#20 Irrigation effects on land surface-subsurface water and energy fluxes simulated by the ACME Land Model

Abstract

The capacity of ACME Land Model (ALM) for representing human impacts is demonstrated using irrigation as an example, which constitutes ~90 of total human freshwater consumptions. In ALM, irrigation starts when soil water content induces soil moisture stress for crops. The soil water deficit is withdrawn from surface water and groundwater (if surface water is not enough to meet the demand). The parameters associated with surface water irrigation and groundwater pumping was calibrated based on a global inventory of census-based surface water and groundwater use compiled by the Food and Agricultural Organization (FAO). Results show that irrigation ahs greatly interrupted the local water balance, with significant increase in evapotranspiration (ET) and decrease of runoff and groundwater storage. Although the global mean effects are small, irrigation has considerable impacts on the seasonality of stream flow. Through coupling to the MOSART river transport model, surface water irrigation leads to significant decrease of streamflow, especially in low-flow conditions, resulting in increased risks of hydrological droughts. Results also show that upstream water use could affect water resource in downstream regions, which poses challenges in Transboundary Rivers. In regions with groundwater as the dominant water source, return flow from irrigation increases streamflow at the expense of groundwater depletion. Sensitivity studies show that the aforementioned effects especially for ET depend largely on the irrigation methods such as sprinker, flood, and drip irrigation techniques. Our results emphasize the importance of accounting for irrigation effects and irrigation sources/methods in ACME for improving climate simulations and for assessing human influence on the water cycle.