How much would it cost to grow a crop using desalinized ocean water? This post is a rough (back of the envelope) calculation.

According to the FAO document Crop Water Needs, a wide range of crops can be grown on 600 mm of water per crop season. Now 2 acre feet of water (per acre of crop) per crop season is just over 600 mm of water, or about 651,700 gallons of water per acre per crop.

This article from San Francisco discusses new desalination plants in California. Two cost estimates are given: as low as $2000 per acre-foot of water and as high as $3000 per acre-foot of water. That is the on-going cost of the water, not the cost to build the plant. The $2000 figure is the most recent cost estimate, but let’s be conservative and use $2500 per acre-foot as our starting point.

The cost of 2 acre-feet of water per crop season adds $5,000 to the cost of the crop. If some rainfall is combined with some irrigation, perhaps half that amount of water could be used, adding $2,500 instead to the cost of the crop.

An acre of wheat produces anywhere from 35 to 70 bushels per acre, depending on the type of wheat. A moderate expectation for yield is about 50 bushels per acre. Dividing $2,500 into those 50 bushels gives us an additional cost of $50.00 per bushel. The $5,000 dollar estimate for water doubles that value to $100.00 additional cost per bushel.

What is the current commodity price for wheat? It’s around $5.45 per bushel. So the cost of wheat would increase by 9 to 18 times.

Another way to look at the numbers, though, is the absolute cost per pound of wheat. A bushel of wheat is 60 lbs. At $100 dollars more per bushel, the price per pound increases by $1.67. If that price were passed on to the consumer, a 5-lb. bag of wheat flour would increase in cost by $8.33. At $50 more per bushel (for half the water from irrigation), the increase is $4.17 per pound of flour. That is a severe price jump, but if the world were ever to face a severe food crisis, it would be possible.

The least thirsty crops are probably bean, pea, onion, and cabbage. It might be possible to grow these crops on one acre-foot of water per crop season per acre, reducing the above costs for wheat by half. But that calculation still adds substantially to the cost of the crop. It might be worth it for onions, since these are used more for flavor than as a staple food.

The clear conclusion is that crops cannot be grown with desalinated water, on a large scale. The cost would make it impossible for the farmer to sell his crops at a profit. The cost would make the food grown available only to the wealthy. I suspect that, in the distant future, food will be available on a two-tier system. The wealthy will have access to foods grown with desalinized water, foods that require lost of land for small amounts of nutrition (red meat, tree fruits and nuts). The main body of the population, up to and including the upper middle class, will have to change their eating habits substantially, as population growth and competition for land use require us to get our food from the crops that provide the most nutrition on the least land and with the least water.

But for now, when there is not enough rainfall or irrigation in a region to support a crop, the production of that crop must be moved to a nation or region with sufficient rainfall or cheap irrigation. So if the West Coast drought continues much longer, agriculture in that region of the U.S. might collapse. The same can be said if the great underground aquifers in the Midwest are pumped dry by excessive irrigation. The Midwest might no longer be the breadbasket of the nation. Substantially lower yields (due to a lack of rain or failure of irrigation) will make farming in those regions no longer economically possible.