This work was initiated in order to improve the evapotranspiration data used in the HBV model. Evapotranspiration is calculated consecutively by the Penman-Monteith equation using three-hourly SYNOP observations transformed to values of net radiation, water vapour deficit and data necessary for evaluating aerodynamical resistance and surf ace resistance. Transpiration, rainfall interception and a simple treatment of winter evaporation are included. Soil moisture is used for calculating the surface resistance and it is updated three-hourly with the soil moisture accounting routine of the HBV model regarding the contributions from rainfall and snow melt. Then soil moisture is reduced due to total evapotranspiration.

Two main parts have been developed and are described here:1. A program for interpolation of missing SYNOP observations and2. The evapotranspiration model.Evapotranspiration is calculated for six SYNOP stations used in the HBV model. Using literature parameter values for open land and forest, the calculated transpiration, interception evaporation and snow evaporation are found to assign reasonable values. Only limited tests against measured evapotranspiration have been made, such as some comparisons with winter data from the NOPEX main site in Norunda north of Uppsala. A comparison is made with evapotranspiration data obtained from calibrations of the HBV model. The performance of the evaporation values in the HBV model remains to be tested.