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Are Reservoirs A Waste?

A lot of water debates in the western United States these days boil down to the question of whether desert cities or desert farms are a worse idea. On the one hand, growing water-intensive crops like alfalfa in places where it doesn't rain seems to make little sense. On the other hand, cities (and their suburbs) use a lot of water for lawns, golf courses, and swimming pools, none of which is exactly necessary to sustain life. The problem, of course, is that there are people who stand to lose their livelihoods if either of these uses gets cut.

Still, farmers and suburbanites should agree on at least one thing: Desert reservoirs are a waste. Evaporation rates are a function of temperature and lack of humidity, and the deserts of the West are nothing if not hot and dry. Lake Powell and Lake Mead, the two biggest reservoirs on the Colorado, lose an average of 1.4 million acre-feet each year to evaporation. That's almost one-tenth of the Colorado River's annual flow and more than twice the annual water use of Los Angeles.

It'd be a much better idea to store that water underground. Naturally-existing aquifers provide a logical place to do this, and there are several proven ways—involving injection wells and porous-bottomed ponds—to put water back into them. And, once the water's underground, very little of it escapes. The managers of the Kern County water bank, one of the largest in the country, calculate that they lose just 2 to 5 percent of the water they put into their system—mostly as the result of evaporation while the water is sitting in ponds soaking into the ground. The percentage lost is essentially the same whether the water stays in storage for two years or ten. By comparison, Lake Mead and Lake Powell lose around 5 percent of their water each year. If you're using them to smooth multiyear wet/dry cycles, that adds up fast.

As climate change makes rainfall patterns more variable, dry regions around the world are going to find themselves needing to store water for years at a time. The majority of that storage should be happening in underground aquifers rather than aboveground lakes. It won't make water skiers very happy, but for everyone else there will be a whole lot more water to go around.

--Rob Inglis, High Country News