Hydrologic effects of logging in western Washington, United States

Abstract

Abstract. Possible changes in streamflow associated with logging were analyzed for 23 western Washington catchments with drainage areas from 14 to 1600 km 2. Statistically significant trends in annual streamflow minima, uncorrected for climatic influences, are all decreasing and are apparently dominated by a regional climate signal associated with the Pacific Decadal Oscillation, rather than land cover change. Using paired catchment analysis, the number of statistically significant trends detected for the peak flow series is largely within the range of statistical noise. Only in the case of the annual minima were more trends detected than could be attributed to chance, owing in part to the lower relative variability, hence greater detectability of trends in low flows. Investigation of the effect of return period on peak flow changes shows an apparent increase in flood peaks for treatment relative to control catchments, the mean magnitude of which decreases with increasing return interval up to about the 10-year return period. In large part, owing to the small number of catchment pairs available, this analysis cannot be considered conclusive. An alternative approach to evaluating trends in peak flows based on time series residuals of observed flows from hydrology model predictions detected increasing trends in peak flow series, which were largely absent in the paired catchment analysis. This is attributed both to the ability of the model, which acts as' the control, to filter out natural variability and to a larger trend "signal" in the residuals analysis resulting from the ability of the method to fix the vegetation condition in the model control. IntroductionIn the mountainous areas of the northwestern United States, what amounts to a large-scale land use experiment has taken place in the last half-century. The "experiment" has been to remove much of the old growth, primarily coniferous, forest by clear-cut logging and to replace it with younger stands of more uniform age and reduced species diversity. There is a perception that this land cover change has resulted in increased frequency and severity of flooding in the watersheds of the western Cascades, especially during rain-on-snow events, which occur when warm fronts follow periods of cool, wet weather, usually in late fall and earlywinter.

Land-cover impacts on streamflow: a change-detection modelling approach that incorporates parameter uncertainty

“…For larger catchments, precipitation inputs and catchment conditions may vary greatly in time and space. Bowling et al (2000) noted this issue in a paired catchment study of larger catchments where about 25% of the precipitation events occurred only in one of the paired catchments. For larger snow-dominated catchments this limitation might be less severe (Troendle et al, 2001;Moore & Scott, 2005).…”

Grays River Watershed and Biological Assessment, 2006 Final Report.

“…The model has been used also to study the interactions between climate and hydrology (Wigmosta et al 1995;Arola and Lettenmaier 1996;Nijssen et al 1997) and the potential impacts of climate change on water resources (Leung et al 1996;Leung and Wigmosta 1999;Wigmosta and Leung 2001). Furthermore, the model has proven to be an important tool Storck et al 1998Bowling Storck 2000;Bowling and Lettenmaier 2001;Wigmosta and Perkins 2001;Vanshaar and Lettenmaier 2001;Vanshaar et al 2002;Waichler et al 2005). Waichler et al (2005) conducted a detailed test of the DHSVM in small watersheds in the H. J. Andrews Experimental Forest, Oregon, examining treatment effects on streamflow.…”

“…Based on these hydrologic alterations, there is a general perception that the land-cover changes resulting from logging have resulted in an increased frequency and severity ing in watershe fected by timber harvest land use. However, an evaluation of this body o h indicates that this perception is not always evident from hydrologic data or modeling scenarios (Bowling et al 2000). There is a demonstrated, consistent increase in watershed water-yield in harvested basins, due largely to a decrease in evapotranspiration in clear-cut areas (Bowling et al 2000).…”

“…However, an evaluation of this body o h indicates that this perception is not always evident from hydrologic data or modeling scenarios (Bowling et al 2000). There is a demonstrated, consistent increase in watershed water-yield in harvested basins, due largely to a decrease in evapotranspiration in clear-cut areas (Bowling et al 2000).…”

“…The result is that there is little, if any, surface runoff in naturally forested watersheds, with most recharge to surface waters occurring slowly via interflow or groundwater pathways. Hillslope processes in the Pacific Northwest tend to be dominated 3.39 T ave the potential to alter the hydrologic regime of forested watersheds in the Pacific Northwest (Harr et al 1975(Harr et al , 1982Harr and McCoriso arche and Lettenmaier 1998;Thomas and Megahan 1998; Beschta e 00; Bowling et al 2000;Bowling and Lettenmaier 2001), as well as influence the ecology of the nati ests (Franklin et al 2002). Based on these hydrologic alterations, there is a general perception that the land-cover changes resulting from logging have resulted in an increased frequency and severity ing in watershe fected by timber harvest land use.…”