Do farmers waste water?

(ht: Brown and Caldwell California Water News)

Do farmers waste water? Vance Kennedy, a retired hydrologist offers his opinion in the Modesto Bee.

There is a common fallacy that flood irrigation by farmers wastes water by using more than is needed for crop growth. As a result, proposals have been made that farmers pay more for their water to discourage that "wasteful" practice.

Why it's not wasteful:

The main source of water to replenish the water table has been through flood irrigation as farmers use more on their land than is required for plant growth. The land filters the water as it sinks into the aquifer. Later, this water is pumped out by the cities and others.

Well, OK, maybe so. I'm assuming Mr. Kennedy has checked to make sure the water seeping in from flood irrigation does wind up in the same aquifer that cities and others pump from.

Aquifers are funny things. A number of aquifers are trapped between aquacludes – layers of rock that are impervious to the flow of water. Think of a layer of sand between two sheets of glass.

If there are no aquacludes between the ground water and the ground surface, you'll have a situation like the classic drawing of the water table. Water seeps down from the surface until it reaches the water table, and you can dig a well to the water table and pull up water for use. Water from flood irrigation would add to this reservoir.

If there is an aquaclude in the way, the water that seeps down will be blocked before it reaches the aquifer. Water in that aquifer gets in at a "recharge zone", usually in nearby hills or mountains, where the aquaclude disappears for whatever reason and there is no more obstruction between the porous rock and the ground surface.

In some areas, you can have layers of porous and non-porous rock, with the result that you'll have several aquifers under a given point. Each aquifer will be fed from a different recharge zone, have different water quality, and probably flow in at least slightly different directions. In such a case, it's even possible to install a casing around a well shaft to block off one or more aquifers in the middle of the well, while allowing water from the rest of the aquifers to flow in.

So, it's not obvious to me that farmers who flood irrigate are necessarily recharging the ground water supply used by anyone else. If a hydrologist says it does, I'll take him at his word, and assume he's done the necessary research to back up that opinion.