Infiltration capacities and surface erodibility associated with forest harvesting activities in the Oregon Cascades

An infiltration capacity and surface erodibility study was con-ducted six years after forest harvesting in the Oregon Cascades. Aportable rainfall simulator was utilized to obtain field measurementson the Coyote Creek and Hi- 15 Watersheds during summer and fall,1977.
Seasonal variations were found to occur in infiltrationcapacities and surface erodibility. Infiltration capacities increasedby 1.4 times from summer to fall, while surface erodibilitycharacteristics, suspended sediment concentration and sedimentyield, decreased from summer to fall. Surface limiting conditionsduring the summer and soil profile controlled conditions in the fallwere hypothesized to explain this seasonal variation.
Nearly all timber harvesting treatments for each study areahad statistically equal summer infiltration capacities in comparisonwith adjacent unlogged areas. In addition, summer surface
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erodibility characteristics on treated areas were typically lessthan those found on undisturbed areas. Only certain skid trails,cable log paths and severely disturbed sites such as tractor win-drowed and burned areas had substantially reduced infiltrationcapacities and increased surface erodibility. However, all areas,including the most severely disturbed, had fall infiltration capacitiesthat exceeded usual and maximum fall precipitation intensities.
Many skid trails and other highly disturbed and compactedareas at Coyote Creek appeared to have greatly recovered sincelogging six years ago. Freezing/thawing, biological activity, andshrinking and swelling of soils may account for this recovery ininfiltration capacities, surface erodibility and soil properties.
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Skid trails and severely disturbed areas may partially account forpeak flow increases and minor sedimentation the first few yearsafter logging. However, data from this study collected six yearsfollowing timber harvesting do not support the premise that con-tinued increases in peak flows are caused by changes in infiltrationcapacities, except perhaps for a tractor windrowed and burned area.
Predictive models for infiltration capacity (normally (Hs-
i
tributed) and surface erodibility characteristics (requiring normal-izing transformations) were not found using regression techniquesbecause of large amounts of variance. Variation in estimates of
infiltration capacities and surface erodibility for individual plots andbetween study areas was identified.