Comprehensive evaluation of hydrological drought characteristics and their relationship to meteorological droughts in the upper Tarim River Basin, central Asia

Comprehensive evaluation of the characteristics and mechanisms of droughts is of great significance to drought risk prediction and prevention. The 3-monthly scale Standardized Runoff Index (SRI-3) and 3-monthly scale Standardized Precipitation Evapotranspiration Index (SPEI-3) were employed herein to characterize hydrological and meteorological droughts, respectively, within the four upper subbasins of the Tarim River Basin (TRB) during 1961-2015. The propagation of droughts was also evaluated. The hydrological drought duration (Dd) and drought severity (Ds) were determined by Run theory, and Copula functions were adopted to investigate the hydrological drought probabilities and return periods. The propagation relationships of hydrological and meteorological droughts were assessed. The results indicated that: 1) hydrological drought index (SRI-3) significantly increased in the TRB from 1961 to 2015; the increase was most significant in winter. Meteorological drought index (SPEI-3) exhibited a weak upward trend through time; 2) the characteristics of hydrological droughts varied between the subbasins; increases in the SRI were most significant in the Yarkand and Hotan Rivers, whereas the Dd and Ds of hydrological droughts were higher in the Kaidu and Yarkand Rivers; 3) Frank Copula was the most closely fitted Copula function in the four subbasins of the TRB and yielded average drought return periods of 4.86, 4.78, 3.72, and 5.57 years for the Kaidu, Aksu, Yarkand, and Hotan River Basins, respectively. The return periods in the four subbasins were generally less than 10 years from 1961 to 2015; 4) a cross wavelet transform (XWT) exhibited a significant positive correlation between hydrological and meteorological droughts, except for the Yarkand River Basin, which exhibited a significant negative correlation. Besides, the propagation relationship of meteorological droughts to hydrological droughts showed remarkable seasonal variations.