Submitted to: International Conference on Water Resources Engineering Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 15, 1999
Publication Date: N/A
Interpretive Summary: Competition for limited water supplies has escalated substantially over the last decade. Unfortunately there are many misconceptions about water use, particularly as related to irrigated agriculture. The potential water savings that might result from increases in irrigation efficiency are usually overestimated. Many of these misconceptions and erroneous estimates sof water savings stem from misunderstandings about how irrigation performance measures are defined and used. The agricultural engineering profession has not been consistent in its use of terminology and how it accounts for various components of water use. This paper attempts to describe some of the details of water consumption by agriculture and how these influence how one describes the performance of irrigation systems. It goes on to discuss the potential impact of policy decisions based on inappropriately defined and applied concepts regarding water allocations to oagriculture. An example is provided. This discussion should be useful for both individual project and regional water management and should be of interest to government agencies with water management authority, consulting engineers, extension specialists, and irrigation farmers.
Traditionally, all water evaporated and transpired within an irrigated field has been considered part of crop evapotranspiration. The crop coefficients used for irrigation scheduling typically included both crop evapotranspiration and soil evaporation. In the 1970s, the concept of basal ET and basal crop coefficients was developed to separate real crop needs from irrigation system needs. The revised FAO procedure for determining total evapotranspiration separates basal crop ET from wet soil evaporation. Previously published crop consumptive-use relationships may have inconsistently included or not included soil evaporation. Unfortunately, these inconsistent estimates of crop water consumption have been used to establish water duties and water transfers. The effect of the soil evaporation component on irrigation efficiency, water duties, and water transfers is demonstrated with a case study from Arizona.