Short-term hydrological responses to silvicultural treatments withina stream buffer zone: a case study

Abstract

The thinning of stream buffer zones (SBZs) is gaining popularity as a silvicultural management practice in order to reduce the risk of wildfire and insect hazard, provide economic return, and improve the effectiveness of SBZs. In this study, streamflow over a 1-year period was monitored at 2 small paired watersheds (treated vs. reference). The short-term impacts of a partial cutting within a SBZ as well as the relative effects of pastoral, forested, and clearcut areas on changes in downstream hydrology were examined. Upstream pastoral areas had a higher water yield than downstream forested sections during the no-harvest (calibration) period of 6 months. The partial cut (about 50% of the basal area) within the SBZ changed the hydrologic pattern by remarkably increasing the water yield on the treated downstream sections during the 6-month-long postharvest period. The harvest operation also caused an increase in direct runoff at 2 downstream sections, WT2 and WT3 (~200% and ~100%, respectively). No significant changes were observed in the water yield pattern of the control watershed. Because harvested areas within the SBZs constitute a fraction of the monitored sub-watersheds and only partial harvesting (~50% of the basal area) was implemented, the observed increase in flow at the treated downstream sections (100% and 250%, respectively) is unprecedented. The partial harvesting within the SBZs also resulted in a significantly flashier hydrological system. Because silvicultural treatments are part of regular, repeated management operations, these short term (in this study, 6-month-long) but substantial changes in water yield, direct runoff, and flashiness could have important implications for water quality, water resources, and downstream biota.