Agriculture

Abstract

Climate, soil and water all determine cropping patterns in California. Limits of the developed supply of water are being approached; in fact, the serious overdraft of groundwater, particularly in the San Joaquin Valley, shows that the replenishable supply is being exceeded. Some undeveloped runoff flows to the ocean-almost 48 million acre-feet in an average year. Much of that, however, is in the protected rivers of the North Coast. Of the developed water supply, agriculture consumptively uses about 85 percent. Under these circumstances, the question of agricultural water use efficiency in California is crucial. Two sets of forces, physical/biological and economic, determine the amount of irrigation water used by the state's farmers. The basic water requirement for crop growth is for the process of evapotranspiration (ET), which has two components-evaporation from soil and water, and transpiration through the leaves of plants. Most ET losses take place through transpiration, which is directly related to crop production through photosynthesis. Production and water use efficiency may drop substantially if the crop is not well cared for; pest, weed and disease control are important, as is adequate nutrition. Overall efficiency of water use in agriculture is closely related to units of crop produced per unit of water applied. Some crops use more water than others, largely because of differences in growing season and in length of growing period. Also, some irrigation methods use more water than others, with sprinkler and drip systems often-though not always-more efficient. Another factor that determines agricultural water use is its cost, particularly the marginal cost of applying additional water. Many irrigation districts use fixed (peracre) charges or a combination of fixed and variable charges which may be related to average consumptive use, but do not reflect the incremental cost of water. One possibility for reducing on-farm water use would be to charge all or most of the cost of surface water at a variable (per-acre-foot) price. Groundwater pumpers, of course, already pay according to the amount they use. How much water could California agriculture save? Most of the unused water associated with less efficient irrigation is recovered-it returns to the basin water system and is used again. As "recoverable losses, " these are not really losses at all in a physical sense, although their recovery is not costless in an economic sense. There also are very large "nonrecoverable losses, " mostly through ET, most of which could not be reduced without a concomitant reduction in crop biomass. In response to higher water costs and/or decreased supplies, farmers may: (1) change cropping patterns, (2) cease irrigating some cropland, (3) in some cases, cut water applications to a point below the crop's maximum need, or (4) adopt new irrigation technology. The first two alternatives would save water for use elsewhere, but of course would affect crop production; the third would reduce nonrecoverable losses, but might be risky, and the fourth would reduce mostly recoverable losses, hence might not significantly increase basinwide water supplies. Factors that will influence future water use by California agriculture are: the worsening overdraft problem; expected future water price increases; environmental constraints; future demand for agricultural products; the trend toward more efficient, and expensive, irrigation technology; the issue of varying values of water (implying that economic efficiency gains could be made by transferring water from one area to another); and the issue of water subsidies, particularly the question of whether a proposed project would create more, or less, total wealth than it costs.