Seasonal Water Balance and Hydroclimatic Imbalances in the Upper Ikopa Watershed, Madagascar

Ikopa River Upper Watershed, a 4560 km 2 freshwater supply watershed for Antananarivo, had variable hydroclimate. Water balance for 2020–2022 was evaluated with rainfall and air temperature data from the Antananarivo weather station combined with a Thornthwaite-Mather model forced for 36 months. Available water storage was set to 100 mm. All 3 years analyzed were drier than the 1991–2020 Antananarivo-Ivato monthly rainfall normal of 1,403.1 mm. Total annual rainfall was 879.5 mm in 2020 (63% of normal), 626.9 mm in 2021 (45%), and 1,034.3 mm in 2022 (74%). About 83–86% of annual rainfall was received during December through March. Adjusted potential evapotranspiration typically ranged from 50 to 110 mm monthly. Actual evapotranspiration matched or nearly matched potential during rainy season months but was limited by precipitation during the dry season. Water surplus was restricted to late rainy season months; there was a surplus of 355 mm in January 2020 when there was 456.8 mm of rainfall. Water deficit spanned much of the dry season, especially May through October, when water deficit was commonly >50 mm per month. Soil moisture store filled to 100 mm during rainy season periods, then dwindled during dry season months. Differences between conditions in 2021 and 2022 highlight how rainfall seasonality may play a role in determining water availability as well as annual totals. As this evaluation used only one weather station and 3 years of data, it can only characterize seasonality rather than long-term trends. Longer time series and river-flow validation would be required for trend detection and adaptation decisions.