Estimating Spatially Distributed Irrigation Water Requirements for the Lower Mekong River Basin: Present Condition and Climate Change Impacts

The Mekong River is the largest transboundary river in Southeast Asia and the main source of water supply for agricultural irrigation in the Lower Mekong River Basin (LMRB), supporting socioeconomic development and the livelihoods of millions of people. Irrigation accounts for more than 70% of water use in the LMRB, and issues of water use and allocation in the basin attract much attention, especially in dry years. However, there is a lack of systematic and up-to-date estimation of irrigation water requirements (IWR) in the LMRB, for historical and climate change conditions, which are essential for water resources planning and management in the basin. We use gridded meteorological and crop land use data to estimate monthly IWR at 5-arc minute resolution in the five LMRB countries, namely Myanmar, Laos, Thailand, Cambodia and Vietnam, using meteorological observations of 1991-2020 and climate projections for 2031-2060 and 2061-2090 by five global climate models under two climate scenarios, SSP126 and SSP585. Crop water productivity and water footprint are also estimated along with IWR. Under historical climate, the total IWR in the LMRB is estimated to be 20.44 billion m³, with Vietnam having the largest share of 9.18 billion m³, followed by Thailand with 7.54 billion m³. IWR concentrates in the dry season of November-April, accounting for 78.4% of annual IWR. Rice is the main water-consuming crop, accounting for 86.7% of total IWR. Relative to historical climate condition, SSP126 generally leads to slightly decreased IWR, whereas SSP585 leads to a large increase in IWR. Crop water productivity of rice is unevenly distributed in the basin, being higher in Vietnam and Laos and lower in Cambodia and Thailand. Green water footprint of rice is about 3.7 times higher than that of blue water footprint, indicating most of rice water consumption is from precipitation. Under SSP126, there is little change in water footprint, however under SSP585 blue water footprint increases significantly, in 2061-2090. This study provides update-to-date and high-resolution IWR estimates which can support water use and allocation dialogue, policy-making and management in the basin.