Modelling long-term transpiration measurements from grassland in southern England

Observations of latent heat, sensible heat and momentum flux from eddy correlation equipment are presented, along with approximately weekly measurements of soil moisture content. The measurements were made over a pasture field in the south of England for 3 years, covering two dry years, 1995 and 1996 and a more normal year, 1997. The data show that, overall, approximately 80% of the measured net radiation was used for evaporation. During July and August of the two dry years, soil moisture deficits of over 150 mm developed within the top 1400 mm, and at these deficits the evaporation was reduced. During the summer months, overall between 71 and 76% of the net radiation was used for evaporation. During the winter months, the latent heat flux exceeded the net incoming radiation. An energy closure better than 10% suggests that the measurements were well founded. The surrounding trees may, however, have influenced the momentum exchange; the estimated roughness length of 38 mm is considerably more than the commonly accepted 10% of vegetation height. This extensive dataset provides an excellent test for the current UK Meteorological Office Surface Energy Scheme (MOSES) when applied to this site, the results of which are presented here. In its unoptimised form, the model can explain 72% of the variance of the hourly evaporative flux data, and with excellent agreement with the soil moisture measurements. Optimisation can improve the model fit, but only up to a maximum of 76%. The largest remaining errors occur during mid-summer when high temperatures, vapour pressure deficits and soil moisture deficits occur.