Multivariate drought indices to analyse spatiotemporal drought propagation in a semi-arid river basin

An accurate characterization of drought and its propagation in the spatiotemporal domain is essential to develop early-stage drought warning and mitigation systems. Conventional univariate indices may not necessarily represent actual drought conditions, as drought is resultant of interactions between multiple processes. This study is aimed at understanding meteorological drought characteristics and its propagation into hydrological and agricultural droughts using multivariate drought indices for the Krishna River Basin, India. In addition to the standardized precipitation evapotranspiration index (SPEI), two new copula-based integrated indices are developed to characterize hydrological drought (by combining streamflows and groundwater levels) and agricultural drought (by combining soil moisture and vegetation condition). Meteorological drought events are identified based on run theory by imposing thresholds on duration (≥ 6 months), intensity (< -1), and severity (< -4.5). Drought propagation was analysed using calculated correlation values between the SPEI-n (with n = 1,3,6, and 12 months) and the developed multivariate indices. Spatial pattern of correlation values indicated a unique hydrological drought signature that is of short-time (monthly to seasonal) propagation in mountainous catchments and long-time (bi-annual to annual) propagation in lower deltaic catchments. However, agricultural drought has a consistent short-time propagation across the basin acting as a precursor to hydrological drought. Further, hydroclimate factors at sub-basin level were analysed with respect to drought indices to identify the influencing factors of drought propagation. Results conclude that hydrological and agricultural drought propagations are influenced by changes in temperature, precipitation, and land cover respectively. Findings of this study can help in developing contextual plans to deal with drought mitigation in a timely and systematic manner.