基于MODIS数据的黄河源区土壤干湿状况时空格局变化

为分析黄河源区土壤干湿状况时空格局变化，基于2007-2016年日序数第193~257天的MODIS 1 km分辨率地表温度（LST）和植被指数（NDVI）产品反演10年的温度-植被干旱指数（TVDI）。分析结果表明：2008-2010年实测土壤水分与TVDI的线性关系的相关系数达到0.7，说明TVDI可作为干旱指标有效地指示黄河源区的土壤干湿状况。在年际变化趋势上，10年间日序数第193~257天的TVDI均在0.4~0.6之间，属干旱分级中的正常状况，但东南部地区整体处于干旱-重旱。中、低植被覆盖区域的干旱趋势较为相似，10年来旱情均较为严重，TVDI数值均在0.6~1.0间浮动，其中低植被覆盖区所有时相的TVDI在10年内数值均在0.6及以上，表明旱情持续。高植被覆盖区域严重干旱的情况较中低覆盖区域缓解，但是干旱依然在10年间持续发生。旱情的空间变化特征显著，严重旱情多集中于东北部和东南部区域，分布趋势基本与中西部整体土壤水分较充足的客观事实吻合。

Spatiotemporal variation of drought conditions based on MODIS data over the source area of Yellow River

To determine the drought situation over the source area of Yellow River (SAYR) and to analyze the spatiotemporal patterns, the moderate resolution imaging spectroradiometer (MODIS) products, including MOD11A2(8-day land surface temperature)and MOD13A2(16-day vegetation index)in 1 km resolution, were used to obtain the temperature-vegetation drought index (TVDI) over a 10-year period from 2007 to 2016. The results are given as follows. 1) Significant correlation coefficient of 0.7 occurred in the linear fitting between TVDI and in-situ soil moisture measurements, indicating that TVDI could be recognized as an effective drought indicator over the SAYR for monitoring drought. 2) The average TVDI value over the whole study area ranged between 0.4 and 0.6 within the study period, showing a normal condition by drought classification standard. However, the southeastern region revealed various drought conditions. Moreover, grassland in different vegetation coverages in the past 10 years had serious drought conditions. Medium and low vegetation-covered areas showed the similar trend with the TVDI value varying from 0.6 to 1. Severe drought in high vegetation-covered regions was alleviated compared to those with low vegetation coverage. 3) The spatial variation of drought was significant. Severe drought occurred mostly in the northeastern and southeastern regions. The distribution trend basically corresponded to the fact that the overall soil water content in central and western regions was adequate while the regions with severe drought mainly distributed in the northeastern and southeastern regions.