Ground-water response to forest harvest: implications for hillslope stability

Abstract

Timber harvest may contribute to increased landsliding frequency through increased soil saturation or loss of soil strength as roots decay. This study assessed the effects of forest harvest on hillslope hydrology and linked hydrologic change before and after harvest with a simple model of hillslope stability. Observations of peak water table heights in 56 groundwater monitoring wells showed that soil saturation levels on hillslopes differed significantly with harvest intensity at one of the two study locations following 25, 75,and 100 percent harvest. Before the forest was cut 100 percent, the average rainfall needed for 50-percent saturation of the soil was 54 mm, but after clearcutting, soils reached an equivalent saturation with 61 percent less rainfall (21 mm). Hillslope stability model results indicate that shallow soils at both study locations, with slopes generally < 30 degrees (58 percent), were not steep enough to be affected significantly by observed increases in saturation. The stability model indicates that with 100-percent harvest, there is a 7-percent reduction in the factor of safety for slope gradients of 35 degrees (70 percent) with soil depths of 1.25 m. Forest managers may be aided by an understanding that variable hydrologic effects may result from similar forest harvests having different landscape position, land contributing area, and soil depths.

Citation

Johnson, A.C.; Edwards, R.T.; Erhardt, R. 2007. Ground-water response to forest harvest: implications for hillslope stability. Journal of the American Water Resources Association. 43(1): 134-147.