观测场建设标准研究,包括场地尺寸、地势地形条件、地表属性等场地要求;仪器设备配置标准;观测指标与操作规范;数据库构建标准等。
(2) 开展风沙流饱和路径研究,解决尺度问题。风沙流饱和路径是解决土壤风蚀空间尺度转换的关键。不同风速(风力因子)、土壤(土壤抗蚀性因子)、地表覆盖(地表粗糙干扰因子)条件下,风沙流饱和路径有很大的差别,因而风沙流饱和路径具有极大的复杂性。受实验段长度的限制,风洞内难以获得风沙流饱和路径,也就无法获得风蚀因子与风沙流饱和路径之间的定量关系。典型地表的土壤风蚀野外长期定位观测是解决该问题的不二选择。这就要求标准风蚀观测场必须充分考虑风沙流饱和路径长度问题。
(3) 建立具有扎实理论基础、基于动力过程和动态变化过程的风蚀模型,是现阶段土壤风蚀研究的一个重要目标。在国家自然科学基金重点项目的支持下,邹学勇等[4]提出的风蚀因子动力分类和参数化方案,为构建土壤风蚀动力模型迈出了重要一步;同时,风蚀因子和风蚀速率动态变化及其互馈研究已逐步展开。只有风蚀过程清晰,尺度问题得以解决并能够获得大量野外实测数据支持和验证,土壤风蚀模型的理论性和普适性才能得到保证。
The authors have declared that no competing interests exist.
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Classification and representation of factors affecting soil wind erosion in a model [J].土壤风蚀模型中的影响因子分类与表达 [J].DOI URL Magsci [本文引用: 10] 摘要
土壤风蚀是包括风、植被、土壤特性、土地利用方式、降水、微地形等多要素交互作用, 发生在特定地理空间, 具有独特的气流—土壤界面相互作用机制的连续动力学过程。基于统计学理论的土壤风蚀经验模型, 不仅难以避免子模型之间有多个风蚀影响要素的交叉出现, 使子模型之间不能严格地相互独立, 导致建模理论基础存在不足, 而且不能客观反映土壤风蚀的动力学过程。在厘清土壤风蚀基本概念, 分析国际土壤风蚀影响因子和土壤风蚀模型研究历史与存在不足的基础上, 提出一个新的基于风蚀动力学理论的土壤风蚀模型理论框架, 以及在此模型框架下土壤风蚀影响因子的分类与表达。阐述了该模型框架和影响因子分类与表达的合理性, 并对土壤风蚀影响因子分类与表达的研究途径进行了探讨。
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根据国外土壤风蚀的发展和研究成果, 本文将土壤风蚀研究历史划分为四个阶段。第一阶段大致在本世纪二十年代以前, 为土壤风蚀研究的萌芽阶段, 以土壤风蚀现象的认识和描述为主; 本世纪三十年代到六十年代中期, 为土壤风蚀科学研究大发展阶段, 实现了由定性描述到定量研究的突破; 六十年代中期到八十年代中期, 为土壤风蚀研究理论的检验与完善阶段。自本世纪八十年代中期之后, 土壤风蚀研究进入了全球性研究阶段。
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1998年初中国科学院知识创新工程得到了国家的批准,在建设国家创新体系的战略背景下,风沙物理研究得到了新的发展机遇。为了适应国家创新体系的要求,风沙物理学首先重新凝练了学科方向和目标,改善研究手段、拓展研究内容、创新研究思路,使我国风沙物理学研究的国际影响不断增加。笔者从风沙颗粒运动、风沙流、风沙边界层、风沙地貌动力学、土壤风蚀、防沙工程原理和风沙电等几个方面比较全面地回顾了中国科学院知识创新工程实施以来,我国风沙物理学研究所取得的主要成就。
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Research history and trend on soil property factors concerning wind erosion [J],土壤风蚀中有关土壤性质因子的研究历史与动向 [J].
国外土壤风蚀中有关土壤性质因子的研究历史可分为四个阶段:第一,定性描述阶段;第二,定量实验研究阶段;第三,运用综合因子建立和完善"通用风蚀方程式"阶段;第四,土壤风蚀土壤亚模型和预测系统的建立和动态研究阶段。国内对土壤风蚀的研究起步较晚,对于不同土类的土壤物理化学性质对风蚀的影响尚没有作深入研究。其历史大致可分为二个阶段:第一,野外观测研究;第二,风蚀的半定量实验研究。目前正由半定量向定量化、标准化、国际化以及动态研究方向发展。
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土壤风蚀可蚀性是风蚀研究的重要内容,也是进行土壤风蚀预报的重要参数。本文对土壤风蚀可蚀性的相关概念进行了阐释,从土壤颗粒及团聚体、土壤水分、土壤结皮、地表不可蚀物质、土地利用方式和空间尺度等6个方面对风蚀可蚀性控制因素的研究进行了综合评述。然后对土壤风蚀可蚀性的指标按照间接指标和直接指标两个类别进行了归纳,同时也对各个指标的监测方法进行了总结。在此基础上,对未来土壤风蚀可蚀性的研究方向进行了展望,讨论了土壤风蚀可蚀性研究需要关注的热点问题。
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The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This paper presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.
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Threshold friction velocity u*t is the friction velocity at which wind erosion is initiated. While u*t is affected by a range of surface and soil properties, it is a function of particle size only for idealized soils. In this paper we present a simple expression for u*t for spherical particles loosely spread over a dry and bare surface. In this expression we consider the balance between the driving forces (aerodynamic drag and lift) and the retarding forces (cohesion and gravity) and assume that the cohesive force is proportional to particle size. It is found that u*t can be expressed as , with Y1 and Y2 being empirical constants. The new expression is both simple and effective.
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Collision data are presented from coloured high-speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V 3 /V 1 630.5–0.6, V n /V 1 63.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud.
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A new model for dust emission by saltation bombardment [J].
In this paper, we present an analysis on the mechanism of dust emission and describe a new model for the prediction of dust emission rate caused by saltation bombardment, based on the volume removal by impacting sand grains as they plough into the soil surface. The model predicts a relationship between the removed volume V and the particle impacting velocity U of the form V 090809 Un, with n = 2 090804 3. The dust emission rate is found to be proportional to u*n + 1, where u* is friction velocity. These model predictions are consistent with wind tunnel and field observations. The model offers a new interpretation of observed data and a simple scheme for the calculation of dust emission.
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Wind tunnel experiments of dust emissions from different soil surfaces are carried out to better understand dust emission mechanisms. The effects of surface renewal on aerodynamic entrainment and saltation bombardment are analyzed in detail. It is found that flow conditions, surface particle motions (saltation and creep), soil dust content and ground obstacles all strongly affect dust emission, causing its rate to vary over orders of magnitude. Aerodynamic entrainment is highly effective, if dust supply is unlimited, as in the first 2-3 min of our wind tunnel runs. While aerodynamic entrainment is suppressed by dust supply limits, surface renewal through the motion of surface particles appears to be an effective pathway to remove the supply limit. Surface renewal is also found to be important to the efficiency of saltation bombardment. We demonstrate that surface renewal is a significant mechanism affecting dust emission and recommend that this mechanism be included in future dust models.
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Impact abrasion of harder by softer materials [J].
Fluorite, halite, periclase, and sylvite have been abraded in a wind tunnel by windblown dust from the "normal" air of the intake room plus or minus introduced silt-sized particles of calcite, barite, or snow. The results indicate that the effective kinetic energy of a projectile and the bond strength of the target-rather than hardness differences, per se-may be fundamental controls of windblast abrasion. It is concluded that, contrary to general belief, some windblast erosion features have probably been produced in response to impact by relatively soft and/or small missiles, such as snow and dust, moving at fairly low wind velocities.
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Abrasion of yardangs [J].
A model for the collision between a sand grain and a body of yardang material is established by using the principles of classical mechanics. A new nondimensional parameter, the "abrasion number" A(n), is derived from the model. The volume removed per impact for different targets is proportional to A(n) approximately. As an indicator of the energy ratio of the impacting material to the target material, A(n) not only reflects the previous "abrasion law" but also introduces two dominant mechanical parameters, Young's modulus and yield stress. We propose A(n) to be a similarity criterion for abrasion experiments. The practicability of the model under natural conditions is examined by an example. It is concluded that the abrasion rates of yardangs in the western Qaidam basin, China, are 0.011-0.398 mm/yr in the period of 1986 to 2010 and remarkable abrasion events generally correspond with strong wind conditions. The model will be helpful for reconstructing the history of aeolian activities from the morphologies of yardangs in the arid regions on Mars.
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A scaling theory for the size distribution of emitted dust aerosols suggests climate models underestimate the size of the global dust cycle [J].
Abstract Mineral dust aerosols impact Earth's radiation budget through interactions with clouds, ecosystems, and radiation, which constitutes a substantial uncertainty in understanding past and predicting future climate changes. One of the causes of this large uncertainty is that the size distribution of emitted dust aerosols is poorly understood. The present study shows that regional and global circulation models (GCMs) overestimate the emitted fraction of clay aerosols (< 2 脦录m diameter) by a factor of 芒聢录2-8 relative to measurements. This discrepancy is resolved by deriving a simple theoretical expression of the emitted dust size distribution that is in excellent agreement with measurements. This expression is based on the physics of the scale-invariant fragmentation of brittle materials, which is shown to be applicable to dust emission. Because clay aerosols produce a strong radiative cooling, the overestimation of the clay fraction causes GCMs to also overestimate the radiative cooling of a given quantity of emitted dust. On local and regional scales, this affects the magnitude and possibly the sign of the dust radiative forcing, with implications for numerical weather forecasting and regional climate predictions in dusty regions. On a global scale, the dust cycle in most GCMs is tuned to match radiative measurements, such that the overestimation of the radiative cooling of a given quantity of emitted dust has likely caused GCMs to underestimate the global dust emission rate. This implies that the deposition flux of dust and its fertilizing effects on ecosystems may be substantially larger than thought.
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Coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain-surface impact process. Experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced, including a diminution of the sub-saltation layer mean wind speed, as the friction speed increases. In this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free-stream shear stresses, and the grain mass flux increases with friction speed u f * less rapidly than u f 3 .
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Experiment on the dynamic photography of the movement of sand-driving wind [J].风沙运动的动态摄影实验 [J]. |
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McEwan I K. Observations of collisions of saltating grains with a granular bed from high-speed cine-film [J].
High-speed photography was used to record saltating sand grains colliding with a horizontal, noncohesive bed of similarly sized grains. Impacting grain/bed interaction is discussed in general. The process, as observed from the films, is then described in terms of the apparent bed contact length (ABCL) and various parameters of the impacting grains and any ejected grains. Examples are given of typical behaviour of bed grains in response to impacting grains of different sizes. Saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments, so that previously buried grains become available for entrainment. This process is discussed in relation to the potential release of dust particles into the airflow.
