| Summary: | The evapotranspiration, one of the main parts of the waterbalance, is not well known in theSwedish mountains. The evapotranspiration is difficult to measure reliably and measurementsare only rarely done. To predict the available water quantity for hydro power generation or forthe risks for darnaging high flows, it is important to know the water balance in the mountains.Since many of the driving variables of the evapotranspiration are dependent on the altitude, itwas investigated in this work if the variation of evaporation in the mountains could beapproximated in simple functional form. The aim of this work was to make clear if theevaporation in the Swedish mountains has a dependence on the altitude and to investigate ifsuch a possible dependence could be used to improve the HBV-111odel. This was done bycalculating the evapotranspiration as a residual in the water balance equation for a number ofhigh-altitude drainage basins in the river Ljusnan, the river Indalsälven and the river Luleälven.The calculations were done with longtime mean values from the years 1961-90. The altitudedependence was calculated by linear regression. The result was inserted in the HBV-model as acorrection factor in the calculation of the evapotranspiration. It was also tested if anyimprovements in the model could be achieved by introducing an altitude dependence of thevariables in the Priestley-Taylor formula. In this case, evapotranspiration was calculated with adaily resolution. Simulations with the HBV-model were performed for the drainage basinsLjusnedal, Torrön and Kultsjön. Evaporation calculated as a water balance residual, decreased24 mm/100 m and year for the river Ljusnan, 54 mm/100 m and year for the river Indalsälvenand 48 mm/ 100 m and year for the river Luleälven. The result of the calculation in the riverLuleälven was condensation in a number of areas. This was probably caused by too fewraingauges in the area. The explained variance <luring simulations with the HBV-modelincreased in the area of Kultsjön if a dependence ...
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