CLEMENT weather and plentiful water mean that Punjab produces an eighth of India's total food grains. But the water table has dropped by ten metres since 1973 and the rate of decline is accelerating on both the Indian and the Pakistani sides of the region. It is a similar story for the north-western Sahara aquifer system (NWSAS), shared by Algeria, Tunisia and Libya. Withdrawals increased ninefold between 1950 and 2008. Springs are drying up and soil salinity has increased.

Such depletion of aquifers is a classic tragedy of the commons. Farmers pump, oblivious of others' actions or the impact of their own. Scarcity stokes this rather than braking it. Worse, much abstracted water is used in inefficient irrigation; compounding that, underpricing means it is often used for watering low-value crops. Powerful farming lobbies have little interest in changing the status quo.

Aquifers, like fish stocks, are most at risk when they cross national borders, making property rights weaker. Groundwater provides about a fifth of the planet's water needs and half its drinking water. In arid countries such as Libya or Saudi Arabia, that figure is close to 100%. Almost 96% of the planet's freshwater resources are stored as groundwater, half of which straddles borders. UNESCO, a United Nations body, estimates that 273 aquifers are shared by two or more countries.

The signing this summer of a treaty between Brazil, Argentina, Uruguay and Paraguay to protect the Guarani aquifer, after a six-year study of the region's underwater resources, has thus come as a nice surprise. It may even be a trend. Mali, Niger and Nigeria are due to sign a provisional deal early next year to set up a body to run the Iullemeden aquifer, where withdrawals have exceeded recharge ever since 1995, endangering the Niger river in the dry season.

The two deals follow a UN resolution in 2008 on creating a legal regime for aquifers (it may become a full convention next year). Lifting sanctions on Libya has had an effect, too. The Libyans say they may stop growing wheat using water from the NWSAS and the Nubian sandstone aquifer system, the world's largest fossil aquifer, which they share with Egypt, Chad and Sudan. An agreement in 1992 set up a body to run this but it has stayed largely dormant. Now sampling and monitoring have resumed, under the aegis of the International Atomic Energy Agency (which has a sideline in environmental monitoring).

Such scientific work is crucial because aquifers are still poorly understood. Until a UNESCO inventory in 2008, nobody knew even how many transboundary aquifers existed. Experts are still refining the count: the American-Mexico border may include 8, 10, 18 or 20 aquifers, depending on how you measure them. Defining sustainability vexes hydrologists too, particularly with ancient fossil aquifers that will inevitably run dry eventually. Estimates for the life of the Nubian sandstone aquifer range from a century to a millennium.