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Analysis on trajectory parameters of saltating grains in wind-blown sand [J].风沙流中跃移轨迹参数分析 [J].
应用高速频闪摄影技术和理论分析相互印证的方法，揭示了沙粒起跃角（α）与降落角（β）跃移长度和最大跃高之比（Ｌ／Ｈ）与α，沙粒运动方向和地面夹角（θ）与时间（ｔ）之间均为非线性关系，只有当沙粒粒径较大和垂直初速率较小时，可近似地将α与β看做线性关系，当Ｕ２Ｉ〈１ｍ／ｓ时，Ｌ／Ｈ≈４Ｃｔｇα，当α〈２５°时，θ与ｔ近似为线性关系。
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Aeolian Sand and Sand Dunes [M]. |
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Forces on a saltating grain in air [J].
A wind tunnel experiment was performed to measure the trajectories of individual sand grains. Then, the acceleration given by the numerical differentiation was used to assess the relative importance o
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Saltation of uniform grains in air [J].
The interaction between a turbulent wind and the motion of uniform saltating grains of sand or soil, so massive as to fail to enter into suspension, is examined on the basis of two complementary hypotheses. The first asserts that the effect of the moving grains on the fluid outside the region to which saltation is confined is similar to that of solid roughness of height comparable with the depth of the saltation layer. The second requires the concentration of particles engaging in the saltation to adjust itself so that the shear stress exerted by the wind on the groundis just sufficient to maintain the sand-strewn surface in a mobile state.
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Simulation of eolian saltation [J]. |
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Saltation layer vegetation canopies and roughness lengths [M]
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Large-eddy simulation of the formation of three-dimentional aeolian sand ripples in a turbulent field [J].湍流风场中三维风成沙波纹形成过程的大涡模拟研究 [J]. |
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Experimental assessment of Owen’s second hypothesis on surface shear stress induced by a fluid during sediment saltation [J].
Abstract A widely used, yet thus far unproven, fluid dynamical hypothesis originally presented by P. R. Owen 50-墆ears ago, states that the surface shear stress induced by a fluid on the ground during equilibrium sediment saltation is constant and independent of the magnitude of the fluid velocity and consequently the particle mass flux. This hypothesis is one of the key elements in almost all current model descriptions of sediment erosion. We measured the surface shear stress in a drifting-sand wind tunnel and found Owen's hypothesis being merely an approximation of the real situation. A significant decrease of the fluid stress with increasing wind velocities was measured for low to intermediate particle mass fluxes. For high particle mass fluxes, Owen's hypothesis essentially holds, although a slight increase of the fluid stress was measured.
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Magnus effect in saltation [J]. |
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The electrostatic force on saltating sand [J].
In blizzards and sandstorms, wind transport of particles is associated with separation of electrostatic charge. Moving particles develop charge of sign opposite the electrostatic charge on stationary surface particles. This electrification produces forces in addition to the gravitational and fluid friction forces that determine trajectories for particles being transported in saltation. Evaluating electrostatic forces requires the electric field strength very near the saltation surface and charge-to-mass ratios for the moving particles. In a low-level blowing sand event we measured an average charge-to-mass ratio of +60 0204C kg0908081 on the saltating particles at 5-cm height and a maximum electric field of +166 kV m0908081 at 1.7-cm height, in wind gusts near 12 m s0908081 at 1.5-m height. The electrostatic force estimated from these measurements was equal in magnitude to the gravitational force on the saltating particles. Including electrostatic forces in the equations of motion for saltating particles may help explain discrepancies between measurements and models of saltation transport.
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Discussion on drag coefficient and lift coefficient of saltation particles in air flow [J].气流中颗粒阻力系数和升力系数的讨论 [J].
对于颗粒在气流中处于跃移状态时的受力问题,许多学者已作过论述。但是,不同学者在建立跃移模型时所选取的力并不完全相同。而且在力的计算方面见解不一,突出表现在阻力和升力的确定上。鉴于此,文章以阻力、阻力系数和升力、升力系数为出发点论述了阻力系数和升力系数的来源与本质,并针对当前广泛使用的阻力和升力公式进行评价,指出这些公式在应用中的缺陷。建议采用阻力系数和升力系数参数,通过实验测量来确定阻力和升力系数,以精确计算颗粒所受的阻力和升力。并对这种方法的合理性进行了讨论。
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Effects of the Magnus and Saffman forces on the saltation trajectories of sand grain [J].
Saltating sand grains are the primary component of airborne sand and account for 75% of all sand transport flux. The saltation height and horizontal distance traveled by sand grains are key factors in sand-control engineering. In addition to gravity and aerodynamic drag, the Magnus and Saffman forces also play important roles in saltation. To quantify the magnitudes of these forces in saltation we used high-speed multi-flash photography to observe the movement of saltating sand grains in a wind tunnel; this proved to be an efficient technique for determining the movement and rotational velocities of grains of natural sand with grain sizes ranging from 0.2 to 0.3mm and shear velocities ( u 68 ) of 0.67, 0.83, and 0.87m s 61021 . The rotational speed of saltating sand grains varied between 200 and 800rev s 61021 ; mean clockwise and anticlockwise rotational speeds were nearly identical, and both increased with increasing saltation height. With saltation heights divided into 1cm intervals, the rotational speeds followed a Lorentzian distribution. Calculations based on a saltation model showed that the maximum increases in saltation height and in horizontal distance due to the Magnus force were 10.2 and 24.9%, respectively. The rate of increase of both parameters increased with increasing lift-off angle. The maximum increases in saltation height and horizontal distance of sand grains caused by the Saffman force were only 4.6% and 3.7%, respectively.
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| [42] |
Transport mass of creeping sand grains and their movement velocities [J].
[1] Aeolian sand transport is an important component of material circulation above terrestrial surfaces and can include processes of creep, saltation, and suspension. The complex movement of material and energy during aeolian transport has meant that these processes have previously been examined in isolation. Although a significant amount of research has been conducted on aeolian sand transport, this focused primarily on saltation. As a result, there are few data available on sand grain creep, primarily due to a lack of theoretical models and the difficulty of direct measurements. In this study, we present novel methods and instrumentations to accurately measure the transport mass and the velocity of creeping sand grains. Using data collected with above instrumentations and a ladder sampler at four friction velocities (u*090009=0900090.26, 0.35, 0.47, and 0.56090009m090009s0908081) in a wind tunnel, we studied the transport mass of creeping sand grains and their movement velocities, as well as other key parameters of aeolian sand transport. Four major conclusions can be drawn from this study: (1) The transport mass (q0) of creeping sand grains increases with increasing frictional wind velocity (u*). The relationship between these variables is represented by the power function q0090009=0900090908080.053090009+0900099.195u*2.800. (2) The contribution of creep to total aeolian sand transport decreases with increasing frictional wind velocity. Creep contributed 57% of total aeolian transport at the lowest frictional wind velocity but only 19% at the highest velocity. (3) The threshold frictional wind velocity for entrainment was 0.158090009m090009s0908081. (4) The movement velocity of creeping sand grains ranged from 0 to 0.14090009m090009s0908081, but more than 70% of recorded velocities were less than 0.02090009m090009s0908081. Although the results of this study require further validation, they provide a strong basis for future research and help deepen our understanding of aeolian sand transport.
|
| [43] |
A bedload trap for aeolian sand transport [J].
This paper describes a bedload trap designed to separate bedload from saltation load in aeolian environments. The trap is installed below the sand surface and features a chimney that can be adjusted to the height of the surface. The chimney houses an internal wall to separate saltation load from bedload. Bedload particles are funneled to a piezo-electric sensor that converts grain impacts to voltages that can be sampled at very high frequencies (44,000聽Hz in this example). Grains are then collected in a container that is easily retrieved so that sand samples can be obtained for weighing and subsequent grain size analysis. An algorithm to isolate single grain impacts is described. The version of the trap presented here is intended for the study of the initiation of grain motion.
|
| [44] |
Measurement of sand creep on a flat sand bed using a high-speed digital camera [J].
The characterization of aeolian sand creep motion remains incomplete. In this study, a top-view imaging system was constructed in a wind tunnel experiment to record sand creep motion on a flat sand bed. In addition, a set of ‘mesoscopic’ parameters was defined based on the creep trajectory that was reconstructed by a particle tracking algorithm. Statistical results show that the parameters representing the lifespan of the creep motion follow a lognormal distribution, whereas those representing the sideways grain motion deviate from the lognormal distribution. As the wind velocity increases, the sideways motion is strengthened and the lifespan of the creep motion is shortened, indicating a stronger transition from creep to saltation. Furthermore, under a ‘pure rolling’ hypothesis, the creep trajectory can be transformed into two numbers of rolling cycles from the perspectives of space and time. The ratio of the two numbers is useful in identifying the specific mode of the creep motion.
|
| [45] |
Recent developments in the dynamics of wind erosion [ |
| [46] |
Dust transport and the question of desert loess formation [J].
ABSTRACT Although contemporary dust storms are frequent in arid and semi-arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid-latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub-tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re-examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short-term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 渭m are transported mainly in long-term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust-transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid-latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi-arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert-marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over-estimate the longer-term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi-arid areas and is finer than typical loess.
|
| [47] |
|
| [48] |
Desert Dust in the Global System [M]. |
| [49] |
A theoretical interpretation of the “giant” dust particle conundrum [J].
The effects of electric force were used to interpret a well known conundrum about the long-term (or long-distance) dust transport in the atmosphere.
|
| [50] |
A theoretical model on the relation between wind speed and grain size in dust transportation and its paleoclimatic implications [J].
In studies of paleoclimate and paleoenvironment based on loess-palaeosol deposits, the rigorous relations between climate elements and the properties of loessic sediment are scarce because the dynamic processes controlled by the related physical laws are often ignored or arbitrarily simplified. In the current study, the horizontal distance of a suspended dust particle is re-estimated using classical mechanics and the relation between wind speed and the grain size of primary loess is then established. The actual application of this new theoretical relation is given by an example of Duowa profile, western Chinese Loess Plateau. The numerical results imply that the Qaidam Basin could be a likely dust source of the Chinese Loess Plateau.
|
| [51] |
Local, proximal and distal Saharan dusts: Characterization and contribution to the sedimentation [ |
| [52] |
The nature and tropospheric formation of iberulites: Pinkish mineral microspherulites [J].
To date, we do not know to what extent these iberulites are present in time and space or whether they can serve as markers for environmental or paleoclimatic analyses or even lead to uncertainty in radiative transfer models.
|
| [53] |
Long-range transport of giant particles in Asian dust identified by physical, mineralogical, and meteorological analysis [J].
