Energy Fluxes above Three Disparate Surfaces in a Temperate Mesoscale Coastal Catchment

This study is part of the long-term catchment-scale hydrological observatory, HOBE, situated in the Skjern River catchment covering 2500 km 2 on the western coast of Denmark. To gain a more detailed knowledge of how evapotranspiraƟ on is controlled by the local surface and atmospheric processes, eddy-covariance systems have been installed over an agricultural fi eld, over a spruce [Picea abies (L.) H. Karst.] plantaƟ on, and on wet grassland. Measurements started in fall 2008, and the fi rst annual series showed large diff erences in evaporaƟ ve response among the surfaces. The annual sum was about 500 mm for the wet grassland and spruce plantaƟ on, while it was about 300 mm for the irrigated agricultural site. In winter, the actual evapotranspiraƟ on rate of the grassland and the forest were much larger than the available energy evaluated from the radiaƟ on balance, while at the same Ɵ me large-scale sensible heat fl ux directed toward the ground was measured. At the agricultural site, the evapotranspiraƟ on rate was controlled by crop development with a leaf area index ≥3 being the threshold where actual evapotranspiraƟ on reached the potenƟ al rate. At the forest site, transpiraƟ on rates were severely limited due to stomatal control, which could be related to soil moisture and vapor pressure defi cits. The intercepƟ on evaporaƟ on was very important for total evapotranspiraƟ on. At the meadow, summer low fl ow in the river coincided with drying out of the meadows, which limited evapotranspiraƟ on. In late summer, evapotranspiraƟ on rates at the meadow and forest sites again increased signifi cantly compared with radiaƟ ve available energy, and again sensible heat fl ux directed toward the ground was observed.