The water regime variability in most catchments is frequently influenced not only by the changes of the vegetation cover in the annual cycle but also by its development in the time span of decades. That means that the resulting evapotranspiration depends not only on the actual climatic situation but also on the soil moisture. The simulations of the rainfall-runoff process have been used with the intention to follow the possible role of the developing land cover. The differences between the observed and simulated flows in relatively long periods can be considered as an appropriate tool for the assessment of the water regime changes, in which the evapotranspiration demand variability is a significant phenomenon.