Giant particles transported over long distances are generally of limitedconcern in atmospheric studies due to their low number concentrations inmineral dust and possible local origin. However, they can play an importantrole in regional circulation of earth materials due to their enormous volumeconcentration. Asian dust laden with giant particles was observed in Korea on31 March 2012, after a migration of about 2000 km across the Yellow Sea fromthe Gobi Desert. Scanning electron microscopy (SEM) revealed that 20% ofthe particles exceeded 10 μm in equivalent sphere diameter, with amaximum of 60 μm. The median diameter from the number distributionwas 5.7 μm, which was larger than the diameters recorded of 2.5 and2.9 μm in Asian dust storms in 2010 and 2011, respectively, and wasconsistent with independent optical particle counter data. Giant particles(10 μm) contributed about 89% of the volume of thedust in the 2012 storm. Illite–smectite series clay minerals were the majormineral group followed by quartz, plagioclase, K-feldspar, and calcite. Thetotal phyllosilicate content was ~52%. The direct long-rangetransport of giant particles was confirmed by calcite nanofibers closelyassociated with clays in a submicron scale identified by high-resolution SEMand transmission electron microscopy. Since giant particles consisted of clayagglomerates and clay-coated quartz, feldspars, and micas, the mineralcomposition varied little throughout the fine (5 μm),coarse (5–10 μm), giant-S (10–20 μm), and giant-L(20 μm) size bins. Analysis of the synopticconditions of the 2012 dust event and its migration indicated that themid-tropospheric strong wind belt directly stretching to Korea induced rapidtransport of the dust, delivering giant particles. Giant dust particles withhigh settling velocity would be the major input into the terrestrial andmarine sedimentary and ecological systems of East Asia and the westernPacific. Analysis of ancient aeolian deposits in Korea suggested the commondeposition of giant particles from Asian dust through the late QuaternaryPeriod. The roles of giant particles should be reviewed with regard toregional circulation of mineral particles and nutrients.
|
| [54] |
The overshoot and equilibration of saltation [J].
Downwind of the leading edge of a saltating surface, the saltation process develops toward an eventual stream wise equilibrium. The nature of this development is investigated in a portable wind tunnel by measuring the vertically integrated streamwise flux Q of saltating sand grains as a function of distance x from the leading edge of the erodible surface, at wind speeds between 8 and 13 m s芒聢聮1. It is found that Q(x) increases from zero at x = 0 to a maximum value at x around 7 m, before decreasing toward an eventual equilibrium value (about half the maximum value) when x is greater than about 15 m. These results are in qualitative agreement with two recent numerical simulations of the approach of saltation to equilibrium. The experiment indicates the minimum length of wind tunnel required for studying the saltation process in its equilibrium state, and the order of magnitude of the measurement errors if shorter tunnels must be used.
|
| [55] |
Wind erosion within a simple field [J].
ABSTRACT Elementary analysis of the wind erosion process within a flat and uniform field is presented. A relationship is derived that describes the variation of the horizontal component of mass flux, fx, downwind of a distinct field boundary. The derived functional relationship is then verified and completed through the use of field data. The generalized form of the equation contains two parameters, fj^j^ and b, that describe different aspects of the wind erosion process. The quantity f^^^ is the maximum horizontal flux for a given height. The length scale b represents the distance at which f,^ attains a value of 63.2% of f^x-
|
| [56] |
Large scale variability of wind erosion mass flux rates at Owens Lake Part I: Verticle profiles of horizontal mass fluxes of wind-eroded particles with diameter greater than 50 μm [J].
A field experiment at Owens (dry) Lake, California, tested whether and how the relative profiles of airborne horizontal mass fluxes for >50-0204m wind-eroded particles changed with friction velocity. The horizontal mass flux at almost all measured heights increased proportionally to the cube of friction velocity above an apparent threshold friction velocity for all sediment tested and increased with height except at one coarse-sand site where the relative horizontal mass flux profile did not change with friction velocity. Size distributions for long-time-averaged horizontal mass flux samples showed a saltation layer from the surface to a height between 30 and 50 cm, above which suspended particles dominate. Measurements from a large dust source area on a line parallel to the wind showed that even though the saltation flux reached equilibrium 090804650 m downwind of the starting point of erosion, weakly suspended particles were still input into the atmosphere 1567 m downwind of the starting point; thus the saltating fraction of the total mass flux decreased after 650 m. The scale length difference and ratio of 70/30 suspended mass flux to saltation mass flux at the farthest down wind sampling site confirm that suspended particles are very important for mass budgets in large source areas and that saltation mass flux can be a variable fraction of total horizontal mass flux for soils with a substantial fraction of <100-0204m particles.
|
| [57] |
The flux profile of a blowing sand cloud: A wind tunnel investigation [J].
The flux profile of a blowing sand cloud, or the variation of blown sand flux with height, is the reflection of blown sand particles that move in different trajectories, and also the basis for checking drifting sand. Here we report the wind tunnel results of systematic tests of the flux profiles of different sized sands at different free-stream wind velocities. The results reveal that within the 60-cm near-surface layer, the decay of blown sand flux with height can be expressed by an exponential function: q h= aexp(鈭 h/ b), where, q h is the blown sand transport rate at height h, a and b are parameters that vary with wind velocity and sand size. The significance of coefficient a and b in the function is defined: a represents the transport rate in true creep and b implies the relative decay rate with height of the blown sand transport rate. The true creep fraction, the ratio of the sand transported on the surface ( h=0) to the total transport varies widely, decreasing with both sand size and wind speed. The flux profiles are converted to straight lines by plotting sand transport rate, q h, on a log-scale. The slope of the straight lines that represents the relative decay rate with height of sand transport rate decreases with an increase in free-stream wind velocity and sand grain size, implying that relatively more of the blown sand is transported to greater heights as grain size and wind speed increase. The average saltating height represented by the height where 50% of the cumulative flux percentage occurs increases with both wind speed and grain size, implying that saltation becomes more intense as grain size and/or wind velocity increase.
|
| [58] |
Theoretical predict ion of creep flux in aeolian sand transport [J].
The creep motion in aeolian sand transport is studied using two typical granular flow models. We focus on the expression of creep flux. It is theoretically revealed that creep fraction (the contribution of creep to the overall sand flux) changes with wind velocity and grain size.
|
| [59] |
Near-bed mass flux profiles in aeolian sand transport: High-resolution measurements in a wind tunnel [J]. |
| [60] |
Vertical profiles of aeolian sand mass flux [J].
Vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel. Considering lower sampling efficiency of the sand trap in the near-bed region, this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap. The experiments were conducted with two test sands and five different stream velocities. In the upper part of the vertical profile, the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near-bed region. The measured longitudinal profiles are similar to the measured vertical profiles. Linking both profiles and the modes of sand transport, it is possible that saltating sand grains give rise to the well-known exponential decay distribution of mass flux, and that creeping and reptating grains force a deviation from it. A simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data.
|
| [61] |
The pattern of grainfall deposition in the lee of aeolian dunes [J].
Abstract ABSTRACT A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density. The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits. The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum.
|
| [62] |
Review of the mechanism of dust emission and deposition [J].粉尘释放和沉积机制的研究进展 [J]. |
| [63] |
Mineral Dust: A Key Player in the Earth System [M]. |
| [64] |
|
| [65] |
Precipitation scavenging [ |
| [66] |
Dynamics of wind erosion: I. Nature of movement of soil by wind [J]. |
| [67] |
A wind erosion equation [J].
The amount of erosion, E, expressed in tons per acre per annum, that will occur from a given agricultural field can be expressed in terms of equivalent variables as: E = f(I', K', C', L', V) where I' is a soil erodibility index, K' is a soil ridge rougness factor, C' is a climatic factor, L' is field length along the prevailing wind erosion direction, and V is equivalent quantity of vegetative cover. The 5 equivalent variables are obtained by grouping some and converting others of the 11 primary variables now known to govern wind erodibility. Relations among variables are extremely complex. Charts and tables have been developed to permit graphical solutions of the equation. The equation is designed to serve the twofold purpose of providing a tool to (i) determine the potential erosion from a particular field, and (ii) determine what field conditions of soil cloddiness, roughness, vegetative cover, sheltering by barriers, or width and orientation of field are necessary to reduce potential erosion to a tolerable amount. Examples of these applications of the equation are presented. Weaknesses in the equation and areas needing further research are discussed.
|
| [68] |
|
| [69] |
TEAM: Taxas erosion analysis model [ |
| [70] |
A model for prediction aeolian and drift and dust entrainment on scales from paddock to region [J].
This paper describes a Wind Erosion Assessment Model (WEAM) for the estimation of sand drift and dust entrainment in agricultural areas. Both the sand drift and dust entrainment parts of the model are physically based, utilising a combination of established and recent theoretical and experimental results. Key components of the model include the Owen equation for the saltation flux; the observed and theoretically predicted proportionality between saltation flux and dust entrainment by saltation bombardment; theoretical and experimental results on the amelioration of wind erosion by nonerodible roughness; and new experimental results on the suppression of erosion by surface moisture. The size distribution of the particles on the soil surface (in their natural state) is used as a primary parameter. The model is restricted to a description of the mobilisation of sand and dust in erosion source areas, and specifically excludes treatment of 2 groups of related processes: dust transport away from source areas and its ultimate deposition; and evolution of surface properties, by the wind erosion process itself, by other weathering processes, or by management intervention. The results of the model are compared with data from a portable wind erosion tunnel, and with direct wind erosion measurements at paddock scale. By offering a synthesis of available physical knowledge of sand drift and dust entrainment, the model also indicates key areas of uncertainty.
|
| [71] |
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| [72] |
A wind erosion prediction system to meet the user need [J].
Single-sprinkler wind-distorted distribution patterns are simulated utilizing drop trajectory computations, and compared with measured patterns. It is found that the exact formulation of the drag coefficient of single drops is not critical for applications focusing on water distribution. A drag correction factor, k is introduced to account for the effect of the incidence angle between drag force and orientation of a jet segment. The agreement between measured and computed patterns improves considerably when the proper value of k has been used. Three measures of similarity of computed to measured single patterns are put forth. The most comprehensive measure, denoted by sigma, is a normalized sum of squares of differences between the measured and computed local application rates. The values of k that produce the best agreement in terms of sigma seem to be an increasing function of the range of a sprinkler. Single patterns computed with the best values of k were used to calculate uniformity coefficients for a realistic spacing of the medium-pressure sprinklers under consideration. The results suggest that the computational method can be used to forecast, with an error of only a few percent, the uniformity of application under wind speeds as high as 8 m/s. Considering the effort involved in field measurements, this seems a viable alternative.
