Usangu rainfall variability analysis: Unveiling spatio-temporal trend, distribution and concentration of rainfall in a semi-arid region

Abstract

Rainfall unpredictability in semi-arid climates affects the quality and quantity of water available for socioeconomic activities, ecological maintenance and developmental projects. Understanding rainfall characteristics such as trend, distribution, and concentration is important for managing water resources and planning to mitigate impacts from rainfall uncertainty. This study was conducted in the Usangu catchment located in a semi-arid environment with three competing users agricultural land upstream, and the National Park and hydropower plant. A multidisciplinary approach that integrated statistical methods, hydrological analysis and geographic information system (GIS) tools was used to unveil a spatio-temporal trend and variability of annual rainfall in Usangu. The results indicated a significant trend for all stations except Mafinga station. The highest decrease was at Kimani (S=-6.13) and the highest increase at Matamba (S = 2.93). There is a distinct distribution of rainfall between the highlands (800-1,200 mm/year) and lowlands (500–800 mm/year). Rainfall is concentrated in short periods indicated by higher precipitation concentration index (PCI) values ranging from (18.8 to 23.6). There is less dispersion of rainfall at station N based on the coefficient of variation (CV = 17.1%) and high dispersion at Kimani (CV = 33.6%) but the majority of the catchment experienced consistent rainfall patterns. According to the standardised anomaly index (SAI), oftentimes the catchment experienced normal conditions (32 years out of 42 years) which accounts for 77%. Also, extreme drought conditions are more dominant in the lowlands than in the highlands. The results of this study are useful for long-term planning of water resources utilization in Usangu.