Evapotranspiration 1961–1990 in Finland as function of meteorological and land-type factors

Basin-wise values of mean annual total evapotranspiration for the period 1961–1990 were obtained from the water balance equation and examined using following factors: the sum of effective temperature (over the land area), the relative area of open peatlands, the amount of growing forest stock (m3 ha–1), the proportion of dense stands (i.e. the reduction in forests where evapotranspiration increases with the volume of growing stock less than usual, due to the lack of water, reduced vitality of trees and reduced evapotranspiration from the forest-floor vegetation), and lake evaporation, taken from preliminary work. The values of the variables were first obtained in 10 km x 10 km grid-squares, and then averaged over the basin areas. 96% of the total variance of the water balance evapotranspiration was explained. The mean standard error is 14.6 mm, and the maximum 30.7 mm. Evapotranspiration was determined separately for fields, open peatlands, lakes and forests. As compared with that of fields, evapotranspiration from open peatlands was 305 mm higher, and from lakes 170–300 mm higher, with an inreasing difference southwards. In Lapland evapotranspiration from forests was 35 mm higher than over fields, and in other regions 50 mm higher. The change in evapotranspiration from 1961–1975 to 1976–1990 was also studied.