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| [73] |
The effect of roughness elements on wind erosion threshold [J].
A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio Rt, the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present. The roughness density is quantified by the frontal area index 0203. The prediction is Rt = (1 090808 m03030203)0908080005(1 + m02050203)0908080005, where 0205 is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; 0303 is the basal-to-frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point. The prediction is well verified by four independent data sets.
|
| [74] |
Intermittent saltation [J]. |
| [75] |
Grain transport rates in steady and unsteady turbulent airflow [J]. |
| [76] |
Transitional behavior of saltation: Wind tunnel observations of unsteady winds [J].
In sequences of sinusoidal velocity variations, sand transport rate is found to increase as gust frequency increases. This relation becomes more pronounced as gust amplitude increases. In the range of energy-containing atmospheric gusts with periodicities between 6 and 20s, sand transport occurs at rates in excess of that recorded for steady winds of the same mean velocity. This result has significant implications for the prediction of sand transport in unsteady winds.
|
| [77] |
Effect of wind speed fluctuation on sand transport rate [J].风沙流中风速脉动对输沙量的影响 [J].
<FONT face=Verdana>通过对不同工程措施下风沙流中风速脉动特征与输沙量波动研究发现：不同孔隙度栅栏中各高度层瞬时风速的波动性具有很好的相关性，而且相邻高度之间的相关性非常显著。在不同风速下，典型工程措施中同一高度层输沙量具有相对应的变化特征；瞬时风速的波动性主要与其所在高度层沙粒的运动状态和工程效益有关。</FONT>
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| [78] |
Experimental measurement of air flow velocity fluctuation of wind-sand flux [J].风沙流中风速脉动的实验测量 [J].
在风洞中测量了四种风速条件下净风与挟沙风的速度脉动,发现沙粒 运动削弱了平均流速而增加了脉动流速,并且不改变流速脉动的分布规律.挟沙风的脉动速度不再像净风那样随高度增加而单调减小,而是在床面附近变化不大,接 着出现极大值,随后依高度增大又趋于减小;也不像净风那样随平均风速的增大而减小,而是先增大后减小,存在极大值.
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| [79] |
Baas A C W, et al. Critical reflections on the coherent flow structures paradigm in aeolian geomorphology [ |
| [80] |
Coherent structures over flat sandy surfaces in aeolian environment [J].
react-text: 283 Dynamic mutual coupling of wind force, roughness elements and soil anti-erodibility controls the process of soil wind erosion. The factors are changeable in any single wind erosion event, including…" /react-text react-text: 284 /react-text [more]
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| [81] |
Very large scale motions in the atmospheric surface layer: A field investigation [J].
A field observation array for the atmospheric surface layer (ASL) was built on a dry flat bed of Qingtu Lake in Minqin (China) as the Qingtu Lake Observation Array (QLOA) site, which is similar to the Surface Layer Turbulence and Environmental Science Test (SLTEST) site in the Utah (USA) Western desert. The present observation array can synchronously perform multi-point measurements of wind velocity and temperature at different vertical and streamwise positions. In other words, three-dimensional turbulent ASL flows can be measured at the QLOA station and Reynolds numbers as high as can be achieved with steady wind conditions. By careful selection and pretreatment for measured data of more than 1200 h, the QLOA data have been validated to be reliable for high Reynolds number turbulent boundary layer research. Results from correlation and spectral analysis confirm that very large scale motions (VLSMs) exist in the ASL at a Reynolds number up to . Through premultiplied spectral analysis, it is revealed that the spectral energy in the high-wavenumber region decreases with height, similar to turbulent boundary layers at low or moderate Reynolds numbers, while it increases with height in the low-wavenumber region resulting in a log090009linear increase of VLSMs energy with height, which is different from turbulent boundary layers at low or moderate Reynolds numbers. The present analyses support the view that the evolution of the VLSMs cannot be fully attributed to a 090004bottom-up090005 mechanism alone, and probably other mechanisms, including a 090004top-down090005 mechanism, also play a role.
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| [82] |
Vortex shedding and morphodynamic response of bed surfaces containing non-erodible roughness elements [J].
A series of wind tunnel experiments was carried out to investigate particle entrainment from surfaces in which one or more roughness elements were embedded. Thin sand strips were employed to eliminate impact and ejection, and thus isolate entrainment by fluid drag. The pattern of erosion is consistent with the presence of coherent vortices, inclusive of trailing vortices in the wake flow. The shape and orientation of the roughness element strongly influence this pattern. When an upwind supply of saltators is introduced, the majority of particles within the bed are entrained through impact, with the exception of a sand tail to the lee of the roughness element. That is, the effect of coherent structures within the airflow, as related to spatial variation in the fluid drag exerted on the bed surface, is completely overprinted by the saltation cloud and the blocking of particle trajectories by the upwind face of the roughness element. In a repeated set of experiments, the bed was allowed to fully adjust its morphology to the transport system. In this case, particle entrainment did not selectively occur within the zone of wake flow, and by inference the fluid stress across the test surface appeared to be uniform. These experiments support the hypothesis that vortex annihilation occurs on morphodynamically adjusted surfaces. In summary, the system response to the emergence of non-erodible roughness elements on surfaces affected by wind erosion involves a suite of geophysical processes, each of which attains varied levels of dominance within a given morphodynamic domain.
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| [83] |
Time problem in calculating soil wind erosion rate with wind tunnel experiment [J].用风洞实验方法计算土壤风蚀量的时距问题 [J].
通过对延津县沙质壤土原状土进行风蚀风洞模拟实验，探讨了土壤风蚀量计算中的时距问题。结果表明，前人使用“风蚀模数”概念来计算土壤蚀量会导致较大误差。原因在于同一风蚀事件中单位时间内的风蚀量随吹蚀的时间的延续而递减，在风洞实验计算风蚀量时使用“累积风蚀量”概念将会有更精确的结果。
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| [84] |
The role of maximum wind speed in sand-transporting events [J].
A sand-transporting event is the minimum unit of aeolian sand transport process. To understand the role of maximum wind speed in such sand-transporting events, wind speeds were measured at a height of 2聽m above the ground in the arid (Menggen) and semi-arid (Taibus Banner) regions in north China during 2009 and 2011. The sand transport flux of each sand-transporting event was calculated based on theoretical equations. Then, the relationships between the maximum wind speed and the average wind speed, the duration and sand transport flux of sand-transporting events were analyzed. It was found that the maximum wind speed was proportional to the average wind speed of sand-transporting events, with a linear model fit, and was also significantly correlated with the duration of sand-transporting events with a power model fit. The maximum wind speed was also positively correlated with sand transport flux of sand-transporting events according to a power model. The maximum wind speed could therefore represent both wind speed and the duration of sand-transporting events, and play a decisive role in the sand transport process of these events. The sand transport flux of sand-transporting events can be predicted rapidly and conveniently by monitoring maximum wind speed.
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| [85] |
Wind regime and sand transport in the corridor between the Badain Jaran and Tengger deserts, central Alxa Plateau, China [J].
The Alxa Plateau in north China is characterized by frequent sand dust storm activity, desertification, various blown sand hazards and extensive sand dunes. Three of China’s major sand deserts, the Badain Jaran (BJ), Tengger and Ulan Buh (UB), are distributed in this region. The BJ desert lies to the northwest and is separated from the other two deserts by mountains Yabrai and Alateng. However, the dominant northwest wind could transport sand from the BJ to the other two deserts through several corridors. Locating the sand source for these deserts is fundamental in understanding the formation and evolution of the aeolian landforms. It has been proposed that the sand in the Tengger desert is from the BJ desert. However, evidence supporting the hypothesis is still limited. To estimate the sand contribution of the BJ to the Tengger desert, we measured wind speeds at a 202m height above the ground for the period from November 2010 to December 2011. We then calculated the amount of sand transport and observed the speed of dune migration in the junction part of the two deserts. Sand-transporting winds (826.002m02s 611 ) occurred mostly in spring and winter, and accounted for 16.4% of the total of the year. The prevailing wind directions were NW, WNW and NNW, and were occupied 61.9% of the total frequency of sand-transporting winds. The frequencies of winds decreased with increasing wind speed, and strong wind frequencies (8217.002m02s 611 ) were 5.3% of the sand-transporting winds. In comparison to adjacent areas, the drift potential in the corridor was several times higher, indicating an obvious effect of narrowing. During the period of observation, 752 sand-transporting events occurred with durations from 10 to 194002min (3202h). In the corridor, the sand transport flux was 37202tons02m 611 02yr 611 , an order of magnitude larger than previous estimation, and the annual total amount of sand transported through the corridor was over 502million tons, indicating a substantial sand supply from the BJ to the Tengger desert. Sand transport in spring and winter accounted for 99.8% of annual total. The amount of sand transported by a single sand-transporting event varied greatly, up to five orders of magnitude.
|
| [86] |
Desertificatopn in the Region of Shenfu-Dongsheng coal Field[D].Lanzhou: Institute of Desert Research, 神府东胜煤田地区的沙漠化问题[D] . |
| [87] |
Experimental investigations of the problem of surface roughness[R]. |
| [88] |
Drag and drag partition on rough surfaces [J].
An analytic treatment of drag and drag partition on rough surfaces is given. The aims are to provide simple predictive expressions for practical applications, and to rationalize existing laboratory and atmospheric data into a single framework. Using dimensional analysis and two physical hypotheses, theoretical predictions are developed for total stress (described by the square root of the canopy drag coefficient), stress partition (described by the ratio Τ S /Τ of the stress Τ s on the underlying ground surface to total stress Τ), zero-plane displacement and roughness length. The stress partition prediction is the simple equation τ S /τ= 1/(1+βλ), where λ= C R C S the ratio of element and surface drag coefficients. This prediction agrees very well with data and is free of adjustable constants. Other predictions also agree well with a range of laboratory and atmospheric data.
|
| [89] |
Wind-induced tree sways [J]. |
| [90] |
Vibration behavior of plantation-grown Scots pine trees in response to wind excitation [J].
This paper deals with the dynamic responses of three plantation-grown Scots pine trees to turbulent wind loading. Although individual tree movement was complicated and irregular in the analyzed wind speed range (hourly mean wind speeds at canopy top below 402m02s 611 ), results from the cross-wavelet transform and wavelet coherence calculations demonstrate that wind-induced tree displacement strongly covaried with wind loads exerted on the trees by coherent structures. These wind loads caused largest tree displacements in the along wind direction as well as in the across wind direction. The absorption of energy available from wind loads in the range of the damped natural sway frequencies of the sample trees’ stems was of minor importance for tree displacement. The application of the bi-orthogonal decomposition provided insight into the resulting modal deformations of the sample trees’ stems. Based on measured stem displacement data, the mean mode shapes for the first eight stem vibration modes (four mean mode shapes in along wind direction and four mean mode shapes in across wind direction) could be identified. The shapes of these modes were quite similar to the normal mode shapes of a clamped-free beam. For the analyzed wind speed range it is shown that most of the energy absorbed by the trees from the wind is dissipated by tree sway in the first mode. Higher vibration modes localized on the sample trees’ stems were only of minor importance for the trees’ responses to wind.
