Hydrologic effects of logging in western Washington, United States

Possible changes in streamflow associated with logging were analyzed for 23 western Washington catchments with drainage areas from 14 to 1600 km. Stat istically significant trends in annual streamflow minima, uncorrected for c limatic influences, are all decreasing and are apparently dominated by a re gional climate signal associated with the Pacific Decadal Oscillation, rath er than land cover change. Using paired catchment analysis, the number of s tatistically significant trends detected for the peak flow series is largel y within the range of statistical noise. Only in the case of the annual min ima were more trends detected than could be attributed to chance, owing in part to the lower relative variability, hence greater detectability of tren ds 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 mode l predictions detected increasing trends in peak flow series, which were la rgely absent in the paired catchment analysis. This is attributed both to t he ability of the model, which acts as the control, to filter out natural v ariability and to a larger trend "signal" in the residuals analysis resulti ng from the ability of the method to fix the vegetation condition in the mo del control.