Improvements in Agricultural Water Management
U.S. Department of Agriculture, Agricultural Research Service
The agricultural community was always been at the mercy of water supplies. Now, with the prospect of larger variations in temperature and precipitation brought about by climate change, farmers are faced with even greater uncertainties about water supplies for crops, forage, and livestock. In the next 30 to 50 years, we could witness large-scale changes in agriculture resulting from the continued depletion of our natural resources—land and energy as well as water—and our heightened awareness that some agricultural practices are driving environmental change. This paper discusses the role of agriculture in improving water management under conditions of climate uncertainty.
THE PRESSURES ON AGRICULTURE
We have known for some time that there is not enough land, water, and fossil fuel to go around. Despite this knowledge, our adoption of conservation practices has been woefully inadequate. As the population continues to multiply (predictions are that the world population will reach 12 billion people in the next 30 to 35 years), the pressures on agriculture will increase.
Until now, inexpensive energy and high labor costs have driven the mechanization of agriculture. Thanks to the low cost of petroleum, food and fiber production more than doubled between 1950 and 1970. But the days of cheap fossil fuels are numbered and prices will skyrocket as supplies dwindle. The increasing scarcity of fossil fuels could have grave consequences for U.S. agriculture, which has become increasingly reliant on inexpensive petroleum products for fuel, pesticides, and fertilizers. The quality of life
that we've come to expect here in the United States—the inexpensive food supplies—may not continue.
THE CONTRIBUTION OF AGRICULTURE TO ENVIRONMENTAL CHANGE
Since the beginning of the industrial revolution, the abundance of carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, and other gases in the atmosphere has increased. The buildup of these gases may alter global temperatures, the worldwide distribution of precipitation, and the quantity and quality of our water resources—all of which impact the productivity of our crop land, range land, and forest land. Yet, agriculture is not only impacted by environmental change, it is also a cause of that change. A recent EPA report shows, for example, that agriculture contributed an estimated 26 percent to the increase in atmospheric gases in the 1980s; agriculture accounted for 13 of the 18 percent increase in atmospheric methane. Ruminant animals, rice paddies, fertilizers, cultivated natural soils, biomass burning, and land use conversion are all sources of agriculture's contribution to environmental change.
Though we know agriculture is contributing to the increase in atmospheric gases, our understanding of the links between agriculture and climate variability is still limited. Understanding how agriculture affects and is affected by environmental change requires improved understanding of basic hydrologic processes and improved water supply forecasting techniques.
THE NEED FOR NEW KNOWLEDGE
We need to improve our quantitative understanding of basic hydrologic processes: ground water recharge, snow accumulation, rainfall variations with elevation, evaporation, and how all of these processes influence streamflow. We also need to develop improved fundamental process models that link appropriate hydrologic processes to relative factors in environmental change.
We need improved predictions of future water supplies. We need to improve the accuracy of water supply forecast models through new developments in hydrologic process models and new technologies, such as remote sensing, geographic information sys-
tems, digital elevation techniques, and real-time data. We have some of the pieces of this puzzle, but we need to start using forecasting models in agriculture.
Atmospheric science has made great progress in the last 25 years in understanding atmospheric conditions and developing an improved capability to make predictions for very short time periods. But we need to improve our predictions of precipitation, temperature, and evaporation for a month or more in advance. Existing meteorological information could be used more often to provide statistical probabilities that certain events will occur—for example, to provide the probable beginning and duration of rainy seasons or the statistical likelihood that a drought will continue for a specific period. Such knowledge is useful in planning the introduction of drought-resistant cultivars, selecting the crop to be produced, introducing new crop rotation systems, and selecting improved agricultural practices. We also need to improve predictions of interannual or decadal variations in temperature, precipitation, and evaporation. Though such predictions have been made, accurate predictions of the onset, duration, cessation, or likelihood of recurrence of drought for site-specific locations are not available.
AGRICULTURAL STRATEGIES FOR COPING WITH ENVIRONMENTAL CHANGE
Crop production under environmental change is affected by two competing tendencies. As carbon dioxide levels go up, yield will more than likely increase. But the likely increase will be counterbalanced if precipitation declines, in which case we could find that production decreases or remains the same. The wise course for dealing with this uncertain future is to increase water use efficiency. More efficient water management technology is evolving at an accelerated pace, but implementing new technologies may require decades and will usually require additional investments in physical and supportive infrastructure. National water policies may in some cases hinder the implementation of new technologies.
