Several recent columnists and writers for this paper have argued that cotton and alfalfa are "water-intensive" crops and should not be grown in California.

Let me first dispel the myth about cotton being such a water-intensive crop. The consumptive water use by a crop basically depends upon four factors: the percentage of the field covered by green foliage; the length of the growing season; the temperature during the growing season; and the humidity during the growing season.

In the San Joaquin Valley, the consumptive water use by cotton is about 2.8 acre-feet per acre. An acre-foot is the volume of water needed to cover an acre of land to a depth of one foot. By comparison, corn uses 2.1 ac-ft/ac; rice uses 4.0 ac-ft/ac; alfalfa uses 3.7; deciduous orchard (apricots, peaches, almonds, walnuts etc.) uses 3.4; grapes use 2.5; and sugar beets use 3.1 ac-ft/ac.

From the farmer's perspective, one of the most important metrics is the revenue generated by one acre of cropland. For any given crop, this varies from year to year, of course, depending upon the prices of agricultural commodities.

In 2012, one acre of cotton generated $1,780 while an acre of corn returned $1,300 and an acre of alfalfa $1,460. Mature tree crops such as almonds and walnuts returned about $6,100 per acre. However, the farmer has a very large investment in a producing orchard or vineyard.

Furthermore, after planting an orchard, the farmer has to wait 3-5 years before he harvests his first crop. During this time, the farmer continues to supply the orchard with fertilizer and water.

Between 2002 and 2012, the acreage of cotton in California was cut nearly in half to 345,000 acres. Much of this former cotton land was converted to orchards, in part because of the higher dollar returns that orchards provide.

In contrast to cotton, however, orchards require water all year to keep the trees alive. On the other hand, cotton land can be fallowed if water is unavailable.

The soils in the San Joaquin Valley vary widely, and despite the higher returns that tree crops such as almonds generate, orchards cannot be grown everywhere.

In the Tulare Lake Basin, for example, orchards cannot tolerate the high soil salinity and high water table that exist there. The high soil salinity also precludes the growing of many field crops, such as tomatoes and alfalfa. Cotton, however, is relatively salt tolerant.

Your writers have also expressed dismay that alfalfa hay is the largest field crop, by acreage, grown in the state. The single top-valued agricultural commodity produced in the state in 2012 was milk with a farm value of $6.9 billion.

Because milk is such a perishable commodity, milk is always produced close to the market in which it is consumed. Milk-producing cows need alfalfa. California has an ideal climate for alfalfa production because rain rarely falls during the growing season, and rain can ruin hay that has been cut.

California's 80,500 farms and ranches received $44.7 billion for their output in 2012. The last thing these farmers need is uninformed urban environmentalists dictating what crops they can grow.

Donald F. Anthrop. is professor emeritus of the Environmental Studies Department at San Jose State University