Global hydrological simulations quantify impact of irrigation return flows on downstream water supply.
Irrigation is critical to global food production, and groundwater supplies nearly half of all irrigation water. Groundwater levels are declining in aquifers in important agricultural regions—including parts of India, China, and the U.S. Runoff and percolation resulting from inefficient use of this unsustainable (non-renewable) groundwater withdrawal increase surface water supply in several major agricultural regions, including the High Plains aquifer in North America, the North China Plain, and parts of north-west India. One proposed solution to the over-extraction of groundwater resources is to increase irrigation efficiency. Researchers under a multi-institutional Cooperative Agreement led by Stanford University examined the implications of hydrological simulation modeling that can quantify the downstream impacts of improving irrigation efficiency and reducing non-renewable groundwater use.
Inefficient use of irrigation water leads to large amounts of non-renewable groundwater entering surface water supply by way of agricultural runoff, where it can be re-used for irrigation downstream. This local and downstream re-use of non-renewable groundwater amounts to about 10% of total global irrigation water use. Reduced pumping of non-renewable groundwater therefore reduces the water supply not only directly for the groundwater users, but also indirectly for those downstream who rely on the re-use of this water through the surface water system. Improving irrigation efficiency by reducing unintended runoff significantly decreases dry-season river flow, and thus water supply, in major irrigated-agriculture river basins around the world.
Researchers constructed global hydrological simulations to quantify the contribution of irrigation return flow to downstream river discharge. They used a gridded global water balance model which simulates global irrigation water demand and supply, and tracks that ‘inefficient’ fraction of applied irrigation water that percolates to groundwater or runs off the land surface. They found that that a significant fraction of unsustainable groundwater withdrawn for irrigation, but ‘lost’ to inefficiencies, is re-used for downstream irrigation. They also found that ecologically important river low-flows can be highly dependent on ‘inefficient’ return flows from non-renewable groundwater use across many agricultural regions. These results highlight the need for careful consideration of both the potential benefits (e.g. reduced water demand) and negative impacts (e.g. reduced ecological low-flows) of changing irrigation efficiencies when searching for solutions to water stress challenges.
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Integrated Assessment Research
This work was supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research Program, Integrated Assessment Research Program, Grant no. DE-SC0005171.
Grogan D, D Wisser, A Prusevich, RB Lammers, S Frolking. “The use and re-use of unsustainable groundwater for irrigation: A global budget,” Environmental Research Letters. 12 (2017) 034017. doi: 10.1088/1748-9326/aa5fb2.
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