INFLUENCE OF CLIMATE ON IRRIGATION

Abstract

The paper presents a procedure for estimating the influence of surface air temperature change on plant potential evapotranspiration and water supplies assuming rainfall and other climate parameters remain unchanged. On the basis of this procedure, it is estimated that 1 °C increase in temperature will produce an average annual increase in potential evapotranspiration in the western U.S. of about 40 mm. It also is estimated that the average annual runoff will decrease by about 6–8 percent for the same temperature increase, assuming no change in precipitation.If global temperatures increase, the spatial and temporal variabilities of climate are also expected to increase. Globally, rainfall is projected to increase, but the locations and extent of desert areas are expected to change. Weather is expected to become more extreme and variable. Water requirements in arid areas are projected to increase. Planning is needed to mitigate through improved management strategies the likely adverse effects of climatic change on water supplies and on crop production.There is a need for a standardized reference crop or vegetative cover for modelling the effect of climatic change on irrigation. The ET of perennial rye grass or of Alta fescue grass as measured by the large weighing lysimeters at Davis, California, could be used as a standard index. An equation requiring only measured (or estimated) values of temperature is recommended for global modelling of climate and its effect on irrigation.A relationship is proposed which expresses annual watershed runoff, R, as a function of the sum of the positive monthly values of precipitation minus evapotranspiration. This relationship requires further investigation and refinement by testing it for a wide range of climatic conditions. For example, it could be investigated at a particular geographic location under a range of elevation induced climate conditions. If the relationship proves to be as reliable as indicated from the limited data used for this study, it could become a simple and useful tool for evaluating the influence of climate on runoff from watersheds. Only measured rainfall and temperature data are required for application of the equation.The continuing CO2 build-up in the atmosphere is reported to increase crop yields, water use efficiencies, and stomatal resistances. However, reported changes in leaf area and stomatal resistances as a result of CO2 atmospheric increases vary considerably among various experimental studies (Rosenberg et al., 1990). It is thus difficult to draw specific conclusions from these results. The differences are at least partly due to the limitations of experimental conditions which include the effects of greenhouse-type enclosures. It is, therefore, recommended that additional studies be conducted on the effects of interactions between environmental CO2 and plant mechanisms in an open agricultural environment.