Runoff and sediment production from harvested hillslopes and the riparian area during high intensity rainfall events

Abstract

Forest harvesting often decreases soil infiltration capacity, leading to rapid and increased delivery of surface runoff, shallow subsurface runoff, and sediment to streams. While the general harvest area is typically the largest area of disturbance, relative to forest roads or skid trails, less is known about the degree to which the general harvest areas act as sources or sinks for runoff and sediment transport. This includes a need to improve understanding of the potential for runoff and sediment delivery from harvest areas through riparian buffers to streams during infrequent, high intensity precipitation events, which are predicted to increase due to climate change. In this study, we used rainfall simulations to investigate surface/shallow subsurface runoff, and sediment transport from plots during extreme precipitation events within a steep, headwater catchment in the Rocky Mountains. Simulations consisted of one hour of high intensity rainfall (I60: 70–80 mm h−1), representative of an ~100 year, or greater, storm event for the northern Rocky Mountain region. Our objectives were to compare runoff rates, sediment concentrations, and sediment yields between the general harvest area, along the edge of the riparian buffer at the interface with the harvested area, and within the riparian buffer. Surface/shallow subsurface runoff rates were greatest in the riparian buffer relative to the harvest area, especially when soil conditions were dry. Mechanical soil disturbance during forest harvesting appeared to result in higher infiltration rates and vertical, preferential flow relative to the riparian buffer. However, sediment concentrations in runoff from plots in the general harvest area were ~15.8-times greater than in the riparian buffer and ~4.2-times greater than at the harvest-riparian edge. Comparatively, sediment yields in the general harvest area were ~2.0-times greater than in the riparian buffer and ~1.2-times greater than at the harvest-riparian edge. Quantitative and qualitative evidence suggests differences in runoff and sediment between the harvest area, harvest-riparian edge, and riparian buffer were due to site differences in hydrophobicity, surface roughness, soil water content, and sediment supply. While we observed moderately high variability with only modest replication, the spatial patterns in the amount and timing of runoff, sediment production, and their relationships with soil moisture were consistent and monotonic along the gradient from harvested areas through riparian buffers. This highlights the need for additional research to explore if similar patterns appear evident after forest harvesting in other hydro-climatic settings.