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| [91] |
Damping by branching: A bioinspiration from trees [J].
Man-made slender structures are known to be sensitive to high levels of vibration due to their flexibility which often cause irreversible damage. In nature, trees repeatedly endure large amplitudes of motion, mostly caused by strong climatic events, yet with minor or no damage in most cases. A new damping mechanism inspired by the architecture of trees is identified here and characterized in the simplest tree-like structure, a Y-shaped branched structure. Through analytical and numerical analyses of a simple two-degree-of-freedom model, branching is shown to be the key ingredient in this protective mechanism that we call damping-by-branching. It originates in the geometrical nonlinearities so that it is specifically efficient to damp out large amplitudes of motion. A more realistic model, using flexible beam approximation, shows that the mechanism is robust. Finally, two bioinspired architectures are analyzed, showing significant levels of damping achieved via branching with typically 30% of the energy being dissipated in one oscillation. This concept of damping-by-branching is of simple practical use in the design of very slender and flexible structures subjected to extreme dynamical loadings.
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| [92] |
Turbulence in plant canopies [J]. |
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A frequency lock-in mechanism in the interaction between wind and crop canopies [J]. |
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Aerodynamic roughness length related to non-aggregated tillage ridges [J].
Wind erosion in agricultural soils is dependent, in part, on the aerodynamic roughness length (lt;igt;zlt;subgt;0lt;/subgt;lt;/igt;) produced by tillage ridges. Although previous studies have related lt;igt;zlt;subgt;0lt;/subgt;lt;/igt; to ridge characteristics (ridge height (lt;Igt;RHlt;/Igt;) and spacing (lt;Igt;RSlt;/Igt;)), these relationships have not been tested for tillage ridges observed in the North African agricultural fields. In these regions, due to climate and soil conditions, small plowing tools are largely used. Most of these tools produce non-aggregated and closely-spaced small ridges. Thus, experiments were conducted in a 7-m long wind tunnel to measure lt;igt;zlt;subgt;0lt;/subgt;lt;/igt; for 11 ridge types covering the range of geometric characteristics frequently observed in south Tunisia. Experimental results suggest that lt;Igt;RHlt;/Igt;lt;supgt;2lt;/supgt;/lt;Igt;RSlt;/Igt; is the first order parameter controlling lt;igt;zlt;subgt;0lt;/subgt;lt;/igt;. A strong relationship between lt;igt;zlt;subgt;0lt;/subgt;lt;/igt; and lt;Igt;RHlt;/Igt;lt;supgt;2lt;/supgt;/lt;Igt;RSlt;/Igt; is proposed for a wide range of ridge characteristics.
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| [95] |
Soil properties affecting wind erosion [J].
Single-sprinkler wind-distorted distribution patterns are simulated utilizing drop trajectory computations, and compared with measured patterns. It is found that the exact formulation of the drag coefficient of single drops is not critical for applications focusing on water distribution. A drag correction factor, k is introduced to account for the effect of the incidence angle between drag force and orientation of a jet segment. The agreement between measured and computed patterns improves considerably when the proper value of k has been used. Three measures of similarity of computed to measured single patterns are put forth. The most comprehensive measure, denoted by sigma, is a normalized sum of squares of differences between the measured and computed local application rates. The values of k that produce the best agreement in terms of sigma seem to be an increasing function of the range of a sprinkler. Single patterns computed with the best values of k were used to calculate uniformity coefficients for a realistic spacing of the medium-pressure sprinklers under consideration. The results suggest that the computational method can be used to forecast, with an error of only a few percent, the uniformity of application under wind speeds as high as 8 m/s. Considering the effort involved in field measurements, this seems a viable alternative.
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| [96] |
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| [97] |
Wind erosion: Predictions of aggregate abrasion coefficients [J].
ABSTRACT Among the major factors controlling wind erodibility of soils are the abrasion coefficients of the soil crust and aggregates. Here, the abrasion coefficient is defined as the abraded soil loss per unit area for each unit mass of abrader passing a unit across-wind width and has units (l/m). However, measuring abrasion coefficients directly in the field is not convenient. So, in field plot studies, energy to crush aggregates (CE) has been measured and related to intrinsic soil properties. In this study, wind tunnel tests were used to develop a prediction equation for aggregate abrasion coefficients as a function of Ln(CE) of aggregates and crusts. KEYWORDS. Wind erosion, Abrasion, Aggregates, Crush energy. abrasion coefficients for soil clods and crusts are needed. Because the coefficients are temporal properties and also vary widely among soils, many laboratory and field plot measurements would be needed to develop prediction equations for this variable. Unfortunately, measuring abrasion coefficients directly is not convenient, because specialized, expensive equipment, such as wind tunnels, is required. Thus, in the WEPS structure, dry stabilities of clods and crust were selected as the temporal variables for daily updating in the model, because they are much easier to measure, by both the researchers developing the prediction equations and users seeking to validate the model in the field. The remaining problem is then to determine the abrasion coefficients as functions of the dry aggregate and crust stabilities. A series of wind tunnel studies was carried out to determine this relationship and is the focus of this report.
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| [98] |
Temporal aspects of the abrasion of microphytic crusts under grain impact [J].
Abstract Wind-tunnel simulations of the response of two moss crusts to grain impact indicate that, given sufficient time, these surfaces will deteriorate under very low wind velocities only slightly above u * t for the loose, saltating grains. In parallel with these experiments, the frequency distributions of ultimate strength and penetration energy were determined for each of the two crust types via penetrometry. Pohlia was found to be stronger than Tortula ; but, even so, both of these crusts had ultimate strengths 20-350 times higher than the force delivered by a single grain impacting each surface at a velocity of 1 ms 鈭1 . In comparison, the modulus of deformation and penetration energy data were very similar for the two surface types, especially for the weakest areas of crust development. This observation is in accord with the wind-tunnel simulations that also found no consistent difference in the response of these two crust types to impact. In comparison with crusts formed by clay and salt, fibrous microphytic crusts are morphologically complex and typically weak. The notable elasticity of these surfaces does reduce the force of grain impact, and thereby provides some protection against rupture. One of the central conclusions of this study suggests that not only is the particle kinetic energy at impact important in crust breakdown, but also tiny fractures at points of localized stress concentration contribute to a progressive reduction in the integrity of the filament net. In some of the experiments conducted as part of this study, up to 50 or more minutes of constant bombardment was required to produce small abrasion marks on selected areas of the microphytic crust. This study prepares a foundation for future experiments needed to examine the breakdown of complex crusts formed in nature. Copyright 漏 2002 John Wiley & Sons, Ltd.
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| [99] |
Dynamic controls on wind erosion and dust generation on west-central Free State agricultural land, South Africa [J].
Abstract The west-central part of South Africa's Free State Province falls within the transition zone between South Africa's sub-humid, temperate grasslands to the east, and the semi-arid Karoo and arid Kalahari to the south and west, respectively. The area is characterized by low rainfall (typically 500-塵m or less) with high variability, but environmental conditions allow widespread dryland commercial agriculture (maize, sunflowers and stock farming). However, human activity promotes wind erosion and the area is susceptible to dust emissions. This study is the first to quantify the degree of wind erosion on the agricultural soils in the region under prevailing winter to spring climatic conditions and land management practices. Using arrays of cup anemometers, dust deposition traps and saltation impact sensors (Safires), measurements were made of the key erosivity and erodibility drivers that control the degree of wind erosion. Results demonstrate that significant quantities of dust are mobilized, particularly during the months of September and October. Thresholds of wind erosion are shown to respond particularly closely to changes in surface and aerodynamic roughness ( z 0 ) with the amount of collected dust correlating well with measures of wind erosivity that weight the impact of higher wind speeds. Given the importance of surface roughness in controlling erosion thresholds, results show that the opportunity exists for well designed farming practices to control wind erosion. However, it is likely that climatically driven environmental change will impact on some of the identified controls on erosion (wind power, moisture availability) with the result that the wind erosion hazard is likely to increase within this marginal environment. Copyright 漏 2010 John Wiley & Sons, Ltd.
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| [100] |
Saltation threshold on Earth, Mars and Venus [J].
ABSTRACT New formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low-air-density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus.
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| [101] |
Influence of moisture on erodibility of soil by wind [J].
CiteSeerX - Scientific documents that cite the following paper: In04uence of moisture on erodibility of soil by wind
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| [102] |
On resistance of eolian sandy soil moisture to wind erosion [J].风沙土水分抗风蚀性研究 [J]. |
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Wind erosion of soil (in Czech with English summary) [M]. |
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McGowan H A. Approaches to modelling land erodibility by wind [J]. |
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The use of erosion pins in the study of slope evolution [M] |
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Soil erosion by wind from bare sandy plains in western Rajasthan, India [J].
ABSTRACT This study was conducted on a bare sandy plain at Bikaner and a partially stabilised sandy plain at Chandan, during 1978. The results showed a loss of as much as 615 tonnes ha -1 and 325 tonnes ha -1 of soil from Bikaner and Chandan respectively during a 75-day period in April-June. A high exponential correlation between wind velocity and soil loss was observed at both sites. Particle sizes in the range 0.10-0.25mm were observed to be most erosive, while particles less than 0.05mm were least erosive.-from Authors
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| [109] |
Recent and past erosion rates in semi-arid Kenya [J].
react-text: 472 Characteristics of flow over a variety of Kenyan hillslopes were measured during artificial rainstorms on 5 m-long plots. An irregular sheetflow developed, but no rills formed, and in general the hillslopes were free of rilling. Uniformity of flow depth across the plot increased as the flow deepened and as vegetation cover declined. -from Authors /react-text react-text: 473 /react-text
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| [110] |
The determination of erosion rates from exposed tree roots in the Piceance Basin, Colorado [J].
Abstract Tree roots, exposed by hillslope erosion in the Piceance basin of Colorado, were inspected to determine average net erosion rates during the last four centuries. Twenty pinyon pine and juniper root sections were obtained from each of five sites in this region. The date when a root was first exposed by erosion was determined, for 83 of the sections collected, by one of three methods: 1. time of initial cambium dieback; 2. interpretation of annual ring growth pattern; and 3. the earliest occurrence of reaction wood. Analysis indicated no significant difference in erosion rates between the five study sites. However, a strong difference in erosion rates was noted between north-facing (0-56 mm/yr) and south-facing (1.18 mm/yr) slopes. No significant difference in erosion rates were found between various south-facing aspects and local steepness of slopes. Significant differences were noted between erosion rate and the length of time the root was exposed to erosion. Rates of erosion on south-facing slopes in the pinyon-juniper community during the last four centuries are as follows: period (years ago) 0-99 100-199 200-299 300-399 erosion rate (mm/yr) 1-79 0-49 0-33 0-22. Although there appears to be a slow increase in erosion rates during the three earliest centuries the dramatic increase during the last century may have been substantially augmented by the introduction of cattle into this region approximately one hundred years ago.
