Sediment Losses associated with degree of watershed disturbance resulting from clearcut harvesting with Best Management Practices

Abstract

Forestry best management practices (BMPs) are effective in reducing water quality and quantity impacts from silvicultural activities and have been shown to be a cost effective means for controlling nonpoint source pollution in complex and variable forested watershed conditions. Nine small (~2.5 ha) watersheds in East Texas were previously instrumented in 1980 to evaluate clearcut harvesting and intensive site preparation on sediment losses, at the time that BMPs had not been adopted in Texas. These same nine watersheds were reinstrumented in 1999 to examine the silvicultural effects on water quality of contemporary silviculture with applications of BMPs. Four large (70-135 ha) watersheds were also instrumented in 1999, with small watersheds nested in them to contrast water quality and site disturbance impacts on stand-sized watersheds. Site disturbance parameters such as degree of surface soil disturbance (% bare soil, % dominant vegetation type, % litter, and % logging slash) and degree of soil compaction (bulk density) were measured on all watersheds in both study periods to determine their relationship to watershed sediment losses. In addition, soil infiltration rates were measured on the nine small watersheds pre- and post-treatment during the 1980 pre-BMP study. First-year post-treatment sediment losses generally decreased with decreasing intensity of site disturbance, from 2,937 kg/ha with the most intensive treatment without BMPs to 225 kg/ha in the most intensive treatment with BMPs. Soils were not compacted sufficiently by harvest to decrease infiltration rates or reduce site productivity. BMPs are designed to reduce site disturbance particularly along sensitive riparian areas. In the current study, streamside management zones (SMZs) were shown to be effective in mitigating harvest disturbance in riparian areas and thus reducing sediment losses. Sediment losses from silvicultural activities were associated with extent of BMP implementation and degree of watershed disturbance.