The key to successful dryland farming is using systems and practices that can take better advantage of favorable years. Three
components of water conservation for dryland production are retaining precipitation on the land, reducing evaporation loss, and using crops that best fit rainfall patterns. Retaining precipitation on the land starts with preventing runoff. For example, furrow diking, conservation bench terraces, and precision land leveling have proven effective. We need to increase the use of these methods. For reducing evaporation, the most promising practice is application of a mulch or residue cover. Crop residues are the only practical source of mulching, which makes crop rotations and predictions of precipitation and snow on a monthly or annual bases extremely important. Matching cropping systems with rainfall patterns—essential in dryland farming for ensuring a good probability of producing a harvestable crop—requires predictions of climate variability on an interannual or decadal basis. There is a potential to change our cropping patterns, but it is very difficult to make adjustments if we do not know the direction or magnitude of precipitation change.
Irrigated agriculture consumes 80 percent of our fresh water supply. Although it accounts for only 11 percent of our total crop land (16 percent of total harvested crop land), it provides 51 percent of the nation's total crop value (though that value declines to 38 percent if you look at total marketable products sold). The point is that irrigated agriculture not only consumes a significant portion of our water supplies, it provides a major contribution to the nation's agricultural economy.
All irrigation systems could be operated more efficiently to help solve the problem of limited water supplies. Many individuals have the misconception that irrigation efficiency depends only on irrigation system design. However, an important aspect of improving water use efficiency is improving irrigation system management, operation, and maintenance. Automated irrigation scheduling can have a major impact on improving on-farm irrigation efficiency. Excessive seepage of irrigation water from canals constructed in permeable soil—a major cause of high water tables and saline soils in many irrigation areas—can be reduced by lining the canals. Finally, automation of delivery systems can sometimes provide a higher level of water control: many delivery systems encourage over irrigation because water is supplied for fixed periods or at fixed amounts, irrespective of seasonal variations in on-farm water needs.
In addition to increasing irrigation efficiency, another method for coping with limited water supplies is to intercept drainage and sewage effluent waters. The drainage or reclaimed water can be blended or used separately for irrigation or other purposes after it has been fully used for crop production.
Whatever the strategy chosen for adapting irrigated agriculture to limited water supplies, the strategy must be matched to the crop, management, and labor conditions.
Mixed Range Land and Crop Land
The interaction between rand land and crop land is regulated primarily by climate, available irrigation water, and management. Weather modification, water harvesting, snow harvesting, livestock management, water spreading, and other techniques are available for improving water-use efficiency of range and crop plants. However, environmental impacts of altering meteorological parameters such as precipitation have not been determined scientifically, and intensive, well-designed experimentation is still needed to evaluate properly the potential usefulness of weather modification techniques.
Many range lands are now invaded by brush and need vegetative modification to control runoff and improve infiltration. Successful brush control, reseeding, reforestation, and other range land improvement actions must be integrated into the total water supply management system.
THE NEED FOR CHANGES IN AGRICULTURAL INSTITUTIONS
The institutions involved in allocating and delivering water can influence the effectiveness of water use in irrigated agriculture. Some economists suggest that a change in the pricing scheme for water is needed so that water will be allocated more efficiently. Others argue that jumping immediately to increased water pricing would ignore the need for an institutional structure that will allow improved decisionmaking with respect to water use. Water policy and law, when properly employed and enforced, become not the single solution to improving water use for increased production but an essential ingredient of efficient and effective management.
Many authorities are recommending that greater attention should be focused on improving existing irrigation systems. Others
suggest that irrigation systems should not be changed unless the economic benefits have been considered and increased education and training have been provided along with advanced technology. Sound education and technical assistance programs can reduce the gap between research and actual improved irrigation practices.
Some authorities suggest that increased public financial assistance is required before any significant shift toward water conservation and environmental protection can occur. On the other hand, technical assistance, interest-free or low-interest loans, or direct cost sharing can only be provided where benefits to the public sector are documented.
Generally, new and improved irrigation systems that integrate automated irrigation water management systems and water measurement need to be developed. Such systems can simplify or reduce the number of management decisions to be made by the water managers and irrigators. The result is improved efficiency of water application, reduced energy use, and environmental protection.
The next 30 to 50 years could bring large-scale changes in agriculture. Even without environmental changes, agriculture will be faced with the seemingly impossible challenge of providing for twice as many people—each person with increasing demands—as exist now. With a relatively fixed land base and water supply and with diminishing reserves of petroleum and mineral resources, our options for increasing agricultural production are severely limited. Uncertainties in future water supplies, precipitation, and other climatic variables could be the straw that breaks the agricultural camel's back.
No single technology can solve all of the water quantity and quality problems confronting agriculture. However, water conservation technologies, when properly selected and implemented, can improve water use and management of crop land, range land, and forest land. Improvements in water management are required at all levels of water use. These management improvements will require bold changes in institutions and organizations, water policy and law, farming systems, education and training programs, and research and development. We must all recognize the critical role of water resources in human life. Improvements in agricultural water management are required both to cope with environmental change and to ensure environmental protection.