Long-Term Drought Variability in Southwestern Uganda: A Comparative Analysis of Ground and Satellite Rainfall Data for Mbarara

Abstract

This study analysed drought occurrence in Mbarara District using ground-based observations and satellite-derived rainfall estimates from the Climate Hazards Infrared Precipitation with Stations (CHIRPS) data (for the period 1981–2024) and the Climate Prediction Center Morphing Technique (CMORPH) datasets (1998–2023). A comparison between satellite-derived and in-situ rainfall data was performed using simple linear regression and error metrics, while seasonal trends were assessed using the non-parametric Mann-Kendall and Theil-Sen methods. Results show that CHIRPS strongly agrees with ground-based observations (R2 ∼ 0.93, RMSE ≈ 32.6 mm, PBIAS = 6.5%), while CMORPH exhibited weaker agreement (R2 ∼ 0.47, RMSE ≈ 42.9 mm, PBIAS = −17.0%). Drought characterisation using the Standardised Precipitation Index (SPI) reveals distinct drought episodes experienced during 1984–1993, 1999–2004, 2009–2013, 2017, and 2022–2024. Analysis of seasonal rainfall variability shows a non-significant decrease in rainfall during the September-October November (SON) season. This study provides one of the longest station satellite-validated drought analyses for southwestern Uganda using CHIRPS data. The observed patterns and episodic drought events highlight a need to strengthen adaptive strategies in agriculture and water resource management to mitigate the socioeconomic impacts of drought in the region.