Hydrological drought evolution with a nonlinear joint index in regions with significant changes in underlying surface

Abstract

It is critical important to accurately evaluate hydrological drought evolutions based on a composite index for the sustainable development of water resources. In this study, a Nonlinear Joint Hydrological Drought Index (NJHDI) is constructed by combining surface water and groundwater to better reflect the multivariate phenomenon in characterizing hydrological drought. The bi-dimensional copula function is applied to describe the complicated and nonlinear relationship between surface water and groundwater due to its dependence structure and flexibility. To reflect the changes in underlying surface, the time-varying parameters are applied in the Soil and Water Assessment Tool (SWAT) model to simulate the surface water and groundwater more accurately. Based on the NJHDI, the spatio-temporal hydrological drought evolutions as well as their potential influencing factors are analyzed in a case study of the Yellow River Basin (YRB). Results indicate that: 1) compared to the single hydrological index, the NJHDI is more sensitive and effective to capture historically droughts; 2) hydrological droughts became increasingly serious during the last few decades, especially in parts of the basin located in the Loess Plateau areas where the ecosystem is extremely fragile; 3) climate and LULC changes have both contributed to the changes in hydrological droughts but their contributions vary significantly spatially. In general, the NJHDI-based hydrological drought evolution can provide more reliable information in hydrological drought decision-making owing to considering changes in underlying land surface, and combining the surface water and groundwater with nonlinear combination method.