Estimating evaporation and surface resistance from a wet grassland

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This paper describes the investigation into evaporation loss at a wet grassland in south-east England. The investigation has two phases. The first attempts to derive values of surface resistance, rs, over a period of soil moisture decline which, when used in the Penman Monteith equation, would enable the computation of actual evaporation loss. The second investigates why the computed values of actual evaporation exceed potential ones. Values of potential evaporation, Eo, were computed from Penman Monteith by setting rs = zero (reference conditions of a wet canopy), while values of actual evaporation, 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 resistance 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 increasing to a value of 155 sm-1 at a soil moisture content of 19%. There were three time steps from the total eight, in which actual evaporation loss computed from the soil moisture balance approach was higher than the potential rate which was unexpected. It is discussed that these results could either be due to problems of time lag or spatial heterogeneity which could introduce errors into the soil moisture balance approach, or that the potential rate of evaporative loss from the wet grassland is being underestimated by the Penman Monteith equation. © 2000 Elsevier Science Ltd. All rights reserved.

Bibliographical metadata

Original languageEnglish
Pages (from-to)599-603
Number of pages4
JournalPhysics and Chemistry of the Earth Part B: Hydrology, Oceans & Atmosphere
Issue number7-8
Publication statusPublished - 2000