Estimation of short-term actual crop évapotranspiration

A method is presented for estimating the hourly actual évapotranspiration (ET) from short natural vegetation or agricultural crops. The method consists of equating the ET flux equations based on the generalized Penman-Monteith (GPM) combination method and a humidity gradient (HG) method. By equating the GPM and HG expressions, a single unknown parameter, either the bulk surface resistance (rs) or aerodynamic resistance (ra), can be determined. In the procedure, the value of the resistance factor is adjusted until the daily ET time series curves from the two methods approximately coincide. An overview of the technical approach and the results of a comparison between the new method and an eddy covariance system at the University of Florida at Gainesville are provided. To illustrate the utility of the method an example is presented in which the average daily ET was determined for a growing season of common bean {Phaseolus vulgaris L.) at Juana Diaz, Puerto Rico. In this example the surface resistance was measured (i.e., stomatal resistance and leaf area) and estimated using the proposed method. A third method was also evaluated in which the surface resistance was estimated using the equation of Ortega-Farias and Fuentes (1999). All three methods were in close agreement.