Temporal and spatial characteristics of ecological drought in the Inland River Basin and its driving factors

Most of the previous studies only used the index of vegetation growth to characterize ecological drought. This study built a Standardized Ecological Water Deficit Index (SEWDI) , analysed its space-time evolution characteristics from the perspectives of time, space and spatio-temporal coupling, extractes feature variables by three-dimensional spatio-temporal clustering, analysed typical ecological drought events, the effect of teleconnection factors on ecological drought was investigated using cross wavelet analysis . The reliability of SEWDI was quantitatively evaluated by comparing the r values of Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), Standardized Soil Moisture Index (SSMI), self-calibrating Palmer Drought Severity Index (scPDSI), and SEWDI with Standardized Solar-induced Fluorescence (SSIF) at various scales in time.The results demonstrated that: (1) Ecological drought in the Inland River Basin showed a weakening trend from 1982 to 2015. Seasonal drought showed a wetting trend in the west and a drought trend in the east, and the wetting trend is most evident in spring. It may be related to the policy of returning farmland to forest in the eastern part of IRB in Inner Mongolia Autonomous Region. (2) Three-dimensional recognition method could extract more detailed characteristic variables of ecological drought events. A total of 133 ecological drought events occurred in the Inland River Basin during 1982-2015, and ecological drought events had a longer duration and intensity after the twenty-first century. (3) June 2005 to February 2007 was the most severe ecological drought event on record, which lasted for 21 months. The drought intensity was 44.65 × 10⁵ month·km², and the drought area was 23.79 × 10⁴km². The event occurred in June 2005, intensified in September 2005, attenuated in June 2006, re-intensified in August 2006 and re-attenuated (extinction) in February 2007. The trend of drought migration spread to northeast (4) Among the teleconnecting factors, Among the teleconnection factors, ENSO has the most complex and greatest influence on ecological drought in the Inland River Basin. (5) In addition to having a high correlation with other drought indexs, the SEWDI can also more accurately reflect the effects of drought on vegetation. When ENSO events occur, IRB precipitation decreases significantly. However, the effect of soil moisture on vegetation is ignored in this study, which aims to provide ideas for calculating ecological drought index of small watershed. How to accurately calculate ecological water deficiency in different soil regions is the focus of future work.