Irrigation Under Stress

Irrigation is a critical means of converting otherwise nonproductive acreage in the western United States to farmland. Nationwide, it plays a large part in increasing agricultural yields. According to agricultural census data, about 45.3 million acres, 13 percent of the nation's cropland, were irrigated in 1982. Another 4.5 million acres of pastureland were irrigated. Irrigated acreage accounted for 100 percent of the rice, 70 percent of the orchards, more than half of the vegetables, and about one-third of the cotton that was planted.

In 1984, the value of the output produced per acre was nearly 2.4 times higher under irrigation than under dry land farming. In part, the difference stemmed from the fact that higher-value crops are more likely to be irrigated. Also, crop yields tend to be higher with irrigation. For instance, average yields on irrigated acreage are 97 percent higher for wheat, 67 percent higher for cotton, 54 percent higher for corn, and 33 percent higher for soybeans.

Yet despite these advantages, irrigated agricultural acreage in the United States has declined in recent years. From a peak of just over 50 million acres in 1978, the total number of irrigated acres had dropped 2 percent by 1982 and nearly 10 percent by 1984.

The decline has been due in part to the general malaise that has gripped the agricultural sector in recent years. But other longer-term factors are placing stress on irrigation:

• increased competition for existing supplies of surface water
• rising costs of new supplies of surface water
• increased marketing of irrigation water for other purposes
• declining groundwater tables
• environmental concerns

Though these stresses are expected to increase, they could be offset by efforts to increase the efficiency of irrigation systems through a growing number of cost-effective measures. Crop and energy price levels and the effects of climate change are also likely to have important impacts on the future of irrigated agriculture in the United States.

Surface supply competition

Potential new irrigators are having to contend with heavy demand on limited surface water supplies. In 1975, according to data from the Second National Water Assessment carried out by the U.S. Water Resources Council, total water use exceeded average streamflow in 24 of the 53 western water resource regions. These 24 water-scarce regions accounted for many of the better growing areas and about two-thirds of the West's irrigated acreage.

Even some of the farmers who have been irrigating with surface waters for a number of years may have problems. Water rights for federal and Indian lands were not specified when, in the nineteenth century, the states were granted jurisdiction over all non navigable waters in the public domain. Water for these lands was ignored as the western states granted water rights based on the principle of "first-in-time, first-in-right." These oversights have become the source of considerable anxiety. Federal courts have supported the view that when the federal government withdrew lands for any purpose, it implicitly withdrew sufficient unappropriated water to achieve the purposes intended for the lands.

Moreover, since the water rights date back to the nineteenth and early twentieth centuries when these lands were set aside, they command a high priority over use of the West's waters. Although few of these claims have been quantified, they are potentially vast and in many cases can be met only by diverting water currently used for irrigation.

In recent years, higher values placed on instream uses of water—fishing, power, recreation, wildlife habitat, navigation—have made it more difficult to increase diversions from many western streams for consumptive uses. Traditionally, water developers in the West assumed, at least implicitly, that water left in a stream had no value. Irrigators and others were granted rights to withdraw water from a stream as long as their water uses did not impair the rights of more senior users. Where rights were granted for instream uses, the rights were generally subordinate to and could be preempted by new offstream uses. But growing public concern about the environment since 1970 and the rise in the value of water for hydropower have resulted in legislative and administrative actions to protect many of the remaining streamflows.

Costly new supplies

Many of the proposed water supply projects suggested in recent years have yet to move beyond the planning stage. High interest rates and budgetary concerns have made federal funding more difficult to obtain. More important, the costs of developing new water supplies now often appear high in comparison with the benefits to be derived.

Three factors make increasing costs for new offstream supplies inevitable. First, because the best sites are developed first, the costs of additional units of storage rise as the capacity of the system increases. Second, as storage capacity increases, a stream's safe yield—the quantity of water that can be supplied with some high degree of probability increases, but only at a diminishing rate. And third, as the water in a stream becomes scarcer through diversion, its marginal value goes up and thus the opportunity cost of storing it and diverting it goes up.

Water marketing

In spite of the growing opportunities for improving the returns to irrigation water—that is, obtaining higher crop yields per unit of water used—more and more farmers presented with the opportunity are opting to sell their water to municipal and industrial users.

Water marketing is becoming common throughout the western states, and several states are introducing institutional and legal changes to facilitate voluntary water transfers. In 1982, California legislation declared water marketing to be a beneficial use and therefore not cause for loss of a water right. Four years later the state went one step further by making state conveyance facilities available for transferring water to new users. In 1987 Oregon and Nebraska approved legislation to encourage water marketing, and several other states considered similar moves.

At the federal level, Congressman George Miller of California introduced a bill in May 1986 to permit marketing of water provided by the Federal Central Valley Project. Regional action was taken two months later when the Western Governors Association adopted a resolution endorsing improved water use efficiency as a major goal and the promotion of voluntary water transfers as a primary means of achieving it.

How will the trend toward transfers affect irrigation? The much higher water values that are often characteristic of nonagricultural uses have led some observers to predict that the practice of irrigation may eventually cease over large geographic areas.

