A novel approach for the estimation of hydrological drought index incorporating precipitation-runoff relationship

From a physical perspective, hydrological drought is usually induced by precipitation deficit. In previous studies, the estimation of hydrological drought index, such as Standardized Runoff Index (SRI), relies solely on runoff series, and does incorporate the physical information of precipitation-runoff relationship. In this paper, a novel approach is proposed for estimating SRI by incorporating precipitation-runoff relationship. Under a two-stage hierarchical model, the monthly runoff distribution for calculating SRI is no longer directly estimated by fitting a theoretical distribution to runoff series, but derived from the conditional distribution of runoff given precipitation covariates using the total probability equation. An application case study indicates the proposed method has an acceptable performance in estimating the monthly runoff distribution. This approach offers a physical understanding of the response of hydrological drought to meteorological drought, and runoff generation mechanism plays a crucial role in the propagation from meteorological drought to hydrological drought.