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| [111] |
Field observation of farmland wind-erosion around Beijing [J].北京市农田风蚀的野外观测研究 [J].
<FONT face=Verdana>针对半湿润区农田风蚀较弱的特点,采用集沙仪加风速廓线仪和插杆黏捕加电镜分析两种手段对北京市农田的风蚀特征进行了野外观测。结果表明,3类主要风蚀农田中,耙平翻耕地风蚀强度最大,玉米留茬地次之,不耙平翻耕地最弱。随着风速增大,各类农田地表的风蚀强度均呈增加的趋势,并且都表现出较好的指数规律性。风蚀物含量垂向分布遵循幂函数递减规律,风蚀物最大粒径和平均粒径垂向分布亦遵循幂函数和指数函数递减规律,风蚀物粒度组成以粒径小于100 μm的悬移质为主。因此,本研究所得结果反映的是一种以悬移质为主的风沙流结构特征,而这可能是半湿润区最常态的一种的农田风蚀状况。</FONT>
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| [112] |
Estimation on farmland wind-erosion and dust emission amount in Bashang of Hebei province by grain composition contrast [J].基于粒度对比法的坝上农田风蚀与粉尘释放量估算 [J].
为了计算当前年内的农田风蚀量，该研究利用农田耕作层土壤粒度组成较均一，风蚀使表层可蚀性颗粒减少，不可蚀颗粒含量相对增加这一特点，通过比较一个风蚀季结束后，农田耕作层表层与下层可蚀性颗粒与不可蚀颗粒相对含量的变化，提出了一种估算当前年内土壤风蚀量和粉尘释放量的方法，并给出了风蚀量与粉尘释放量的计算公式。利用此方法对河北坝上地区主要农田类型土壤风蚀量和粉尘释放量进行计算。计算结果表明，2013年研究区农田风蚀量为960～5 700 g/(m2·a)，平均为2 852.14 g/(m2·a)，平均风蚀深度为0.21 cm/a，从强度上划分属于重度风蚀。农田平均粉尘释放量为768.16 g/(m2·a)，约占农田平均风蚀量的29.00%。粉尘释放量与风蚀量之间有显著的线性相关关系，翻耕耙平地的风蚀量和粉尘释放量显著大于留茬地。该方法的估算结果与前人采用其他方法得到的结果以及实地观测得到的结果基本吻合。
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| [113] |
Study on the method of quantitative observation on soil wind erosion in field [J].野外土壤风蚀定量观测方法的研究 [J].
[目的]介绍野外土壤风蚀定量观测的新方法。[方法]在综合分析土壤风蚀量评估主要方法和手段优缺点的基础上，提出一种野外土壤风蚀量定量观测的新方法———风蚀圈法。[结果]风蚀圈的最大优点是简单实用、容易携带，可长期放置在野外，实现土壤风蚀的定量观测；其镶嵌在土壤中，表面基本与地面持平，可防止局部微地形对风蚀过程的影响；底部尼龙布透水通气性好，可保持风蚀圈内土样与周围土壤状态一致，观测数据接近实际；最大弊端是放、取土时部分土样容易跑出圈外，影响观测值的准确性。[结论]风蚀圈法是野外观测比较实用的方法，可用于测定单位面积的土壤风蚀量，实现风沙区风蚀季节的定量连续监测。
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| [114] |
A review of field measurement on wind erosion [J].国内外风蚀监测方法回顾与评述 [J].
为了准确认识风沙运动规律、建立风蚀模型，必须对风沙过程进行野外实地监测。鉴于风沙过程的复杂性，目前风蚀监测的方法多种多样。根据监测目的和原理的差异，将现有的主要风蚀监测方法分为风蚀量监测和输沙率监测两大类分别评述。风蚀量监测主要包括集沙盘法，降尘缸法，风蚀盘法，风蚀圈法，示踪法，遥感法，侵蚀针法等；输沙率的监测主要使用集沙仪和沙粒传感器直接测量，采用的仪器有MWAC,BSNE，WITSEG及SENSIT等。同时，总结了各种风蚀监测方法的特点和监测仪器的优缺点及其应用范围，根据研究目的、立地条件的不同给出了各自可采用的方法及设备的建议，并提出了今后风蚀监测的发展方向。旨在提高风蚀监测的科学性、研究成果的可靠性和不同方法之间的可比性。
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| [115] |
Comparative study of soil drifting in the field and in a wind tunnel [J]. |
| [116] |
Properties of soil which influence wind erosion: I. The governing principle of surface roughness [J]. |
| [117] |
Properties of soil which influence wind erosion: II. Dry aggregate structure as an index of erodibility [J]. |
| [118] |
Properties of soil which influence wind erosion: III. The effect of apparent density and erodibility [J]. |
| [119] |
Properties of soil which influence wind erosion: IV. State of dry aggregate structure [J]. |
| [120] |
Properties of soil which influence wind erosion: V. Mechanical stability of structure [J]. |
| [121] |
Factors that influence clod structure and erodibility of soil by wind: I. Soil structure [J]. |
| [122] |
Factors that influence clod structure and erodibility of soil by wind: II. Water stable structure [J]. |
| [123] |
Factors that influence clod structure and erodibility of soil by wind: III. Calcium carbonate and decomposed organic material [J]. |
| [124] |
Factors that influence clod structure and erodibility of soil by wind: IV. Sand, silt, and clay [J]. |
| [125] |
Factors that influence clod structure and erodibility of soil by wind: V. Organic matter at various stages of decomposition [J]. |
| [126] |
Field measurement and scaled-down wind-tunnel model measurement of airflow field over a barchan dune [J].
Airflow is measured over a barchan dune in the field and over a scaled-down model in a wind tunnel. The change of the flow speed over the stoss side is represented by the change of speed-up ratio. According to the field measurement, the wind profiles within 0-3m above the stoss can be divided into two segments. The lower segment, about 0.66 m thick, is the inner-boundary layer, within which the friction velocities derived from the wind profiles increase from the upwind inter-dune region to the upper stoss, and then decrease near the dune top. This change, together with the changes of airflow field, speed-up ratio and sand flux, is related to the morphological change and contributes to the stable shape and height of a barchan dune. In the wind tunnel, airflow varies in a similar way as in the field, with the speed-up ratios constantly higher than 1.0 and increasing along the stoss slope. While the segmentation of wind profiles also occurs in the wind tunnel, friction velocities derived from the wind profiles decrease along the stoss, indicating a very thin inner-boundary layer above the wind tunnel model where the detailed wind-speed change becomes difficult to measure using the present instruments.
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| [127] |
Use of Be-7 and Cs-137 measurement to document short and medium-term rates of water-induced soil erosion on agricultural land [J]. |
| [128] |
Validating the use of 137Cs measurements to estimate rates of soil redistribution by wind [J].
Wind erosion has degraded over one-half billion hectares of land worldwide. Cesium (Cs) has been used as a tracer to study long-term rates of soil redistribution by water and, to a lesser extent, by wind. Early studies assumed that the decline in Cs activity for a potentially eroded soil relative to that for an uneroded soil was linearly proportional to soil loss. More recently, models have emerged that consider the effects of soil cultivation and the particle surface area-dependent partitioning of Cs on soils. We investigated the partitioning of Cs in wind-eroded sediments and with soil surface samples sieved into contiguous ranges of particle sizes. We also compared the Cs activities and stratification of several adjacent soils with known wind erosion and deposition histories. Finally, we tested Cs-based soil loss models with measured data from sites with documented histories. Cs activities and mean particle diameters of aeolian samples agreed well with the Cs activities and respective mean diameters of the sieved surface soil samples. Good agreement between model estimations and measured data indicated that Cs models developed to estimate soil redistribution by water were also applicable to soil redistribution by wind provided that the models contained an appropriate particle size correction parameter.
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| [129] |
Wind erosion on Deliblato (the largest European continental sandy terrain) studied using 210Pbex and 137Cs measurements [J].
The objective of this paper is to estimate the difference in wind erosion between two extreme situations: sandy soil permanently covered by grass and the nearby frequently ploughed area highly susceptible to wind erosion. The spatial pattern of soil erosion rate was investigated using 137 Cs and 210 Pb ex tracing technique. The spatial pattern of erosion rate obtained within the studied area reveal influence of topography as well as direction of prevailing winds on mobilization and transport of the soil particles.
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| [130] |
Erosion rates of different land uses and sediment sources in a watershed using the 137Cs tracing method: Field studies in the Loess Plateau of China [J].
A good understanding of the erosion rate and sediment source is an important prerequisite for the soil and water conservation, and the prevention and reduction of flood hazard. The objectives of this
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| [131] |
Preliminary results of using 137Cs to study wind erosion in the Qinghai-Tibet Plateau [J].
The world fallout of caesium-137 (137Cs) associated with nuclear weapons testing during the 1950s and 1960s has provided a valuable man-made tracer for studies of soil erosion and sediment delivery. But relatively few researchers have used it to estimate wind erosion. In this paper, the137Cs technique is introduced into the study of wind erosion and its modern processes in the Qinghai-Tibet Plateau. Two137Cs reference inventories of 982·11 and 2376·04 Bq m612was established preliminarily, which distribute in the south and mid-north parts of the study area respectively. By analysing the patterns of137Cs depth profiles from sampling sites, the aeolian processes of erosion and deposition along nearly 40 years has been revealed, i.e. the shrub coppice dunes(S1) and semi-fixed dunefields (S3) had experienced the alternation of erosion and deposition, while the grasslands (S4, S6 and S7) and dry farmlands (S5) suffered erosion only. By using the137Cs model, the average wind erosion rates for shrub coppice dune (S1), semi-fixed dunefields (S3), dry farmlands (S5) and grasslands (S4, S6 and S7) were estimated to be 84·14, 69·43, 30·68 and 21·84 tha611a611respectively, and for the whole Plateau, averaging 47·59 ha611a611which can be regarded as the medium erosion standard. These results derived from137Cs for the first time have significant implications for the further research of wind erosion and desertification control in the Qinghai-Tibet Plateau.
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| [132] |
Characteristics of 137Cs Deposition in Steppe Area [J]. |
| [133] |
Wind tunnel test and 137Cs tracing study on wind erosion of several soils in Tibet [J].
