In a study that highlights the complexity of improving water usage, Danielle Grogan and colleagues at the University of New Hampshire showed that even with 100% irrigation efficiency, demand for unsustainable groundwater would only halve.

"Groundwater that becomes runoff can enter surface water systems," said Grogan. "So to ignore groundwater is to ignore a significant part of our food system, and also an important player in bolstering our surface water system."

Groundwater supplies nearly half of all water used in agriculture, and some 20% of this is thought to be unsustainable. Often too much groundwater is pumped out of the ground when used for irrigation, and the remainder runs off into surface water systems such as rivers and reservoirs.

Classical methods to assess irrigation efficiency are thought to be ineffective at large scales because they deal with individual fields and do not account for the transfer of water between fields. As a result, researchers are discussing different methods that could better estimate irrigation efficiency. But these discussions have focused on surface water, while neglecting the importance of groundwater and groundwater runoff, say Grogan and colleagues.

To assess the usage of unsustainable groundwater, Grogan and colleagues used a water-system model that could simulate the demand for irrigation water, and track where the water came from and where it went. They also ran several computer simulations to assess the impact of changing the efficiency of irrigation.

It turned out that nearly as much unsustainable groundwater is used when it runs off a field, joining the surface water system, as when it is pumped onto a field in the first place. To put it another way, surface water sources that are thought to be more sustainable are often fed by unsustainable groundwater.

"Some water users who think they are relying only on surface water are actually using groundwater," Grogan explained. "If unsustainable groundwater supplies run out – as a non-renewable resource eventually will, given continued use – this also means that the users who think they are only relying on surface water may face reductions in water supply that they did not expect."

Grogan added that surface water is used for much more besides irrigation – power generation, drinking, transport and various other practices could also be affected by depleted groundwater.

"I hope that more studies will seriously consider the interaction between groundwater and surface water," said Grogan. "In particular, I think the conversation on groundwater should focus on our reliance on groundwater use and re-use – instead of just focusing on the initial use, which is currently where the conversation tends to end. Additionally, plans for hydro-infrastructure and ecological in-stream flow requirements should consider the role that groundwater plays in bolstering surface water supplies."

The study is published in Environmental Research Letters (ERL).

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