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J.1 '\ 2.3 Consumptive Use The amount of water used in producing crops is known an "consumptive uSe" or "evapotranspiration." It is the loss of water in vapor form from plants and soil surface. All aerial parts of plants may lose some water by evaporation, but most of the water is lost by transpiration through the stomata which are concentrated mostly in the leaves. Evaporation is the loss of water vapor from the soil or from exterior portions of plants where water may have accumulated from irrigation, dew, or exudation from the interior parts of plants. Evapotranspiration is a major component of the hydrologic cycle. Therefore, it is involved to some degree in nearly all hydrologic studies, and as such 'in the planning and development of water resources in a' river basin. It fo~s the foundation for the planning and design of most irrigationprojects.and generally forms the basis for determining the , surface and subsurface storage requirements, the capacity of the delivery ~ system, and general operation practices; further, it is needed for manage- ment-of water quality, control of soil salinity, negotiation of water compacts and treaties, and litigation and adjudication of water rights. The continuing and great need for evapotranspiration data has resulted .in numerous basic, empirical and crude methods of estimating it. Empirical and crude met~ods are often used due to data limitations and the time- c~nsuming and expensive nature of making detailed measurements. However, the method of estimation should be selected such that it represents the best alternative at a given time and reliance is made on basic science, physics and engineering principles. The Jensell-Haise method (Jensen and Haise, 1963) was used to estimate evapotranspiration. This method is the lost appropriate given the limited climatic data base that exists in the ~lores Project Area. . -l