A Novel Index for Integrative Drought Assessment in Agricultural Reservoirs

Effective water resource management in agricultural reservoirs is crucial for water security, yet climate change intensifies drought challenges. Meanwhile, existing drought indices are not designed for monitoring drought conditions in agricultural reservoirs, lacking key hydrological and operational variables such as reservoir storage and inflow. While these indices may effectively represent specific droughts, they do not capture the full dynamics of reservoir-based droughts. To address this gap, all relevant hydroclimatic factors influencing reservoir conditions must be integrated for a more comprehensive drought representation. We developed a Multivariate Drought Index (MDI) to assess hydrometeorological droughts in 404 agricultural reservoirs across five major watersheds (Hangang, Geumgang, Nakdonggang, Yeongsangang, and Seomjingang) of South Korea, integrating precipitation, inflow, and reservoir storage to represent the full drought causal chain. Using Principal Component Analysis (PCA), MDI was validated against the Standardized Precipitation Evaporation Index (SPEI) and historical droughts (1973–2015). The results showed MDI detected earlier and longer droughts than SPEI with droughts in Nakdonggang watershed lasted 81 months for MDI compared to 22 months for SPEI. The MDI aligned with recorded drought events, confirming its reliability. The study suggests that relying on a single drought index might not fully capture drought severity and magnitude. The proposed MDI offers a comprehensive assessment of reservoir-based drought and its potential for scalability makes it adaptable to different geographical regions, temporal scales, and water resource systems.