A modeling study of irrigation effects on global surface- and groundwater resources under a changing climate

Abstract:

In this study, we investigate the effects of irrigation on global surface water (SW) and groundwater (GW) resources by performing simulations of the Community Land Model 4.0 (CLM4) at 0.5-degree resolution driven by downscaled/bias-corrected historical climate simulations and future projections from five General Circulation Models (GCMs) from 1950-2099. For each climate scenario, three sets of numerical experiments were configured: (1) a control experiment (CTRL) in which all crops are assumed to be rainfed; (2) an irrigation experiment (IRRIG) in which the irrigation module is activated; and (3) a groundwater pumping experiment (PUMP) in which a groundwater pumping scheme coupled with the irrigation module is activated. The parameters associated with irrigation and groundwater pumping were calibrated based on a global inventory of census-based SW and GW use compiled by the Food and Agricultural Organization (FAO).

Our results suggest that irrigation could lead to two major opposing effects on SW/GW: SW depletion/GW accumulation in regions with irrigation primarily fed by SW, and SW accumulation/GW depletion in regions with irrigation fed primarily by GW. Furthermore, irrigation depending primarily on SW tends to have larger impacts on low-flow than high-flow conditions of SW, suggesting that intensive irrigation water use has the potential to further exacerbate low-flow conditions, increasing vulnerability to drought. By the end of the 21^st century (2070-2099), climate change significantly increases (relative to 1971-2000) irrigation water demand in the regions equipped for irrigation across the world. The increase in demand combined with the increased temporal-spatial variability of water supply will cause more severe issues of local water scarcity for irrigation. Regionally, irrigation has the potential to aggravate climate-induced changes of SW/GW although the effects are negligible when averaged globally. Our results emphasize the importance of accounting for irrigation effects and irrigation sources in regional climate change impact assessment.