Submitted to: American Society of Agronomy Monograph Series
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2003
Publication Date: 5/1/2007
Citation: Kimball, B.A. 2007. Global change and water resources. In R.J. Lascano & R.E. Sojka (eds.), Irrigation of Agricultural Crops, Agronomy Monograph No. 30, 2nd edition. Amer. Society of Agronomy, Crop Science Soc of Amer., Soil Science Soc. of Amer., Madison, WI, pp. 627-653, color plates 17-1, 17-2, 17-3, 17-4.
Interpretive Summary: The increasing atmospheric CO2 concentration is likely to affect future crop growth and water relations directly, while possible global warming and changing precipitation patterns could affect irrigation requirements and water supplies. At the same time, there is ever increasing demand and competition for water, which may be exacerbated by such global environmental changes This paper evaluates how crop water use may be affected by several opposing global change factors, and reviews other studies about the possible implications for irrigation water requirements. Global change is likely to increase crop water use slightly, but depending on how precipitation patterns may change, irrigation water requirements and irrigation water supplies may be substantially affected. This research should benefit water resource planners, as well as growers and consumers of food and fiber.
Technical Abstract: The influence of global change on future water resources is difficult to predict because various components are likely to be affected in opposing ways. Global warming would tend to increase evapotranspiration (ET) rates and irrigation water requirements, while increasing precipitation would both decrease irrigation water requirements and increase water supplies, although regional pattern changes are very uncertain. The direct effects of elevated CO2 on plants likely will cause increases in stomatal resistance, which will also tend to reduce ET; but at the same time, the elevated CO2 will stimulate increases in plant leaf area and canopy temperature, both of which increase ET. The sensitivity of "reference" ET for alfalfa to the several opposing future influences was examined using a standard form of the Penman-Monteith equation, and slight increases in crop ET (2-6%) were predicted for plausible scenarios of future temperature and CO2. However, irrigation requirement is the difference between crop water use and available water from precipitation and soil storage, and the latter two likely will also be affected by global change. Some modeling studies predict that irrigation requirements will increase substantially, on the order of 35% for the U.S. overall, but with wide variability. Streamflow is also projected to increase. However, with global warming, greater proportions of annual precipitation may fall as rain, and snowpacks may melt faster. The latter implies loss of a huge free snowpack "reservoir" that presently stores winter precipitation at higher elevations that can be used for summer irrigation at lower elevations. However, the effects on irrigation requirements and on water supplies are very uncertain due to the uncertainties in projected precipitation patterns, as well as in human economic and social factors. It behooves future water resource planners and future growers to try to be as flexible as possible.