干旱是全球分布最广、发生频率最高、持续时间最长、影响范围最大、造成的经济损失最为严重的一种自然灾害，干旱也是所有自然灾害中影响因子最为复杂、人类了解最少、监测最为困难的一种自然灾害，干旱监测是世界性的难题。干旱可以发生在任何气候带上，但干旱、半干旱地区是全球干旱灾害发生最频繁的地区。干旱发生特征和规律因地区的不同会有很大的差异，不同地区对干旱监测方法不同。目前，世界各国干旱监测主要利用基于气象、水文、农业和卫星遥感等观测资料建立的各种干旱指数开展，已经有150多种干旱指数。植被状况指数VCI是应用最为广泛的一种卫星监测干旱的指数，研究和业务应用结果表明，VCI对全球各地的干旱均有较好的反映，已经应用在美国国家大气海洋局（NOAA）日常干旱监测业务中，中国国家卫星气象中心干旱卫星遥感监测服务产品也是以VCI为基础。我国干旱半干旱地区主要分布在新疆、甘肃、青海、陕西、宁夏以及内蒙古自治区的中西部，这里降水少且不稳定，降水变率大，是中国干旱发生频率最高的地区。干旱严重制约着当地经济发展和人类生活质量的提高，使本身非常脆弱的生态环境趋于恶化。为了了解条件植被指数VCI对西北地区不同气候区干旱的监测能力，以上述6省（区）为研究区，利用1982—2003年22年NDVI数据，计算了研究区域22年来逐月的VCI，对比分析了不同气候区VCI与降水距平的关系。结果表明，VCI在空间和时间上较好地反映了西北大部分气候干旱发生、发展和空间分布，是干旱监测的较好指标，但在干旱和极端干旱地区，VCI经常出现异常偏高现象，不能反映干旱气候区常年干旱的基本特点。通过对西北不同生态系统之间NDVI特点和各生态系统间NDVI年变化及其年际变化规律的研究，设计了VCI改进方案，提出了改进的条件植被指数RVCI。通过对22年来逐月RVCI与VCI的对比，RVCI客观地反映了干旱气候区常年干旱特点，较VCI有显著改进。

Drought is the world's natural disaster that is the most widely distributed, most frequent, most long lasting, with the biggest impact and causing the worst economic losses. Among all the natural disasters, drought is also the one whose causes are the most complicated and least understood and whose monitoring is the most difficult. Indeed the drought monitoring is well known as a hard problem. Drought can happen at any time on any climate zone. Arid and semiarid regions are where drought occurs the most often. But a humid region with abundant rainfall can also have drought from a temporary climate change. As the regions changes, characteristics and patterns of drought may vary considerably. Its monitoring methods are also different. At present, drought monitoring throughout the world is primarily based on the indices from meteorological, hydrological, agricultural and satellite remote sensing data. The vegetation condition index (VCI) is one of the most widely used indices for satellite monitoring of drought. As shown by researches and operational applications, VCI is a good reflection of world wide drought condition. It has been used in routine drought monitoring by U.S. National Ocean and Atmosphere Administration (NOAA). Chinese National Satellite Meteorological Center also bases its monitoring service on VCI. Arid and semiarid areas in China are mostly distributed in the northwest part including Xinjiang, Gansu, Qinghai, Shannxi and Ningxia, as well as in Inner Mongolia. Here the precipitation is small and unstable, and the variability of precipitation is large. Drought occurs most frequently in these areas, serious affecting the human life and the economic development, and causes the deterioration of the already fragile ecology environment. In order to better understand the capability of the VCI in monitoring drought at different climate region in northwest China, this article chooses the above six provinces as study region, uses the NDVI data during the 22 years of 19822003 to calculate the monthly VCI in the study region, and analyses the relationship of VCI with the percentage precipitation anomalies at different climatic region during the 22 years. Our study shows that VCI reflects the occurring, development and distribution of drought quite well in space and time in most climate regions in northwest China, and thus serves as a good indicator for drought monitoring. However, in the arid and extremely arid areas, VCI has shown to be abnormally high and does not reflect the perennial drought condition in these areas. We found that this is caused by the extremely low rate of vegetation covering in these areas. Through the study of the NDVI characteristics, annual change and interannual change patterns of the various ecosystems in northwest China, improvement procedures are formulated and an improved vegetation condition index, called RVCI, is introduced. Through the monthly comparison between RVCI and VCI during the 22 years, it is shown that RVCI remarkably improves VCI to better reflect the perennial drought condition of the arid areas. 