River and Reservoir Water Availability Guides Global Irrigation Modeling
Irrigation supports agricultural production, but its widespread use can perturb the regional and global water cycles. Given its potentially disruptive nature, simulating the effects of land use and water use in Earth system models requires accurately representing irrigation. Scientists developed a novel method to improve the representation of irrigation processes in the Energy Exascale Earth System Model (E3SM). This new method couples the existing land surface model with the river and water management models to dynamically constrain the supply of irrigation water based on the available river and reservoir water. The new two-way coupled method better represents global groundwater recharge and level decline, while capturing the seasonal dynamics of irrigation water allocations that reflect local water conditions. Constraining the irrigation water supply based on available surface water results in more realistically simulated irrigation efficiency.
The two-way coupled irrigation scheme represents an important step to accurately model the interactions between human water use and the global water system. It lays a foundation for future studies aimed at enhancing scientific understanding of how global water resources constrain energy production and use, as well as how socioeconomic decisions, such as land use, influence water resources. Better irrigation representation is key to evaluating the feasibility and impacts of large-scale bioenergy production.
This work introduces a major enhancement into E3SM, replacing the one‐way coupled irrigation scheme with a two‐way coupled scheme that better captures the interactions between the irrigation-influencing land, river, and water management models. The two‐way coupling scheme alters the modeled hydrological and irrigation processes in many important ways. First, the surface water constraints in the new coupling scheme result in less surface water withdrawal and less return flow. This reduced surface water withdrawal (by more than 50% in some arid river basins) results in a 5–10% reduction in evapotranspiration and up to a 60% reduction in return flow. Second, constraining the irrigation supply to use only surface water available from rivers and reservoirs in addition to groundwater reduces irrigation water withdrawal by about 30% at a global scale. Third, the two‐way coupled scheme can capture the seasonal dynamics of irrigation water allocations. Groundwater contributes more to global irrigation allocations from February to June, but the monsoon-dominated increase in summer river discharge in major basins shifts the main source of irrigation water to surface water after June.