Evapotranspiration: progress in measurement and modeling in agriculture

This article provides a focused survey of progress in crop evapotranspiration (ET) measurement and modeling, with particular emphasis on the aspects of interest to the irrigation profession. The significant advances in understanding and quantifying crop ET during the past few decades are largely due to our increased ability to measure near-surface climate variables and surface energy and momentum exchanges, complemented by progress in soil and plant sensor technology. However, ET measurement is not commonly practiced, and modeling is mostly preferred. Much theoretical progress in ET modeling originated with the 1948 work of Penman and the subsequent modification to the Penman-Monteith (P-M) equation and to multi-layer and sparse canopy models. These advances strengthened confidence in using the combination equation and encouraged a significant step forward through the adaptation of the P-M equation to provide a standard estimate of reference crop ET for use in the long-established, two-step, crop coefficient (Kc) methodology. Recently, there has been a continued progress in this field via the one-step application of the P-M equation to estimate crop ET directly using effective stomatal resistance rather than Kc. This article concludes by drawing attention to a general need to improve crop water productivity by reducing non-beneficial soil evaporation and, in this context, the potential value of using improved methods and models to partition ET and to aid scheduling limited irrigation.