A Copula-Based Composite Drought Index for Enhanced Drought Monitoring and Analysis

This research developed a composite drought index based on the copula theory, standardized precipitation, evapotranspiration and groundwater Index (SPEGI). SPEGI innovatively integrated surface water variables, that is, precipitation and actual evapotranspiration, and groundwater variables, that is, groundwater water storage changes. Subsequently, it was compared with standardized precipitation evapotranspiration index (SPEI), standardized soil moisture index (SSMI), and standardized groundwater index (SGI), and cross-validated with observed crop yield to validate its applicability. The Tarim River, recognized as China's largest inland river, has its water resources primarily supplied by its headstream. The headstream of the Tarim River Basin (TRB) is increasingly subjected to intense and prolonged droughts under climate change, which serves as the study area. Finally, the study assessed drought characteristics in the headstream of TRB using SPEGI. The results demonstrated that Gaussian copula provided the best fit for constructing joint probability distributions at a 1-month scale, whereas the Gumbel copula was most suitable at the 3-, 6-, and 12-month scales. The SPEGI effectively identified both meteorological and groundwater drought features and demonstrated a better correlation with crop yields compared to SPEI, SSMI, and SGI. Additionally, the drought characteristics in mountainous regions exhibited high frequency, short duration, and low intensity, whereas the plains exhibited low frequency, long duration, and high intensity. This research provided an innovative perspective for drought research, considering both surface water and groundwater and focusing on improved regional drought assessment and strategic development.