The soils of alpine meadows and alpine grassland steppes, aeolian soils, coarse-grained soils, and farm soils cultivated from alpine grasslands in Tibet are typical soils that are suffering from different degrees of soil erosion by wind. Based on field investigations, wind tunnel experiments, and a 137Cs trace study, this work tested the erodibility of these soils by wind, simulated the protective functions of natural vegetation and the accelerative effects of damage by livestock, woodcutting, and cultivation on erosion, and estimated erosion rates from 1963 to 2001. The results indicated that alpine meadows have the strongest resistance to wind erosion, and that undamaged alpine meadow soils generally sustain only weak or no wind erosion. Alpine grassland steppes with good vegetation cover and little damage by humans exhibit good resistance to wind erosion and suffered from only slight erosion. However, soil erodibility increased remarkably in response to serious disturbance by livestock and woodcutting; wind erosion reached 33.03 t ha 611 year 611. The erodibility of semi-stabilized aeolian soil and mobile aeolian soil was highest, at 52.17 and 56.4 t ha 611 year 611, respectively. The mean erosion rates of coarse-grained soil with various levels of vegetation coverage and of farm soil were intermediate, at 45.85 and 51.33 t ha 611 year 611, respectively. Restricting livestock, woodcutting, and excessive grassland cultivation are the keys to controlling wind erosion in Tibet. In agricultural regions, taking protective cultivation and management to enhance surface roughness is a useful way to control wind erosion.
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| [134] |
Estimation of wind erosion rates by using 137Cs tracing technique: A case study in Tariat-Xilin Gol transect, Mongolian Plateau [J].
Wind erosion is one of the major environmental problems in semi-arid and arid regions. Here we es- tablished the Tariat-Xilin Gol transect from northwest to southeast across the Mongolian Plateau, and selected seven sampling sites along the transect. We then estimated the soil wind erosion rates by using the ~(137)Cs tracing technique and examined their spatial dynamics. Our results showed that the ~(137)Cs inventories of sampling sites ranged from 265.63±44.91 to 1279.54±166.53 Bq·m~(-2), and the wind erosion rates varied from 64.58 to 419.63 t·km~(-2)·a~(-1) accordingly. In the Mongolia section of the transect (from Tariat to Sainshand), the wind erosion rate increased gradually with vegetation type and climatic regimes; the wind erosion process was controlled by physical factors such as annual precipitation and vegetation coverage, etc., and the impact of human activities was negligible. While in the China section of the transect (Inner Mongolia), the wind erosion rates of Xilin Hot and Zhengxiangbai Banner were thrice as much as those of Bayannur of Mongolia, although these three sites were all dominated by typical steppe. Besides the physical factors, higher population density and livestock carrying level should be responsible for the higher wind erosion rates in these two regions of Inner Mongolia.
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| [135] |
On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology [J].
It is shown that the relative concentrations of any two radioisotopes with appreciably different half-lives, in a particular rain, do not depend on the local meteorological conditions, but only on the latitude and altitude at which the air mass was irradiated and can therefore be used as labels to trace the history of air-masses. The study of such isotope ratios in individual precipitations can, therefore, lead to useful meteorological information.
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| [136] |
Total deposition velocities and scavenging ratios of 7Be and 210Pb at Rokkasho, Japan [J].
The atmospheric concentrations and deposition fluxes of 7 Be and 210 Pb were observed biweekly in Rokkasho, Japan on the Pacific Ocean coast at the northern end of Honshu Island, from March 2000 to March 2006, to clarify their regional features. Seasonal variation pattern of atmospheric 7 Be concentrations had double peaks, and that of 210 Pb had a single peak. Deposition fluxes of 7 Be and 210 Pb showed the same patterns. The total deposition pattern of 7 Be was similar to that commonly seen on the Pacific Ocean side of northern Honshu Island, while the pattern of 210 Pb was similar to that commonly seen on the Japan Sea side. The lack of high spine mountains windward in Rokkasho may be the cause of this ambiguity in the winter monsoon season. Total deposition velocities and scavenging ratios of 210 Pb were similar to those of 7 Be from spring to fall, and showed that both nuclides had a similar removal process from the atmosphere. However, the scavenging ratios of 210 Pb were slightly larger than those of 7 Be in winter, indicating different behaviors for both nuclides in the scavenging process. The scavenging ratios of both nuclides inversely correlated with precipitation rate, and the ratios in winter were larger than in the other seasons.
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| [137] |
Soil erosion processes on cultivated slope on Loess Plateau using 7Be tracer [J].黄土高原农耕地坡面侵蚀过程的7Be示踪试验研究 [J]. |
| [138] |
The Redistribution of 7Be Deposition to the Land Surface and Its Application on the Soil Erosion Processes of the Slopes[D]. 7Be沉降在地表分配规律及其在示踪坡面侵蚀过程中的应用研究[D] . |
| [139] |
Using beryllium-7 to monitor the relative proportions of interrill and rill erosion from loessal soil slopes in a single rainfall event [J].
Abstract Quantifying the relative proportions of soil losses due to interrill and rill erosion processes during erosion events is an important factor in predicting total soil losses and sediment transport and deposition. Beryllium-7 (7Be) can provide a convenient way to trace sediment movement over short timescales providing information that can potentially be applied to longer-term, larger-scale erosion processes. We used simulated rainstorms to generate soil erosion from two experimental plots (565m × 465m; 25° slope) containing a bare, hand-cultivated loessal soil, and measured 7Be activities to identify the erosion processes contributing to eroded material movement and/or deposition in a flat area at the foot of the slope. Based on the mass balance of 7Be detected in the eroded soil source and in the sediments, the proportions of material from interrill and rill erosion processes were estimated in the total soil losses, the deposited sediments in the flat area, and in the suspended sediments discharged from the plots. The proportion of interrill eroded material in the discharged sediment decreased over time as that of rill eroded material increased. The amount of deposited material was greatly affected by overland flow rates. The estimated amounts of rill eroded material calculated using 7Be activities were in good agreement with those based on physical measurements of total plot rill volumes. Although time lags of 45 and 11 minutes existed between detection of sediment being removed by rill erosion, based on 7Be activities, and observed rill initiation times, our results suggest that the use of 7Be tracer has the potential to accurately quantify the processes of erosion from bare, loessal cultivated slopes and of deposition in flatter, downslope areas that occur in single rainfall events. Such measurements could be applied to estimate longer-term erosion occurring over larger areas possessing similar landforms. Copyright 08 2010 John Wiley & Sons, Ltd.
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| [140] |
Beryllium-7 atmospheric deposition and soil inventory on the northern Loess Plateau of China [J].
617Be concentrations in rainfall were artificially high due to 7Be dry deposition.617Be deposition input showed the highest in summer and the lowest in winter.617Be inventories in undisturbed soil were obviously unimodal over the year.
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| [141] |
A wind tunnel experiment to explore the feasibility of using beryllium-7 measurements to estimate soil loss by wind erosion [J].
Sandy loess from the Wind-Water Erosion Crisscross Region on the Loess Plateau of China, an area with severe wind erosion, was collected for use in a wind tunnel experiment, to explore the feasibility of using Be-7 measurements to estimate the amount of soil lost through wind erosion. Wind erosion selectively removes the finer particles of soil. Use of procedures for estimating soil loss from Be-7 measurements developed for water erosion, which do not take account of this selective removal of fines, is therefore likely to result in overestimation of the amount of soil lost through wind erosion, because Be-7 is preferentially associated with the finer fractions of the soil. The results of the experiment, supplemented by measurements undertaken on two field plots in the study region demonstrated a well-defined power function relationship between S-e/S-o and A(Be) (where Se is the specific surface area of the soil at the eroded site; So is the SSA of the original soil and A(Be) is the Be-7 activity remaining at the eroded site), with an exponent of similar to 0.75. It is proposed that a particle size correction factor P', based on the term (S-e/S-o)(0.75), can be incorporated into the procedure for estimating soil loss by wind erosion from Be-7 measurements. The estimates of soil loss obtained using the refined procedure were in close agreement with the measured values. Use of the Be-7 measurements to estimate soil loss without incorporating the particle size correction factor P' resulted in overestimation of the soil loss by similar to 14%. When P' was incorporated, the overestimation was reduced to similar to 2%. (C) 2013 Elsevier Ltd. All rights reserved.
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| [142] |
Tracer techniques in aeolian research: Approaches, applications, and challenges [J].
Aeolian processes, the entrainment, transport and deposition of sediments by wind, impacts climate, biogeochemical cycles, food security, environmental quality and human health. Considering the multitude of interactions between aeolian processes and all the major components of the Earth system, there is a growing interest in the scientific community to quantify the wind-related sediment movement process and redistribution rates at different spatial and temporal scales. However, this quantification is rather challenging, due to the complexities of physical mechanisms involved in aeolian processes and the inherent fundamental differences from the rather well-studied processes controlling fluvial erosion. Traditional techniques, such as erosion plots and surveying methods for monitoring wind erosion, are capable of quantifying sediment movement on small scales but they have a number of limitations in terms of the representativeness of the data obtained, spatial and temporal resolution and the patterns over extended areas, and the costs involved. The demand for alternative methods of soil loss and sediment redistribution assessment, in order to complement and enhance the existing methods, has directed attention to use tracing approaches for monitoring rates and spatial patterns of sediment redistribution at various scales. A comprehensive synthesis of available information from different scientific disciplines on aeolian tracer techniques, their applications and limitations are important in understanding the role of aeolian processes and their interactions with the Earths systems in changing climate and management scenarios. The objective of this paper is to provide a scientific review of the current tracer approaches in aeolian studies, including fallout radionuclides, rare earth elements, sediment finger printing and soil magnetism, as well as giving an introduction of the potential tracers that are in development.
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| [143] |
Spatial distribution of aeolian erosion of soil and its driving factors in China [J].中国土壤风力侵蚀空间格局及驱动因子分析 [J]. |
| [144] |
Wind erosion hazard assessment of Mongolian Plateau by using FMC fuzzy cluster method [J].蒙古高原土壤风蚀危险度的 FCM模糊聚类研究 [J].
论文运用模糊c均值聚类（FCM）方法对蒙古高原风蚀危险度空间格局进行了研究。通过对植被覆盖率、地形起伏度、土壤干燥度和风场强度4个主要风蚀环境因子数据进行模糊聚类，结合专家知识，将蒙古高原风蚀危险度划分为6个等级。结果表明，FCM方法可有效地获取风蚀－环境关系知识，为预测性风蚀危险度制图提供依据；蒙古高原风蚀危险度表现为从东到西逐渐加强的空间格局，与NDVI和土壤干燥度的空间格局相似，说明水分条件和植被状况是决定蒙古高原风蚀危险的最主要因素，不同风蚀危险度分区结果与蒙古高原典型景观植被类型区域分布特征基本吻合。
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| [145] |
Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research [J].
Ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings. With proper considerations of their limitations, UAs may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics.
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| [146] |
Sonic anemometers in aeolian sediment transport research [J].
Space–time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous RS. This largely explains why, in previous studies, a poor correlation was found between instantaneous RS measured at 3 m height and saltation flux near the surface, whereas the correlation between wind speed at some height and saltation flux was much better. Therefore, the poor correlation between RS away from the surface and saltation flux does not contradict that saltation flux is caused by RS.
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| [147] |
Mass, momentum and kinetic energy fluxes of saltating particles [ |
| [148] |
Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity [J].
The evaluations indicate that improvements to the instrument production process are required to ensure a standard momentum threshold among individual instruments. Furthermore, the sensor design needs to be reconsidered in order to eliminate the variation in response depending on azimuth direction, so that the sensor is uniformly omni-directional.
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| [149] |
Aeolian sediment transport over gobi: Field studies atop the Mogao Grottoes, China [J].
This paper reports on field studies of aeolian sediment transport over a rough surface-gobi atop the Mogao Grottoes, China, in relation to sediment entrainment, saltation mass flux and transport rate prediction. Wind speeds were measured with five cup anemometers at different heights and sediment entrainment and transport measured with horizontal and vertical sediment traps coupled to weighing sensors, where sediment entrainment and transport were measured synchronously with wind speeds. Four sediment transport events, with a measurement duration ranging between 2.5 and 11h, were studied. The entrainment threshold determined by the horizontal sediment trap varied between 0.28 and 0.33ms611, and the effect of non-erodible roughness elements-gravels increased the entrainment threshold approximately by 1.8times compared to a uniform sand surface. Unlike the non-monotone curve shape of sediment flux density profile over gobi measured in wind tunnels, the flux density profile measured in the field showed an exponential form. Aeolian sediment transport over gobi could be predicted by an Owen-type saltation model:q=Aρ/gu65(u652-u65t2), whereqis sediment transport rate,Ais a soil-related dimensionless factor,u65is the friction velocity,u65tis the threshold friction velocity,gis the gravitational acceleration,ρis the air density. This study indicates that the sediment flux sampling using horizontal and vertical sediment traps coupled to weighing sensors provides a practical method to determine values forAin this model that can provide good estimates of sediment transport rates in gobi areas.
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| [150] |
Numerical simulation on stochastic movement of sands in wind blown sand [J].风沙流中沙粒随机运动的数值模拟研究 [J].
<FONT face=Verdana>通过对描述沙粒垂向运动速度脉动分量的随机微分方程的直接求解，获得了风沙流中沙粒运动的随机轨迹。结果表明，由于沙粒垂向脉动速度的影响，沙粒的轨迹与不考虑其垂向脉动速度的情形存在明显不同。在此基础上，通过对大量轨迹的统计计算，得到了沙粒浓度的分布规律。</FONT>
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| [151] |
Numerical simulation of wind sand movement in straw checkerboard barriers [J].
Pattern formation of desiccation cracks on a layer of a calcium carbonate paste is studied experimentally. This paste is known to exhibit a memory effect, which means that a short-time application of
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| [152] |
Wind tunnel modeling of echo and climbing dunes [ |
| [153] |
Wiggs G F S. Pulse-wire anemometry on rough surfaces, with application to desert sand dunes [J].
The use of pulsed-wire anemometry to measure surface shear stress in rough wall boundary layers is discussed. It is shown that if the size of the probe is not too dissimilar from the size and spacing of the roughness elements, reasonably accurate measurements of shear velocity are possible, provided the (active) tip of the probe is located near the zero plane displacement height. The technique is illustrated via an experiment on the flow over a 1/200 scale model of a “barchan” desert sand dune. Measurements of surface stress using the pulsed-wire probe on a modelled dune are shown to be consistent with those deduced from extrapolated crossed hot wire data and also with the implications of full-scale measurements of sand transport rates. These results, in addition to confirming the viability of pulsed wire anemometry on rough surfaces, have led to the resolution of a long-standing paradox concerning the dynamics of such dunes.
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| [154] |
The role of streamline curvature in sand dune dynamics: Evidence from field and wind tunnel measurements [J].
Using the occurrence of streamline curvature as a starting point, a new 2-D model of dune dynamics is deduced. This model relies on the establishment of an equilibrium between windward slope morphology, surface stresses induced by streamline curvature, and streamwise acceleration. Adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress, whereas convex streamline curvature and deceleration have the opposite effect, the relationships between form and process are investigated in each of three morphologically distinct zones: the upwind interdune and concave toe region of the dune, the convex portion of the windward slope, and the crest-brink region. The applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field.
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| [155] |
Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes [J].
Abstract Topographic interactions generate multidirectional and unsteady air04ow that limits the application of velocity pro03le approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin-type surface-mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp-crested model dunes. Findings complement existing theories on secondary air04ow effects on stoss transport dynamics and provide new information on the in04uence of lee-side air04ow patterns on dune morphodynamics. For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a signi03cant drop in time-averaged measurements of streamwise shear stress at this location. This effect is believed suf03cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and 04ow acceleration cause an increase in 04ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60–70 per cent greater than at 04ow reattachment on the lower stoss of closely spaced dunes. Streamwise 04ow accelerations, rather than turbulence, have greater in04uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed 04ow within the separation cell generates signi03cant surface shear (30–40 per cent of maximum values) for both spacings. This supports 03eld studies that suggest reversed 04ow is competent enough to return sediment to the dune directly or in a de04ected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time-averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and 04ow steadiness increase within 6 h ( h = dune height) of reattachment and approach upwind values by 25 h . A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to 04uvial estimates. The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost-effective design shows promise for validating these 03ndings in natural dune settings. Copyright 08 2003 John Wiley & Sons, Ltd.
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| [156] |
Progress in scale study [J].尺度问题研究进展 [J]. |
| [157] |
Saltation and wind-flow interaction in a variable slope wind tunnel [J].
Over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope, grain size, and friction speed influence bed roughness. Owen (1964) proposed that ja:math where C is a constant, but we found that C depends in a complex manner on grain size and friction speed. C increases from very low values near the threshold to an apparently limiting value at large friction speeds. For an almost uniform 125 渭m sand, the limiting value of C is 0.06. The limiting value decreases, however, with particle size: for a uniform 544 渭m sand, C only approaches 0.03 for friction speeds as high as 1 m/s. With the measured values of friction speed corrected for the effects of slope, our results appear to show that the limiting values of C do not depend on the slope angle, even though the slope affects the individual shapes of particle trajectory. C is perhaps independent of slope angle because the roughness height, and, therefore, C , is more directly related to the amount of momentum extracted from the air by the particles (i.e. the shear stress) rather than to the manner of extraction (particle trajectory shapes).
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| [158] |
Wind tunnel measurement of boundary-layer response to sediment transport [J].
This paper presents an empirical analysis of the boundary-layer response to sediment transport in a wind tunnel with working section 0.7 070705 0.76 070705 12.5 m. With equilibrium transport, both the boundary-layer depth and displacement thickness increase relative to clean air. The logarithmic expression of the Law of the Wall provides a good description of the velocity profile within the lower 20% of the boundary-layer depth, at least. Shear velocity magnitudes (u8) are similar to and scale with those measured in the equivalent clean air flow, though this relation is dependent to some extent on particle shape and size. There is no empirical evidence to suggest that u* drops either to or below threshold, as hypothesized in some deterministic models of saltation. Departure from the Law of the Wall is evident in the flow above the saltation curtain at high velocities (u* > 9 m s-1). Though similar in appearance to an outer wake effect, direct solution of Coles070705 wake parameter indicates that this is not a wake phenomenon. This departure is most likely ascribed to a wind tunnel constraint on the downwind adjustment of a relatively thin boundary layer to the effective roughness associated with the saltation cloud. Froude numbers, computed for the high incident velocities where this departure is most evident, exceed the conservative limit of 10 suggested by White and Mounla (1991) for an equilibrium boundary layer.
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| [159] |
Sand flux and wind profiles in the saltation layer above a rounded dune top [J].
The near-bed airflow and the movement of sand dune sediments by wind are fundamental dune geomorphological processes. This research measured the wind profiles and sand mass flux on the rounded top of a transverse dune at the southern edge of the Tengger Desert to examine how to best predict the vertical profile of sand flux. This work also tested the accuracy of previously developed models in predicting the apparent roughness length during saltation. Results show that mass flux vertical distribution over the dune top is underestimated by an exponential function, overestimated by a power function, but closely matches the predictions made using the LgstcDoseRsp function. Given suitable values of α, β and γ according to the grain size composition, S03rensen equation with the peaked shape of the mass transport curve will well predict the dimensionless mass flux qg/ρu * 3 against dimensionless shear velocity u */ u * t . The modified Charnock model works best of the previously published models tested, with an R 2 of 0.783 in predicting the enhanced roughness over the moving sand surface, as opposed to an R 2 of 0.758 for the Owen model and an R 2 of 0.547 for the Raupach model. For the rounded dune top in this study, C m =0.446±0.016.
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| [160] |
Field observations of wind profiles and sand fluxes above the windward slope of a sand dune before and after the establishment of semi-buried straw checkerboard barriers [J].
Straw checkerboard barriers are effective and widely used measures to control near-surface sand flow. The present study measured the wind profiles and sand mass flux above the windward slope of a transverse dune before and after the establishment of semi-buried straw checkerboards. The 0.202m high checkerboards enhanced the aerodynamic roughness length to larger than 0.0202m, which was two to three orders of magnitude higher than that of the bare sand. The modified Charnock model predicted the roughness length of the sand bed during saltation well, with C m 02=020.13802±020.003. For the checkerboards, z 0 increased slowly to a level around 0.03702m with increasing wind velocity and the rate of increase tended to slow down in strong wind. The barriers reduced sand flux and altered its vertical distribution. The total height-integrated dimensionless mass flux of saltating particles ( q 0 ) above bare sand followed the relationship ln02 q 0 02=02 a02+02b ( u 65t / u 65 )02+02 c ( u 65t / u 65 ) 2 , with a peak at u 65 / u 65t 02≈022, whereas a possible peak appeared at u 65 / u 65t 02≈021.5 above 102m02×02102m straw checkerboards. The vertical distribution of mass flux above these barriers resembled an “elephant trunk”, with maximum mass flux at 0.05–0.202m above the bed, in contrast with the continuously and rapidly decreasing mass flux with increasing height above the bare sand. The influences of the barriers on the wind and sand flow prevent dune movement and alter the evolution of dune morphology.
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