No abstract

No abstract

The effects of clearcut silviculture (road building, clearfelling, cable logging, and site preparation) were evaluated using long-term peakfow records for three small watersheds (60-101 ha) and six large basins (62-640 km 2) in the western Cascades of Oregon, USA. After a calibration period, two of the small watersheds were treated while the third remained untreated (control). Analysis indicated that peakfow increases following treatments were dependent upon peakfow magnitude. Peakfow increases averaged approximately 13-16% after treatment for 1-yr recurrence interval events, and 6-9% for 5-yr recurrence interval events. For the six large basins, multiple linear regression analyses of peakfows relative to: (1) peakfow magnitude; and (2) difference in percent area harvested provided mixed results. While signifcant (p < 0.05) relationships were found in half of the analyses, the explained variance (�r 2) due to harvesting was generally small (1-7%).

We investigated the potential effects of rainfall intensity smoothing by forest canopies on slope stability by modelling soil responses to measured rainfall and throughfall during high-intensity rain. Field measurements showed that maximum intensities of precipitation were generally reduced under forest canopies at two sites in the Pacific Northwest, USA. Modelling soil water porepressure responses of a hypothetical hillslope to the field data resulted in estimates of slope stability that were generally greater under forest canopy than for the same hillslope without forest canopy. Results indicate that smoothing of precipitation intensities may translate into overall greater stability of hillslopes under forest canopies.

[1] The paired catchment approach has been the predominant method for detecting the effects of disturbance on catchment-scale hydrology. Notwithstanding, the utility of this approach is limited by regression model sample size, variability between paired catchments, type II error, and the inability of locating a long-term suitable control. An increasingly common practice is to use rainfall-runoff models to discern the effect of disturbance on hydrology, but few hydrologic model studies (1) consider problems associated with model identification, (2) use formal statistical methods to evaluate the significance of hydrologic change relative to variations in rainfall and streamflow, and (3) apply change detection models to undisturbed catchments to test the approach. We present an alternative method to the paired catchment approach and improve on stand-alone hydrologic modeling to discern the effects of forest harvesting at the catchment scale. Our method combines rainfallrunoff modeling to account for natural fluctuations in daily streamflow, uncertainty analyses using the generalized likelihood uncertainty estimation method to identify and separate hydrologic model uncertainty from unexplained variation, and GLS regression change detection models to provide a formal experimental framework for detecting changes in daily streamflow relative to variations in daily hydrologic and climatic data. We include statistical analyses of climate variation and a two-part evaluation to explore model performance and account for unexplained variation. Evaluations consisted of applying our method to a control catchment and to a period prior to harvesting in a treated catchment to demonstrate that our method was capable of capturing the absence of land use change in an undisturbed catchment and capturing the absence of land use change during a period of no disturbance in the harvested catchment. In addition, we explore the sensitivity of our method to model identification, number of simulations, and likelihood thresholds for model identification. We show that an increase in the number of model simulations does not necessarily result in increased change detection performance. Our method is a potentially useful alternative to the paired catchment approach where reference catchments are not possible and to stand-alone hydrologic modeling for detecting the effects of land use change on hydrology.