Last week I talked about the water problem and how water is unique among economically productive commodities. Most commodities are substitutable – that is, you can use something else in place of the commodity, if you really need to. Few commodities have perfect substitutes; corn is not exactly the same thing as wheat, barley is not a perfect substitute for rye. But if the wheat crop fails, we don’t all starve; we substitute other grain products for wheat and get by. Water, of course, is different. There are some marginal uses for water where you can find substitutes; cars don’t have to use water in their radiators, they could use other fluids. But for most of the things we use water for – drinking and irrigation – it’s water, or nothing.

The other unique property that water has is that it doesn’t get used up. When you burn a barrel of oil, it’s converted into carbon dioxide and other gases and residues – but those residues never coalesce back into oil. Short of rocketing it into space, nothing that we do with water destroys the water itself. If you pour a gallon of water onto your field, it’s absorbed by the soil, taken up by the plants, used to build new plant mass, and moved around – but the water is never destroyed. It ends up coming back to the system in one way or another. The details of where and how it comes back can be very important – but it always comes back. (Generally, by running to the ocean in one form or another, then heated by the sun and vaporized into clouds, then pouring back onto the earth in the form of rain or snow.)

But it might not come back in a convenient form, or at convenient time or place. Many countries that have almost no rainfall nonetheless have prosperous agricultural sectors. They manage this feat by pumping water from the deep aquifers beneath their land area – water that was laid down as long as two million years ago – and using it to irrigate land that receives little or no natural rainfall. Saudi Arabia is one example of a country that uses groundwater mining to prop up its agricultural sector; they have been self-sufficient in wheat production for the last three decades, with almost five million hectares under cultivation. The trouble with mining water, of course, is that the water in those aquifers refills slowly, if at all. Alarmingly, more than half the world’s population now lives in countries that (a) rely at least somewhat on fossil water, and (b) have water tables that are falling.

Countries like Saudi Arabia are phasing out their irrigation programs. For the Saudis, self-sufficiency in grain was a strategic decision, not an economic one; it would have made more sense for them to buy grain on the international market than to prop up farms in the desert. Their shift back to market-based food supplies is being forced on them by the drying up of their reservoir, but no Saudis will go hungry as a result. The picture is not so optimistic in countries like China and India. 80% of China’s grain supply comes from irrigation, and 60% of India’s. Those countries are not as arid as Saudi Arabia, but their natural rainfall does not support the level of irrigation that is in place, and they are rapidly mining out their fossil water reserves.

In the short term, that’s good news for US farmers. American fossil water reserves are diminishing, but only 20% of our grain crop relies on irrigation. Our rainfall supports our agricultural infrastructure, for now – meaning that US grain producers will still be in business when China and India are looking for food. In the short term, American farmers are on the sweet end of the stick.

That’s likely to change, however, and next week we will look at why.