HYDROLOGICAL RESPONSE TO REFORESTATION IN THE GANARASKA RIVER BASIN, SOUTHERN ONTARIO

Headwater slopes of the Ganaraska River basin in southern Ontario were reforested after World War II in an effort to reduce soil erosion and downstream flooding. Hydrological responses to this land use change are examined. Between 1945 and 1990 forest cover in Ganaraska Forest increased from 3.3 to 8.9% of the basin area. Over this period annual runoff decreased by 100 mm, and runoff ratios were reduced. These hydrological changes exceed those predicted from empirical relationships between runoff and forest cover, and likely reflect a combination of increased interception and evapotranspiration, water storage in headwater aquifers, and deep groundwater losses. Reforestation was also associated with significant decreases in high flows and increases in annual minimum daily flows. Annual maximum daily mean specific discharges decreased from 0.36 to 0.11 m(3) s(-1) km(-2) (+/-0.0009 m(3) s(-1) km(-2)) between 1946 and 1962, while annual maximum daily mean specific discharges decreased from 0.0037 to 0.0062 m(3) s(-1) km(-2) (+/-0.10 m(3) s(-1) km(-2)) between 1957 and 1990, as estimated from best-fit regression relationships. The response of runoff ratios and peak flows to increased forest cover was more rapid than that of minimum daily flows. Results suggest that hydrological responses to relatively minor modifications of forest cover can be detected, depending on location of land use change within the basin.