Hydrologic processes and peak discharge response to forest removal, regrowth, and roads in 10 small experimental basins, western Cascades, Oregon

The magnitude, seasonality, and duration of peak discharge responses to forest removal and regrowth and roads in 10 pairs of experimental basins in the western Cascade Range of Oregon are consistent with fundamental water balance and routing concepts in hydrology. Hypothesized effects of forestry treatments on evapotranspiration, cloud water interception, snowpack dynamics, and subsurface flow interception vary predictably by season, geographic setting, amount of forest canopy removal, stage of canopy regrowth, and arrangement of roads in the basin. Posttreatment responses of selected subpopulations of matched peak discharge events were examined over 10- to 34-year posttreatment periods in treated-control basin pairs in a range of geographic settings. Changes in evapotranspiration associated with forest canopy removal and regrowth apparently accounted for significant increases (31-116%) in peak discharges during the first postharvest decade in 8 of 10 treated basins, but the events that were affected were small (<0.22- or 0.28-year return periods) and occurred in the fail (September-November), when soils are in moisture deficit, rather than in spring (March-May), when soils are in moisture surplus. For a given amount of forest canopy removal, initial increases in small, fall events were greater in drier basins than wetter basins, and increases tended to disappear as forest canopies regrew. Changes in cloud water interception apparently offset changes in evapotranspiration in two partially cut basins. Changes in snowpack dynamics apparently accounted for significant increases (25-31%) in winter rain-on-snow events, but other types of winter events did not change, in four of five basins at the H. J. Andrews Experimental Forest. Changes in subsurface flow interception apparently accounted for significant increases (13-36%) in large (>1-year return period) events in seven of eight basins with roads, and, controlling for geographic location, the magnitude of increases was related to the density of midslope roads.