Hydrologic effects of clear-cutting and wildfire on steep granitic slopes in Idaho

Many of the environmental impacts of logging and wildfire are caused by changes in the hydrologic response of slopes after disturbances. This study was conducted to evaluate changes in inflow, storage, and outflow for 3-yr periods before and after clear-cut logging and wildfire on two steep, granitic microwatersheds in Idaho. Clear-cutting alone and clear-cutting plus wildfire increased annual peak snow water equivalent and snowmelt rates an average of 41 and 30%, respectively. The greater volume and rate of snowmelt caused respective increases in the peak piezometric rise and in total piezometric storage, amounting to 47 and 27%. Accordingly, the total volume of subsurface flow intercepted by the roadcut was increased 96% and was accompanied by 27% greater peak flow rates. None of the above responses were detectable on an adjacent watershed that was burned by wildfire alone. Evapotranspiration was reduced on both watersheds after clear-cutting or wildfire, as indicated by increases in the unsaturated soil water content at the end of the growing season, amounting to 44 and 72%, respectively. Accelerated mass erosion on clear-cut slopes and accelerated surface and mass erosion on roads and in channels below roads can result from such changes.