Estimations of potential evapotranspiration as input to runoff calculations with the HBV model are usually given as monthly standard values calculated with the Penman method. Daily changes in the weather conditions can in later model versions be taken into account by the introduction of a temperature anomaly correction of the evapotranspiration. In this study daily values of potential evapotranspiration calculated with the Priestley-Taylor method were used as input to the model. The required net radiation estimations were calculated from routine weather observations including cloudiness. Potential evapotranspiration was calculated on a three hour basis over a 20-year period. Model simulations using different input data on the potential evapotranspiration were made for three drainage basins (3,500-4,300 km2) in Sweden. The Priestley-Taylor evapotranspiration generally gave small improvements of the runoff simulations. The simple temperature anomaly correction method gave improvements of the same size.

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