A simulation model for predicting hourly pan evaporation from meteorological data

The objective of this study was to develop and validate a simulation model of the evaporation rate of a Class A evaporimeter pan (E-pan). A multilayer model was first developed, based on the discretization of the pan water volume into several layers. The energy balance equations established at the water surface and within the successive in-depth layers were solved using an iterative numerical scheme. The wind function at the pan surface was identified from previous experiments, and the convective processes within the tank were accounted for by introducing an internal 'mixing' function which depends on the wind velocity. The model was calibrated and validated using hourly averaged measurements of the evaporation rate and water temperature, collected in a Class A pan located near Cartagena (Southeast Spain). The simulated outputs of both water temperature and Epan proved to be realistic when compared to the observed values. Experimental data evidenced that the convective mixing process within the water volume induced a rapid homogenization of the temperature field within the whole water body. This result led us to propose a simplified version of the multilayer model, assuming an isothermal behavior of the pan. The outputs of the single layer model are similar to those supplied by the multilayer model although slightly less accurate. Due to its good predictive performances, facility of use and implementation, the simplified model may be proposed for applied purposes, such as routine prediction of Class A pan evaporation, while the multilayer model appears to be more appropriate for research purposes. (c) 2005 Elsevier Ltd All rights reserved.