CHANGES IN SOIL MOISTURE AND PORE PRESSURE AFTER HARVESTING A FORESTED HILLSLOPE IN NORTHERN CALIFORNIA

Abstract

ABSTRACT: In 1987, a 0.83-ha zero-order swale was instrumented with 58 pierometersand 25 tensiometers along several hillslope transects. Through 1993, soil moistureconditions were measured by pressure transducers connected to a digital data loggerrecording at 15-minute intervals.In August 1989, the 100-year-old second-growth forestin the swale was felled. Logs were removed by cable yarding and heavy logging equipmentwas excluded from the hillslopes. Increases in peak piezometric levels and soil moisturewere observed following logging.In the shallower, unsaturated portion of the soil profile,the increase was short-lived due to the rapid resprouting of redwood stumps. At thesoil-bedrock interface, increased pore pressures persisted during winter periods throughoutthe 4-yr post-harvest period.In addition to changes in evapotranspiration, pore pressureincreases may be explained by reduced canopy interception, compaction, or the collapse ofsoil pipes.At the base of the swale, pipeflow accounted for virtually all of the stormflow.After logging, soil pipes continued to efficiently route surplus stormflows through an existingpiping network and no slope instabilities were observed.KEY TERMS: hillslope hydrology, piezometer, soil moisture, timber harvest, soil piping.INTRODUCTIONThe impacts of timber harvesting on hydrologic processes are of great concern to landmanagers and the public in the Pacific Northwest. Many scientists have evaluated theeffects of logging on streamflow and sedimentation but much less work has been done onsubsurface flow processes.Slope stability problems arise on steep slopes where subsurfacedrainage is inefficient. The resultant mass erosion events are the major source ofsedimentation in this region.It is now known that logging roads can alter subsurface flowconditions and aggravate slope stability hazards (Megahan, 1972). It is not clear if timberfelling and skyline yarding alone can significantly affect the physical properties that governhillslope drainage processes.This study addresses this issue by comparing pre- and post-harvest pore pressure levels and soil moisture conditions on a steep hillslope within a zero-order basin in coastal northwestern California.