The scientists quantified the amount of water storage required to maximize crop irrigation without depleting water stocks and encroaching on nature. They found that dammed reservoirs could be used to store more than half of the water needed for such irrigation.
However, they stress that large reservoirs already in use should only be part of the solution with other sustainable alternatives being explored rather than new dams being built as these can have damaging impacts on river ecosystems.
“There is an urgent need to explore alternative water storage solutions, but we have to acknowledge that many dams are already in place,” says Rafael Schmitt, a research engineer with the Stanford Natural Capital Project who was the lead author of the study.
“Our research illuminates their crucial role in ensuring food security in the future,” he adds.
Agriculture worldwide often wreaks havoc by depleting and polluting freshwater resources while also damaging natural habitats and leading to deforestation. “Two-thirds of global cropland depends on rainfall and often makes up for its absence by using non-sustainable water resources, such as non-renewable groundwater, or impeding environmental flows,” the scientists note.
The researchers analyzed the amount of freshwater in surface and groundwater bodies generated and renewed by natural hydrological cycles. They also examined the water demands of crops on irrigated and on land fed by rain. Based on their estimates storage-fed irrigation could feed about 1.15 billion people.
“If all 3,700 potential dam sites that have been mapped for their hydropower potential were built and partially used for irrigation, the world’s dams could supply enough water storage to irrigate crops for about 641 million people or 55% of the total,” they argue.
However, once dams’ socio-environmental consequences are taken into account, they become a less appealing option. Importantly, dams can fragment rivers, severely impact acquatic species such as migrating fish, and lead to the displacement of people.
“Dams are also less appealing for irrigation storage because of water loss, expense, and ecological damage related to the need for conveyance to distant agricultural fields, as well as higher levels of evaporation across large reservoirs’ large water surfaces,” the scientists explain.
“Amongst all supply and demand side options to increase food and water security, building more dams should be the last resort,” they stress.
The more environmentally-friendly alternatives for water storage for irrigation the experts recommend include water harvesting with small dams, recharging groundwater systems with excess surface water from winter storms or spring snow melt, and better management of soil moisture on farm fields.
“These decentralized approaches lose less water due to evaporation, require less conveyance infrastructure, and often create co-benefits for local communities and wildlife,” they explain.