Effects on Crops of Reduced and Late Rainfall

Importance of Water . Anyone who has grown a houseplant knows that an adequate supply of water is essential to its growth and survival. Inadequate or poorly timed application of water can result in retarded growth, early or late flowering, reduced quality and quantity of desirable plant parts, and death. A plant that is suffering from water stress is more likely to suffer from other problems as well. It will be more vulnerable to attack from insects and disease organisms and will lose ground in the competition against drought-tolerant weeds.

Because water is essential to all biological processes, there is little room to substitute other inputs to compensate for shortfalls. Although substantial effort has been devoted to breeding crops for drought tolerance, successes have been limited. The term drought resistant has usually meant that the crop can extract water that other species can't, or that the crop survives water stress better than other crops. Because of the fragile nature of this resistance, existing improvements could be inadequate to protect a crop from the wide fluctuations in rainfall and moisture conditions possible under global climate change.

Crop water demand is influenced primarily by the same environmental factors that may change because of the greenhouse effect. As previously noted, increased atmospheric concentrations can decrease demand for water by increasing crop water-use efficiency; but higher temperatures work in the opposite direction to increase water demand in support of higher evapotranspiration rates. The latter effect is likely to exceed the benefits of the former in many crops. Alfalfa is a good example of a crop whose water requirement is temperature-sensitive. The crop requires 5.1 acre feet per year when grown in Kern County, but 6.5 in Imperial County. This change is due at least partly to a temperature differential of about three degrees (Celsius).

Range and Dryland Crops . In California's Mediterranean climate, rainfall and peak water demand for summer and perennial crops are not synchronized under even the best natural conditions. Grass and wildland habitats exist in precarious equilibrium as species compete for limited resources. The sensitivity of resident annual grassland, oak-savanna, or oak-woodland/grass systems to water availability is illustrated by the case of the blue oak (Quercus douglasii ). This tree, the dominant native low-elevation tree in the state, currently suffers from an inability to survive beyond the seedling stage. Researchers believe that this failure to thrive stems from actions of the early Spanish settlers, who introduced annual grasses that were able to outcompete the native perennial grasses. These annuals use more soil water from the region mined by oak seedlings than did the native perennials, with the result that the only oaks standing today are those that germinated during periods of two or three consecutive wet years. The last such period occurred about sixty years ago. A drier environment caused by global warming could conceivably bring about the elimination of the blue oak in California.

The effects of decreased precipitation on rainfed winter annuals grown in California, such as oats, oat hay, barley, or wheat, will likely be different from those experienced by grasslands, owing to the flexibility farmers have in management of these crops. Elimination of species is unlikely, since plantings of winter wheat and specialty crops will follow shifting rainfall patterns. In marginal rainfall areas, crops with low water requirements will be preferred, or, where slopes allow, irrigation will be instituted on an as-needed basis. This type of irrigation is already being practiced by some wheat farmers, who cut small furrows in their fields to allow irrigation when required. Other cropping systems that increase the flexibility of response to drought, and management practices such as water harvesting and field-level storage, will assume new importance.

Although irrigation will raise the cost of growing some traditionally rainfed winter crops, water used early to midway through the rainy season may not represent a net loss to total stored supplies. Winter irrigations may simply reduce some of the excess water that would be lost through high streamflows and flooding.

Timing of precipitation will be just as important to range and dryland crops as total quantity of water. A delay in the onset of the rainy season could result in serious decreases in autumn forage production for domestic livestock and wildlife grazers; annual grassland range plants typically germinate in autumn, grow rapidly until low temperatures nearly stop aboveground productivity during winter, and grow very rapidly in late winter and early spring until soil water is depleted. Other forage grasses would be impacted as well by a late rainy season: some perennials regrow in autumn regardless of fall rains; these species would experience plant mortality due to extensive grazing and low moisture.

Rainfed winter annuals grown without the option to irrigate could be at risk from the late arrival of the rainy season should the temperature requirements of the crop no longer coincide with the availability of water. Wheat grown in the Central Valley, for instance, requires a certain period of cold temperatures to produce optimally. If rains came too late in fall, dryland wheat would ripen late in spring after temperatures had become too high.

Irrigated Crops . Decreased precipitation is likely to affect crop production on irrigated lands mainly through changes in pricing and distribution systems rather than directly as for dryland agriculture. Different results will occur depending on what means society uses to allocate scarce water resources. If strategies (already being employed in some areas when water supplies are low) to reduce the length of the irrigation season are pursued, researchers might breed crops that develop in a shorter time period than those presently under cultivation. If water is allocated through a price mechanism, water costs to agriculture could increase dramatically, and the mix of crops planted in the state could change. Which crops would dominate the mix would depend as much on the elasticities of demand for the crops under consideration as on crop water-use efficiencies.

Regardless of the mechanism chosen to ration water among users, agriculture will need to follow water-conserving strategies. Many such practices are already followed. Pressurized-delivery systems are used to assure even application of water to each crop plant in a field, with the added benefit of reducing competition from weeds and evaporative losses. Some crop rotations include a fallow season during which the soil is stubble-mulched, then disked shallowly to preserve one season's soil water for the next year's crop. A concept known as "deficit irrigation" is thought to have potential for economizing on water without compromising yields. In perennial fruit crops, which have a large water requirement—three to four acre/feet per year—applied water can be reduced by up to 20 percent without yield loss if drought stress occurs at the right points during fruit development. Unfortunately, the long-term effects of this deficit irrigation are not clear, and greater control over the timing of irrigation would be needed by growers in order to implement the strategy.