Modelling actual, reference and equilibrium evaporation from a temperate wet grassland

Abstract

AbstractActual and reference evaporation from a wet grassland in Southeast England was studied over the spring and summer of 1999 (March to September) through changes in surface wetness. The Penman–Monteith (using resistance values for reference grass surface), equilibrium evaporation and Priestley–Taylor models were compared with output from the Bowen ratio energy balance (BREB) method. On field visits, inundation of the grazing marsh was mapped and surface soil moisture monitored in a regular grid using a capacitance probe. During the study period, the extent of flooding fell from approximately 10% to 0% and the surface soil moisture declined from over 38% to 15%. Daily averaged Bowen ratios displayed large variation but were below unity, indicating that latent energy flux was the dominant energy sink. The Penman–Monteith and equilibrium evaporation models underestimated during periods of surface inundation and overestimated when no surface water was present. Computed values of the Priestley–Taylor parameter α showed daily variability, but α was predictable with surface wetness such an average value of α = 1·25 characterized periods of inundation, and an average values of α = 0·80 represented periods of no surface water. The performance of the models in computing actual evaporation was compared with the BREB using the root‐mean‐square error and index of agreement. The optimal model was the Priestley–Taylor model. Copyright © 2004 John Wiley & Sons, Ltd.