The water balance of Sweden Annual mean values (1931-60) of precipitation, evaporation and run-off

Type: Report
Series: RMK 18
Author: Eriksson, Bertil


The correction factors, which were proposed in an earlier report RMK 17 (1980) to be applied to measured precipitation amounts, have been used on the normal annual values for the period 1931-60. A map is presented, where corrected values from about 260 stations have been used for the analysis. To get the normal annual evaporation values the normal runoff values have been subtracted from the corrected precipitation annual sums. A map is drawn showing the pattern of the normal evaporation in Sweden. The map and the values show good agreement in those points, where reliable evaporation values are available. The conclusion is drawn that the corrections of the precipitation data have the correct order of magnitude. However, there are of course great uncertainties in the details of the map of the normal evaporation in southern Sweden in some areas values above 500 mm per year appear. In order to verify these results, another independent method must be used. It  seems appropriate to establish regression equations between evaporation and the summer mean temperature and maybe even other variables like wind velocity, cloudiness, vapour pressure deficit.

Area mean values of runoff, precipitation and evapotranspiration amounts have been calculated from the maps by integration. The mean annual precipitation amount for the whole of Sweden was found to be 745 mm. This value is 28% higher than the value computed from uncorrected data. 18% units are due to corrections for losses due to wind, evaporation and adhesion, when measuring the precipitation amounts. The rest, 10% units, is an effect of the fact that the precipitation stations are too few in higher regions. For the country as a whole is found that somewhat less than 50% of the precipitation evaporates as water vapour into the atrnosphere. In mountanious regions, where the largest precipitation amounts fall, only about 15% dissapear into the air.