Significant impacts of irrigation water sources and methods on modeling irrigation effects in the ACME L and Model

An irrigation module considering irrigation water source and irrigation method has been incorporated into the ACME Land Model (ALM). Global numerical experiments were conducted to investigate irrigation effects and their sensitivity to irrigation water sources and irrigation methods. All simulations shared the same irrigation soil moisture target constrained by a global census data set of irrigation amounts. Irrigation has large impacts on terrestrial water balances especially in regions with extensive irrigation. Such effects depend on the irrigation water source: surface water‐fed irrigation decreases runoff and water table depth, while groundwater‐fed irrigation increases water table depth, and increases or decreases runoff depending on the pumping intensity. Irrigation effects also depend significantly on the irrigation method. Flood irrigation applies water in large volumes within short durations, resulting in much larger impacts on runoff and water table depth than drip and sprinkler irrigation. Differentiating the irrigation water source and method is important not only for representing the distinct pathways of how irrigation influences the terrestrial water balances, but also for estimating irrigation water use efficiency. Specifically, groundwater pumping has lower irrigation water use efficiency than irrigation relying on surface water withdrawal only due to enhanced recharge rates. Different irrigation methods also affect water use efficiency, with drip irrigation being the most efficient followed by sprinkler and flood irrigation. Our results highlight the importance of explicitly accounting for irrigation source and irrigation method, which are the least understood and constrained aspects in modeling irrigation water demand, water scarcity, and irrigation effects in Earth System Models.