Direct measurement of evaporation from grassland at Plynlimon

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.