A model giving daily actual evapotranspiration values from synoptic data has been developed using results from the forest meteorological measuring site Velen in southern Sweden. The following submodels are included:
- Model for evaporation of rain water intercepted by the forest canopy. Parameters are the free throughfall coefficient and water storage capacity of the canopy. The model was developed using rainfall data from 29 troughs sited below and aside the canopy.
- Model for transpiration-from the dry forest canopy. A physiological expression for the canopy surface resistance has been developed using 52 daily transpiration values obtained over the canopy by the Bowen ratio-energy balance method.
Using data from seven synoptic stations during seven warm seasons (April - October) monthly model evapotranspiration sums were compared to monthly data estimated from the hydrological water balance. The agreement was much better than using the traditional Penman formula with synoptic data. Thus, modelling of the special forest processes gives significant improvement of the monthly values. The model can now be run for about 190 synoptic stations in Sweden and comparisons can be made with evapotranspiration measurements made in other Darts of the country.