Estimating evaporation and surface resistance from a wet grassland

This paper describes the investigation into evaporation loss at a wet grass land in south-east England. The investigation has two phases. The first att empts to derive values of surface resistance, rs, over a period of soil moi sture decline which, when used in the Penman Monteith equation, would enabl e the computation of actual evaporation loss. The second investigates why t he computed values of actual evaporation exceed potential ones. Values of p otential evaporation, Eo, were computed from Penman Monteith by setting rs = zero (reference conditions of a wet canopy), while values of actual evapo ration, E, were derived using a soil moisture balance approach. With these values of potential and actual evaporation, the Penman Monteith formula was rearranged to find rs, following the approach by Russell (1980). The resis tance values range from 8 sm(-1) to 155 sm(-1) which are in the same order of magnitude observed by other researchers and follow the expected trend of increase with soil moisture stress. Of the soil moisture range 28 to 19%, values of resistance are below 60 sm(-1) between 28 to 22%, thereafter incr easing to a value of 155 sm(-1) at a soil moisture content of 19%. There we re three time steps from the total eight, in which actual evaporation loss computed from the soil moisture balance approach was higher than the potent ial rate which was unexpected. It is discussed that these results could eit her be due to problems of time lag or spatial heterogeneity which could int roduce errors into the soil moisture balance approach, or that the potentia l rate of evaporative loss from the wet grassland is being underestimated b y the Penman Monteith equation. (C) 2000 Elsevier Science Ltd. All rights r eserved.