Direct measurement of evaporation from grassland at Plynlimon D. D. McNeil Institute of Hydrology, Wallingford, Oxon., U.K.
Abstract. Over the last decade there has been some uncertainty
over the calculation of "actual" catchment evaporation from the long-term difference between rainfall and runoff
for the grassland Wye catchment at Plynlimon, compared to estimates made using the Penman
formula on data from automatic weather stations. Hence, measurements of actual evaporation
were made over a two month period in the late summer of 1992 using the eddy-correlation
technique on a relatively flat but wetter than average site in this upland region. Although the site
was rather restricted for this type of measurement, the quality of data proved typical of other eddy-correlation
measurements made at more aerodynamically suitable sites. The ratio of actual evaporation
to the Penman estimate on dry days in summer was 0.83. This compares with an average
annual ratio, generated from the catchment data for the period 1978 to 1995, of 1.01. The catchment
rainfall value has been improved recently by weighting each gauge using altitude domains.
Citation: McNeil, D. D.: Direct measurement of evaporation from grassland at Plynlimon, Hydrol. Earth Syst. Sci., 1, 447-452, doi:10.5194/hess-1-447-1997, 1997.