The work analyzes the costs of water treatment in four selected water reservoirs. Today, in addition to the production functions of the forest, emphasis is also placed on non-production functions. In order to be able to assign a certain significance to the non-productive function of a forest, we need to know the quantified value of this function. We have chosen the alternative cost method as the most suitable method for quantifying the water protection function of the forest. Using the regression function, we quantified the relationship between the forest cover of water reservoirs and the cost of water treatment. Based on the quantification, we determined the value of the water protection function of the forest for each catchment area of water reservoirs. The results confirmed the dependence between forest cover and costs. In the catchment area with the highest forest cover we also got the highest value of the water protection function of the forest and with decreasing forest cover the value of the water protection function decreased.