Such forecasts seem unwarranted, however. Water transfers occur gradually and are not likely to affect more than a small percentage of agricultural water rights for the foreseeable future. Because of the current dominance of irrigation in western water use, large percentage increases in nonagricultural water uses can he met with relatively small percentage reductions in irrigation use. For instance, nearly 90 percent of the consumptive use of western water is for irrigation. Thus, a 10 percent reduction in irrigation use would almost be sufficient to double the water available for municipal and industrial uses.

Declining groundwater supplies

About half of the irrigation water used in 1984 came from the ground. Unfortunately, since pumping exceeds aquifer recharge in many of the nation's irrigated areas, much of this use relied on nonrenewable supplies. Data collected by the Economic Research Service of the U.S. Department of Agriculture suggest that as of 1983, groundwater supplies underlying more than 14 million irrigated acres were declining between half a foot to more than five feet per year.

While groundwater stocks are vast, economic factors greatly limit the supplies available for irrigation. And as groundwater tables decline, the economics of irrigation with groundwater deteriorates as pumping costs increase, well yields decline, and pumping efficiency falls. Moreover, groundwater irrigation is a very energy-intensive form of agriculture and therefore its profitability is susceptible to the uncertainties of energy prices.

Groundwater use for irrigation has already started to decline in some areas, most notably in the southern High Plains and Arizona. But the impacts on production have been mitigated by the adoption of more efficient irrigation systems and switching to crops offering higher returns to water.

Environmental pressures

Salinity and streamflow reduction are major environmental concerns specifically linked to irrigation. Irrigation results in increasing salt concentrations in the water and soil resources essential to the long-term viability of irrigated agriculture. Indeed, salinity levels in several river basins—the Lower Colorado, the Rio Grande, and the San Joaquin—have become so high that the viability of irrigation is being threatened.

Fortunately, there is a strong complementarity between irrigation practices that conserve water and those that reduce the related environmental problems. Limiting the rate and quantity of water applied to levels that can be fully absorbed within the root zone eliminates the erosion, sedimentation, and water-quality degradation associated with irrigation.

Unfortunately, however, the elimination of runoff contributes to the accumulation of salts within the root zone. To prevent the salts from damaging productivity, additional water must be applied occasionally to leach excess salts from the soil. Even though some of these salts are likely to show up in neighboring streams or underlying aquifers, improved water management practices can go a long way toward reducing the environmental costs of irrigation while conserving scarce water supplies.

Future growth in irrigated agriculture in the western states will depend in large part on increasing the returns to water. One method of achieving this goal is to improve the efficiency with which the water is delivered to the plants by investing in new irrigation systems.

Another method is to adopt improved irrigation scheduling. If water is applied only when it will be used most effectively by the plant, higher yields with less water may be possible. Irrigation scheduling services that utilize models and computers to analyze data on crop responsiveness to water, daily weather records, soil moisture levels, infiltration and evaporation rates, and other factors are now available in many areas to assist fanners in making irrigation decisions. As water costs rise, it may become profitable to irrigate less even if lower yields are a likely result. For instance, water use can be reduced by about one-third with only modest declines in yields by simply irrigating alternate furrows.

Another method for combating rising water prices is to switch to crops that use less water. For instance, some growers have replaced corn with sorghum or wheat as water became more costly.

Agricultural researchers are developing seeds, chemicals, and management practices that will help farmers adapt to higher water costs. Chemicals that reduce the amount of water a plant transpires, seed varieties that provide high yields with lower water inputs, and agronomic and irrigation management techniques that improve yields per unit of water are being developed at experiment stations and in some cases have already been adopted by irrigators.

Energy and crop prices

As already mentioned, economic factors such as energy and crop price levels will be important to the future growth of irrigated agriculture. Since fuel is needed to pump water from the ground and to distribute it under pressure through sprinkler systems, high energy prices are likely to affect irrigators more adversely than other farmers.

All farmers have been affected by the low crop prices of recent years, but irrigators have been affected especially. Their yields and production costs are generally higher, so their profits tend to be more sensitive than those of other farmers to agricultural prices. High crop prices encourage yield-increasing investments, especially irrigation.

High crop prices undoubtedly would strengthen the ability of irrigators to deal with the stresses they are facing. However, even relatively high crop prices are unlikely to offset the trend to transfer potential irrigation water to municipal and industrial uses, which have much higher water values.

The climate factor

The foregoing discussion implicitly assumes the nation's climate will remain essentially unchanged. This assumption becomes increasingly questionable in view of the widely held consensus among climatologists that increasing concentrations of carbon dioxide and other radiatively active trace gases in the atmosphere will lead to a global warming and substantial, but unknown, changes in precipitation patterns.

Global climatic modeling is not sufficiently advanced to predict what these changes might mean for growing conditions in specific regions. Regional water supplies could be substantially curtailed even with no reduction in precipitation because higher average temperatures would cause evapotranspiration rates to rise. On the other hand, water supplies might rise as the global hydrological cycle is accelerated by the warming.

The added uncertainty related to climate provides further justification for institutional changes that will make irrigators as well as other water users more responsive to underlying resource conditions. It also highlights the need for research designed to develop better ways for adapting to increased water scarcity.

The days of rapid growth based on access to low-cost water are over for irrigators. No likely set of policies will do much to increase the flow of public funds for irrigation projects that are criticized for their questionable economics and for their adverse impacts on the environment and instream water uses.