Submitted to: Journal of Irrigation and Drainage Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2008
Publication Date: 1/16/2009
Citation: Clemmens, A.J. 2009. Accuracy of project-wide water uses from a water balance: a case study from southern california. Journal of Irrigation and Drainage Systems 22: 287-309.
Interpretive Summary: Water resources in many parts of the world are increasingly in short supply. Irrigated agriculture is often viewed as a source for additional water supplies for other uses. However, it is important to understand the hydrologic context in which land is irrigated, and thus, what happens to water applied by irrigation and not consumed. In this paper, a detailed water balance in performed on a 200,000 ha irrigation project in Southern California. The paper documents the various sources and destinations for both rain and irrigation water. It also demonstrates that one can determine the accuracy of the volume of water consumed or any other destination by determining the accuracy of each measured water balance component. For this project, water consumed was determined as the remainder in the water balance, and it was estimated to within 5%. These results should be of use to consultants, the World Bank, the Bureau of Reclamation, and other government agencies.
Technical Abstract: A detailed water balance was conducted on the Imperial Valley in southern California for the years 1987 to 1996. The area included all lands within the boundaries defined, including farms, towns, road, etc. This analysis included surface and subsurface inflows, rainfall, surface and subsurface outflows, evaporation, evapotranspiration, municipal and industrial uses, etc. Total water consumption was computed as the remainder in the water balance. The accuracy of this quantity was determined from standard statistical procedures based on the accuracy of the input data. For the Imperial Valley, total water consumption was estimated to within ±4.5%. The various sources of water and destinations were partitioned so that the accuracy of each component could be determined. The method also determines the source of uncertainty through the variance of each measurement. Subsystems were defined so that other quantities of interest could be determined. A canal subsystem water balance was used to determine more accurate estimates of water delivered to farms. A drainage/river subsystem water balance was used to determine the amount of irrigation return flows. Finally, the destinations of irrigation water were delineated and the Irrigation Consumptive Use Coefficient (ICUC) was calculated. The confidence interval, as a ten year average, was 0.64 < ICUC > 0.70. It was shown that the uncertainty of rainfall and how that water is partition has a significant influence on the accuracy of the estimated water consumption and ICUC.