Comprehensive evaluation of hydrological drought and its relationships with meteorological drought in the Yellow River basin, China

Under the background of global warming, the acceleration of water cycle process will aggravate the risk of hydrological drought in the river basin. The Yellow River basin (YRB) is the most severely affected area by drought in China's major river basins, so it is particularly important to comprehensively evaluate the hydrological drought and explore its relationships with meteorological drought in the YRB. In this study, the Standardized Streamflow Index (SSI) was adopted as a hydrological drought index, and the evolution characteristics of hydrological drought were comprehensively evaluated in the YRB from 1961 to 2015. The duration and severity of hydrological drought events were identified based on run theory, and the copula functions with the highest goodness of fit (GOF) were used to investigate the drought return period. Finally, the Standardized Precipitation Evapotranspiration index (SPEI) was adopted as a meteorological drought index, and the relationships between hydrological and meteorological drought were revealed by cross wavelet transform method. The results indicated that: (1) drought showed an increasing trend in the YRB from 1961 to 2015, while the worst drought occurred in 1997; (2) the trend characteristic of drought was different in each subzone; (3) the most severe drought lasted for 32 months, with drought severity of 43.29, and drought return period of 23.26 years; (4) Frank-copula was considered to be the best-fitted copula function in the YRB; and (5) the cross wavelet transform illustrated that there was a positive correlation between hydrological and meteorological drought, and the phase angle relationships indicated that meteorological drought occurred earlier than hydrological drought in the YRB.