State-of-the-Art Evapotranspiration Prediction

This special issue of the Journal of Irrigation and Drainage Engineering focuses on evapotranspiration (ET) and its prediction in the context of irrigated agriculture. Replenishment of ET is the primary purpose of irrigation, and thus ET is a major component in the denominators of equations that establish irrigation water requirements and irrigation diversions. ET represents the component of the irrigation diversion that undergoes a physical phase change, and thus, the one component that physically “disappears” from the liquid water resource. In this regard, accurate quantification of ET is necessary to accurately quantify the disposition and hydrology of the other components of the irrigation diversion. Accurate quantification of diversion components and ultimate water consumption is an absolute necessity in 2005, when our fresh water supplies are stretched, more and more, between inflow stream needs and off-stream usage and consumption, between storage and natural flow, between agriculture and cities, between agriculture and recreation, and among agriculture, environment, and endangered species.

The papers in this special issue on ET span a broad range of focus and method of application. The first three papers in this special issue by Allen et al. describe the crop coefficient method as standardized by the United Nations Food and Agriculture Organization (FAO) for predicting ET on a field-by-field basis, and application and testing of that method including application to a large irrigation district in California. This application is one of the first to compare prediction by a crop coefficient-based method with ET from a large irrigated area as determined from an inflow–outflow water balance.

The next two papers, by Burt et al. and by Mutziger et al., provide an extensive review of literature on measurement and prediction of evaporation from bare soil and testing of the evaporation model standardized by FAO. The sixth paper, by Temesgen and Echings compares and discusses prediction methods for reference ET used in California. Their work includes comparison of California methods with the recently released ASCE report on standardized calculation of reference evapotranspiration.

The last two papers in this special issue “jump scales” through the application of satellite imagery to “map” ET. The paper by Bastiaanssen et al. describes how a full energy balance-based procedure is applied to large land areas to predict ET at relatively high resolution. The paper by Tasumi et al. describes application of similar satellite-based technology to investigate the field-to-field variation in ET from the same crop type. This work provides a new means to view the variation in ET within populations of fields and provides means to compare with and perhaps update traditionally used crop coefficient curves for specific areas.

The Journal hopes that the assemblage of these eight papers with a focus on ET provides a useful glimpse and review of the current state of the art in ET prediction for engineering applications. While the papers are far from being all-inclusive of current applications, they do provide a good representation of current thrusts in improving the applicability and accuracy of quantification of this very large and valuable component of irrigation hydrology—evapotranspiration.