A nonparametric standardized runoff index for characterizing hydrological drought in the Shaying River Basin, China

Under the background of global warming and human activities, drought occurs frequently in the Shaying River Basin (SYRB). It is particularly important for agriculture and water resources management to comprehensively evaluate the evolution of hydrological drought in the river basin. In this study, we used the nonparametric standardized runoff index (NSRI) to investigate the temporal characteristics of hydrological drought in the SYRB from 1956 to 2013. The duration and severity of hydrological drought events were identified based on run theory, and the copula functions with the highest goodness of fit were used to investigate the drought return period. In addition, the double cumulative curve method was used to analyze the main causes of hydrological drought in the SYRB. The results indicated that: (1) From 1957 to 2013, the drought showed an increasing trend in the upper, middle and lower reaches of the SYRB, with frequent alternations of droughts, and the trend characteristic of drought was different in each subzone; (2) the frequency of drought decreased from upstream to downstream, but the duration and severity of drought increased; (3) Frank-copula was considered to be the best fitting two-dimensional copula function in the SYRB and the most severe drought lasted for 25 months, with drought severity of 11.485, and drought return period of 42.14 years; (4) human activities were the main reason for the decrease of runoff in the SYRB and the dominant factor for the intensification of hydrological drought in the basin.