Variability and Trends in Evaporation and Water Balance over Tanzania

Evaporation is a critical component of the hydrological cycle, shaping water resource availability in climate-sensitive regions such as Tanzania. This study examines the spatiotemporal variability and long-term trends of evaporation and the precipitation-evaporation balance (P-E) from 1995 to 2025 using high-resolution ERA5-Land reanalysis data. Monthly spatial patterns, seasonal correlations with temperature and precipitation, and decadal trends were analyzed to assess changes in water availability and evaporative demand. Results highlight strong seasonal and spatial heterogeneity, with evaporation peaks during the wet season (November-May) under energy-limited conditions, while it weakens in the dry season (June-October) under moisture-limited constraints. The P-E balance indicates seasonal surpluses in the rainy periods (March-May and November-December) and deficits in dry months, most pronounced in the semi-arid central plateau. Correlation analysis shows that evaporation is positively linked to temperature across most months, while its relationship with precipitation shifts: negative during dry periods and positive in wet months, reflecting energy-moisture interactions. Trend analysis reveals a significant decline in evaporation across most months, particularly June-October, with the strongest negative trend in November/(approximately −6.2 mm). March, however, shows a consistent increase. These declining evaporative rates, combined with shifting precipitation patterns, suggest a tightening regional water balance with implications for agriculture, surface water availability, and drought resilience. By linking national-scale evaporation dynamics to both local soil-moisture processes and large-scale climate drivers, this study bridges basin-level findings with broader hydrological trends, offering a comprehensive framework for climate-sensitive water resource management in East Africa.