"Internationally shared, or transboundary, aquifers have long played an important role in sustaining drinking water supply and food production, supporting the livelihoods of millions of people worldwide," Yoshihide Wada of Utrecht University in the Netherlands told environmentalresearchweb. "Rapidly growing populations and their food demands cast significant doubt on the sustainability of transboundary aquifers."

Along with Lena Heinrich from the International Groundwater Resources Assessment Centre in the Netherlands, Wada calculated aquifer stress for 408 transboundary aquifers around the world from 1960–2010. Their aquifer-stress indicator included measures of groundwater abstraction, natural groundwater recharge and additional recharge from irrigation as return flow. The pair found that 8% of the aquifers are currently stressed by human overexploitation, with the rate of groundwater pumping increasing substantially over the past 50 years.

According to Wada, it is difficult to assess transboundary aquifers because there is a lack of observed data for factors such as groundwater abstraction and groundwater recharge. Groundwater may also transfer from one country to another in a complex way. "For instance, most of the groundwater recharge may occur in one country, whereas the groundwater may be extensively abstracted in the other countries," he said. "Therefore, we opted to use a global modelling approach and combined it with available country statistics of groundwater abstraction and socio-economic data worldwide with an uncertainty assessment."

The team found several transboundary aquifers with substantial groundwater depletion, including the India River Plain (India and Pakistan), Paleogene and Cretaceous aquifers in the Arabian Peninsula and several aquifers crossing the US-Mexico border.

"Our analysis is unique such that human overexploitation is assessed at transboundary aquifers where data is rather difficult to obtain due to multi-state involvement and their conflict of interest," said Wada.

The researchers found that improving irrigation efficiency might reduce the amount of groundwater depletion for some transboundary aquifers but could aggravate depletion where both surface water and groundwater are used for irrigation by decreasing groundwater recharge from surface water.

"The analysis of irrigation efficiency provides a new insight of how overexploitation may be mitigated," said Wada.

Effective management of transboundary aquifers is difficult, according to Wada, as they may cross international political boundaries over several sovereign countries, and international laws for their preservation are limited.

"Conjunctive use of surface water and groundwater facilitates the management of transboundary aquifers for more sustainable use and provides pathways for minimizing aquifer stress and maximizing the beneficial use of the groundwater resources," he added.

Although human overexploitation of transboundary aquifers is a regional problem it has much larger consequences due to the international food trade. "To reduce human overexploitation, various measures can be applied, involving water recycling, crops that need less water, water management and governance, dietary change, and economic incentives," said Wada. "However, some of these solutions require a substantial amount of economic investment that may not be easily realized for developing countries with limited financial and technological resources."

Wada believes that the global modelling approach can be improved further by integrating the results with observed data, including satellite observations such as terrestrial water storage. This will help to reduce model uncertainty.

Wada and Heinrich published their results in cite>Environmental Research Letters (ERL).

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