Current frameworks for reference ET and crop coefficient calculation

This article describes current and likely near-term future frameworks for calculating evapotranspiration. These include structures for estimating crop coefficients (K_c) primarily centered on the FAO-56 dual K_capproach, with example applications. Emphasis is placed on estimation of parameters and special cases to be considered. Newer, and often preferred, bases for establishing K_cbcurves include thermal units and vegetation indices. Also described and discussed are the application of reference ET calculations using hourly vs. 24-hour timesteps, the use of and conditioning of gridded weather data sets, and the likelihood of movement toward multi-layer and multi-source resistance models for ET estimation. Complementing this is satellite-based determination of ET using both vegetation indices and surface energy balance.