Complementarity Characteristics of Actual and Potential Evapotranspiration and Spatiotemporal Changes in Evapotranspiration Drought Index over Ningxia in the Upper Reaches of the Yellow River in China

Based on energy balance theory, using Theil–Sen median trend analysis and the Mann–Kendall test, this research studied applicability of complementary theory evapotranspiration (ET) over Ningxia in Upper Reaches Yellow River with MOD16 ET product measured data meteorological stations, based which drought index (EDI) was proposed for first time. Moreover, usability EDI also verified its influencing factors were analyzed. The results revealed that there a relationship between AET PET 91.1% area Ningxia, including strictly asymmetrically relationships 69.2% 21.9% total area, respectively. ranged from 0 to 1 useful accurately reflect degree study annual monthly scales. From 2001 2020, average 0.66, smallest January largest May. different time scales had factors. Precipitation most factor EDI, but However, surface non-precipitation water replenishment, such as irrigation, great impact both EDI. application scope complementarity extended time, applied, will provide theoretical basis empirical reference arid semi-arid areas.