Assessing future irrigation demand under climate change and bilateral water sharing in the Mahakali command area

Study region The Mahakali Irrigation Project (MIP) command area, located in the far-western Terai region of Nepal, relies on transboundary water from the Mahakali River governed by bilateral treaties with India. Study focus This study evaluates the impact of climate change on future crop and irrigation water requirements and assesses whether treaty-based water allocations are sufficient to meet projected demand. Bias-corrected outputs from selected CMIP6 models under SSP2–4.5 and SSP5–8.5 scenarios were used to estimate future climate variables. The FAO CROPWAT model was applied to compute crop water requirement (CWR) and irrigation water requirement (IWR) for near (2026–2050), mid (2051–2075), and far (2076–2100) future periods. New hydrological insights Results reveal a strong seasonal redistribution of water demand driven by warming and shifting precipitation patterns. Irrigation water requirement increases sharply during dry and post-monsoon months, with September showing increases up to 564% and February exceeding 6-fold increases under future scenarios. In contrast, monsoon months (July–August) continue to exhibit negligible irrigation demand due to sufficient rainfall. Despite projected increases in annual precipitation, reductions in early monsoon (June) and post-monsoon (October) rainfall, combined with rising evapotranspiration, lead to significant water deficits during critical crop growth periods. Comparison with treaty-based supply (4.25 m³/s in dry season and 28.35 m³/s in wet season) indicates that future irrigation demand may exceed available supply by up to 30–40 m³ /s during dry months, highlighting a substantial seasonal mismatch. These findings demonstrate that timing of water availability, rather than total annual volume, is the dominant control on future irrigation stress, emphasizing the need for adaptive water allocation, storage expansion, and climate-resilient irrigation planning in transboundary river systems.