Evaporation in the Upper Harz experimental basins is computed and compared to the values calculated according to formulae. Potential evaporation is determined for mean daily and monthly values by different methods. For the computation of the terms generally lacking in water balance studies, such as actual evaporation and changes in snow and soil storage, a model was established on the basis of daily values. As a result water balances can also be drawn up for shorter periods such as single years or months.