Calibration equations for streamflow between the control (W-I) and the treatment (W-II) watersheds as an integral part of a paired experimental watershed study in Belgrad Forest are presented. Correlation coefficients of the calibration equations were found to be highly significant. The effect of 11% removal of the standing volume by regular selective cutting in old-growth oak-beech forest ecosystems was discussed. Streamflow hydrographs were developed for March, April, May, and June of 1986, and for the long-term period from 1986 to 1996. Increase in water yield immediately after the treatment was detected as the difference between measured and predicted values. Significant increases occurred in streamflow in March 1986 and in the long-term period (1986–1996).