The measurement of evaporation by meteorological methods

Abstract

Three methods based on energy budget and turbulent transfer formulations may be recommended for the determination of evaporation E from catchment areas. They all involve measurement of available energy R-G, R being net radiative flux and G the heat flux into the surface. When estimates from forests on an hourly to daily basis are made, the contribution from canopy heat storage should be included in G. The energy budget/Bowen ratio method requires no detailed specification of surface properties but involves relatively complex instrumentation to measure vertical differences of temperature and humidity in the air. With the exclusion of eddy covariance, it is potentially the most accurate method and should be applicable for crops and forests, over a wide range of evaporation conditions. The residual energy budget method requires accurate estimates of atmospheric resistance and measurement of surface temperature. Where the latter is available (by airborne radiometer) the method is most reliable for surfaces of low roughness or in conditions of high evaporation. The combination method incorporates both energy budget and bulk relationships to eliminate surface temperature. It introduces surface resistance, which must be parameterized in terms of soil, plant species and atmospheric quantities, with allowance made for sub-canopy fluxes and evporation of intercepted rainfall. The method is suitable for estimating daily or monthly evaporation using standard atmospheric observations. The eddy covariance method is not yet sufficiently developed to measure E reliably at the operational level over long periods of time and over less-than uniform terrain but, of all the methods to be described, it has the greatest potential to do so. Methods based on eddy covariance, wind and other profiles are more appropriate to research investigations and, with the above, are described in the text.