Fitting measured evapotranspiration data to the FA056 dual crop coefficient method

The FAO-56 publication of the UN Food and Agriculture Organization contains guidelines on constructing and applying a 'dual crop coefficient' method to characterize the behavior of evapotranspiration (ET) on a day to day basis. The dual crop coefficient (K_c) method substantially improves the ability to fit simulated with measured data, as compared to the 'single' K_c method, by partitioning evaporation from soil (Es) from transpiration from vegetation. This permits the separate estimation of Es when there are known wetting events from precipitation and irrigation and assists in explaining behavior of measured data. The application of the dual K_c method is relatively straight forward, especially when applied using the straight-line segment method for the basal K_c curve, K_cb. Illustrations are given on fitting the dual K_c method and K_cb curve to daily ET data for irrigated and rainfed corn crops near Mead, Nebraska measured by eddy covariance and sensitivity to various soil and root zone parameters. Assessment of transferring K_cb curve parameters to other fields and years indicates that soil and root zone parameters are relatively transferrable with little modification, whereas lengths of the four crop growth stages do vary from year to year due to differences in cultivar type and possibly differences in weather.