This dissertation is part of the long-term catchment-scale hydrological observatory, HOBE, situated in the Skjern River catchment covering 2500 km2 on the western coast of Denmark. To gain a more detailed knowledge of how evapotranspiration is controlled by the local surface and weather patterns, eddy-covariance systems was installed over the tree dominant surface types in the catchment; an agricultural field, a spruce [Picea abies (L.) H. Karst] plantation and a meadow site.
Measurements started in late 2008, and the full evaporation and energy balances for the years 2009-2011 forms the basis for this study. At the spruce plantation additional separate measurements of transpiration, interception evaporation and forest floor evaporation was performed. Transpiration was measured in the growing season of 2010 using Granier type TDP sap flux probes, interception was measured using net precipitation gauges for the years of 2010 and 2011 and forest floor evaporation was measured on a campaign basis by weighing cut out sections for forest floor.
The cumulative measured evapotranspiration from the three surfaces showed large differences. 2009 was an unusually dry year with much lower than normal rates of precipitation in April, May and June, while 2010 and 2011 where normal years in terms of precipitation. The meadow site and agricultural site had the largest evapotranspiration in dry year of 2009 at 512 and 470 mm respectively, while the normal years 2010 and 2011 saw 446 and 455 mm for the meadow and 406 and 400 mm for the agricultural site. The spruce plantation showed the opposite pattern. In 2009 the ET was 494 mm while in 2010 and 2011 the sum was 545 and 544 mm respectively. In all years the agricultural site had less evapotranspiration than the natural surfaces. In the dry year 2009 the meadow had slightly more evapotranspiration than spruce plantation, while in the normal years 2010 and 2011 the forest had considerably more evapotranspiration than either the Meadows or the Farm.
At the agricultural site, transpiration was the most important component of the evapotranspiration. The rate of evapotranspiration was controlled by crop development and by the available energy. At the meadow site soil evaporation and evaporation from free water surfaces was the most important parts of the evapotranspiration. The rate of evapotranspiration was controlled by the water level in the Skjern River which influenced the ground water level in the meadows and by the available energy. At the spruce plantation transpiration and terception evaporation were both important.
The rate of transpiration was heavily influenced by stomatal control in response to high vapor pressure deficits. In addition soil moisture stress had a limiting effect during prolonged dry periods. Interception evaporation was controlled by the amount and duration of precipitation. During rain, advection was found to comprise about half the energy balance of the spruce plantation in summer
and the majority the energy balance in winter. On average, 19% of the precipitation evaporated during rain in summer, and 12% of the precipitation evaporated during rain in winter. Canopy structure in the forest were found to affect both transpiration and interception evaporation. Young stands with an open canopy structure transpired at about 30% higher rate than mature stands with a close canopy structure; the young stands had an interception evaporation of 31% of the gross precipitation, while the mature stands had an interception evaporation of 34% of the gross precipitation. Within the mature stands there was an edge effect with trees next to access roads and aisles being responsible for a disproportionally large part of the stand transpiration.