Rainfall decline, regime shifts and meteorological drought persistence in the Ngwezi River Basin hydroclimatic zone, Southern Zambia

Rainfall variability is a major driver of hydroclimatic vulnerability in semi-arid basins, yet its long-term behaviour and drought implications remain insufficiently understood in many parts of southern Zambia. This paper treats drought specifically as meteorological drought, defined as a sustained precipitation deficit relative to the local climatology, and does not claim to diagnose agricultural, ecological or hydrological drought directly. This study assessed annual rainfall variability, regime shifts, and drought dynamics across Choma, Livingstone, and Sesheke in the Ngwezi River Basin hydroclimatic zone for 1973–2022. Annual rainfall trends were analysed using the Mann-Kendall test and Sen's slope estimator, while rainfall homogeneity was examined using Pettitt, Standard Normal Homogeneity, and Buishand tests. Drought frequency and persistence were evaluated using the Standardized Precipitation Index (SPI) at 3-, 6-, and 12-month timescales, supported by the Rainfall Anomaly Index, dry/wet month counts, and consecutive dry-month duration. Because the analysis is rainfall-based, the Standardized Precipitation Evapotranspiration Index (SPEI) was not incorporated; drought results are therefore interpreted as precipitation-deficit signals rather than temperature-adjusted water-balance drought. Results show strong spatial differentiation. Sesheke recorded the steepest and most significant rainfall decline (−6.7 mm yr−1; p = 0.0002) and a significant rainfall regime shift in 1993, while Choma showed significant drying and Livingstone remained comparatively stable. Sesheke also exhibited the highest meteorological drought frequency, the longest continuous SPI-12 event (36 months), more frequent negative anomalies, and recurrent dry-period persistence. The findings show that hydroclimatic deterioration in the basin is driven not only by rainfall decline, but also by regime shifts, anomaly persistence, and seasonal contraction. The study identifies Sesheke as the basin's primary meteorological drought hotspot and highlights the need for spatially differentiated drought preparedness and water-management strategies.