| Drought is one of main constraints of agricultural development not only in China but also around the world.The frequent occurrences of drought and the consequent to huge agricultural, industrial,and national economic losses have attracted government's high attention.The development of monitoring drought is thus an important element in reducing risk,increasing industrial and agricultural productivity.Using remote sensing technology to get surface spectrum information to monitor the drought at a very high time resolution and spatial resolution is the hotspot of drought monitoring research.However,the current remote sensing monitoring models lack in the adaptability analysis of region and period.In order to improve the accuracy of remote sensing drought monitoring model,it is necessary to study the adaptability of drought monitoring indices in a specific period of test area.This paper presented a case study for Chongqing,where the occurrence of drought in summer with a high frequency.On the basis of systematic analyses of the remote sensing monitoring researches and the development trend at home and abroad,Vegetation Condition Index(VCI), Temperature-Vegetation Dryness Index(TVDI),Vegetation Supply Water Index(VSWI)and the measured soil relative moisture data were selected to create water content spatial distribution map, using the MODIS data provided by NASA,2005-2007 measured soil moisture data and the practical condition.A comparative analysis of VCI,TVDI,and VSWI was carried out to evaluate the adaptability of drought monitoring models.Then the severe summer drought in 2006 was quantitatively analyzed in temporal and spatial.The main contents and conclusions were as follows.(1)The distribution maps of soil relative moisture in summer in 2005 and 2006 were created using VCI,TVDI,and VSWI,respectively.The results indicated that the trends of summer drought were accord with the local conditions.The retrieval accuracy showed in order of VSWI＞VCI＞TVDI.(2)The validation and adaptability of the drought monitoring models were investigated using the measured relative soil water content.Significant correlationships were found between the retrieval data and the observations.Although,different indices are applicable to different drought conditions.VSWI was the optimum model in the study area during research period,TVDI and VCI has different adaptability in different years.(3)The drought severity levels of year 2006 were examined using VSWI model.Statistical analysis indicated that the drought fell into three successive phases.During pre-drought(from Jun. 26 to Jul.11),the areas of light,middle,and heavy drought occupied 6.27%,1.21%,and 0%, respectively.During the middle stage(from Aug.13 to Aug.28),the areas of light,moderate,and heavy drought occupied 44.80%,13.65%,and 26.79%,respectively.During the post-drought(from Aug.29 to Sep.13),83.07%of soil water regained normal range and the areas of light,moderate, and heavy drought occupied 15.34%,0.88%,and 0.10%,respectively.Meanwhile,the spatial distribution maps showed that the trends of drought condition during the three phases.During the pre-drought,severe drought occurred in the southeast,where 20%area suffered drought.During the middle phase,drought severity extended to the west,the worst severe drought existed in the northwest followed by southwest,drought areas were 98%,and 95%,respectively.During the post-drought,the southeast and northeast regions regained normal conditions quickly.(4)There was a significant drought difference across the land use types.The impact of drought exhibited in order of paddy field＞dry land and other land use type＞forest and grass.For example, during the severe drought of year 2006,in the pre-drought stage,the moist ratio of paddy field was 32.12%,in the middle and post-drought phases,the ratio of severe drought were 42.48%,0.18%, respectively.
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