Assessment of drought evolution characteristics based on a nonparametric and trivariate integrated drought index

Drought index is an important tool for drought research. The development of a reliable and simple integrated drought index is a critical step of current drought monitoring and risk assessments. The Gringorten point position formula was adopted to develop a trivariate standardized drought index (TSDI) based on precipitation (meteorology), runoff (hydrology), and soil moisture (agriculture) information in this study. This index can be used to characterize droughts from an integrated meteorological, hydrological, and agricultural point of view, and explore drought evolution characteristics. The possible driving forces of drought were explored by examining the associations between sunspot activities and large-scale atmospheric circulation anomalies. A typical Loess Plateau watershed, the Wei River Basin (WRB), was selected as a case study. Results showed that: 1) the TSDI is better at characterizing real drought conditions than other traditional single-variable indices; although it is similar to the parametric trivariate drought index based on copula function (Serinaldi and Grimaldi., 2007; Yang et al., 2018), it is much simpler to calculate and more suitable for characterizing drought of different regions; 2) the value of integrated drought index gradually increased in all areas, and its change point occurred in the early 1970s and 1990s mainly due to variations in runoff; 3) the evolution of the integrated drought appeared to be strongly influenced by sunspot activities and abnormal atmospheric circulation factors, where sunspots activities are dominant. This study provides a valuable reference for the development of an integrated drought index, which might be applied to local drought monitoring, preparedness and mitigation.