Comparative assessment of standardized indices for monitoring drought in north-eastern South Africa

Abstract

Drought assessment provides critical data for mitigating drought risk and reducing community vulnerability, particularly in semi-arid regions like north-eastern South Africa. The South African Weather Service (SAWS) primarily uses the Standardized Precipitation Index (SPI) to monitor drought impacts on agriculture and water resources. However, SPI relies solely on rainfall data, neglecting key variables such as evapotranspiration, which plays a significant role in drought development. This study evaluates and compares the effectiveness of SPI, the Standardized Streamflow Index (SSI), and the Standardized Precipitation Evaporation Index (SPEI) in assessing drought conditions in the Luvuvhu River Catchment (LRC). Rainfall and temperature data were obtained from SAWS, while streamflow records were sourced from the Department of Water and Sanitation. The non-parametric Mann-Kendall trend test was applied for drought trends analyses across 1-, 6-, and 12-month timescales. Results indicate that mild droughts were prevalent in the study area shown by all indices at all timescales, while extreme drought events were less frequent but more pronounced events at longer timescales. The evaluation criteria further revealed SPEI as the most suitable index for drought monitoring in the LRC due to its ability to account for both rainfall and evapotranspiration, providing a more comprehensive assessment of drought conditions. These findings showcased the increasing frequency and severity of droughts in the region, emphasizing the need for improved drought monitoring, early warning systems, and adaptive water resource management strategies to mitigate future drought impacts.