Annual water balance of high elevation forest and clearcut sites

Forest cover influences the balance between input of water (precipitation) and output (evaporation and drainage) from the soil. Measurements of precipitation interception by forests and soil water content were used to determine the annual water balance of three pairs of high-elevation lodgepole pine forests and recent clearcuts in the southern interior of British Columbia, Canada from October 2002 to October 2005. Snow and rainfall interception were measured for the forests, soil water content was measured using time domain reflectometry and evaporation and drainage were calculated using a water balance model calibrated for the sites. Annually, interception of precipitation by a forest reduced the amount of water reaching the forest floor by 25%. Biweekly average evaporation rates (transpiration plus evaporated interception and soil evaporation) in the summer were 1.5 to 2.5 mm/d for the forests and clearcuts when the soil was moist. Soil drying during the summer reduced the clearcut evaporation rate to less than half that of the forest. Over the summer, evaporation of intercepted water and transpiration from a forested surface exceeded evaporation from a recent clearcut by about 30%. Removal of forest cover increased the amount of water available for surface and subsurface runoff to streams by up to 50%. A large part of this was due to the reduction in interception of precipitation.