Introducing a new multi-index approach for integrated drought monitoring in South Africa using the National Integrated Water Information System (NIWIS)

Drought is one of the most damaging natural hazards in South Africa, affecting water security, agriculture, ecosystems, and socio-economic development. Existing drought monitoring systems often rely on single indicators or subjective weighting methods that inadequately represent drought propagation across the hydrological cycle. This study presents a Standardised Non-Linear Multi-Index (SNLMI) framework for integrated drought monitoring in the Berg Catchment, Western Cape, South Africa. The framework integrates meteorological, surface water, and groundwater drought indicators represented by the Standardized Precipitation Index (SPI), Standardized Streamflow Index (SSI), and Standardized Groundwater Index (SGI). Monthly precipitation, streamflow, and groundwater level data were standardized following goodness-of-fit analyses using the Kolmogorov-Smirnov, Anderson-Darling, and Shapiro-Wilk tests. The Pearson Type III distribution was selected for SPI, while the Weibull distribution provided the best fit for SSI and SGI. Vine copula models were applied to characterize dependence among the drought indices and quantify compound drought behaviour. The SNLMI successfully captured drought onset, propagation, persistence, and recovery, clearly identifying major drought events, including the 2015–2019 drought. Results showed that groundwater drought often persisted after meteorological conditions had recovered, demonstrating the importance of integrated drought assessment. Compared with conventional Multi-Criteria Decision Analysis (MCDA) approaches used in the National Integrated Water Information System (NIWIS), the SNLMI eliminates subjective weighting while providing a probabilistic and objective framework. By integrating in-situ and Earth Observation datasets, the proposed framework improves drought characterization, risk assessment, and early warning, supporting more effective drought preparedness and sustainable water resources management in South Africa.