Evaluating the effects of timber harvest on hydrologically sensitive areas and hydrologic response

Abstract

Effects of forest treatments on watershed hydrology in terms of water yield and runoff processes is a primary concern. This paper evaluates the effects of long-term land cover change on hydrological processes in the Mica Creek Experimental Watershed (MCEW) in northern Idaho, USA. In the MCEW, effects of clearcut and partial cut harvest practices on streamflow were monitored and evaluated from 1991 to 2013. Firstly, snowpack dynamics in terms of snowmelt timing and average melt rate under different elevations, aspects, land covers, and climate conditions were analyzed at the point-scale with the snow accumulation and melt (SAM) model from the perspective of the energy balance. Then, the grid-based soil moisture routing (SMR) hydrology model was used to provide temporal and spatial hydrological changes in the MCEW. We assigned locations as hydrologically sensitive areas (HSAs) based on the probability of runoff generation (P(Ai = HS)) to capture potential downstream environmental effects of actual land cover change and optimal harvest patterns. The results highlight the strong impacts timber harvest can have on increasing streamflow and, in particular, peak flow. On average, the daily averaged degree of saturation increased by 20% after timber harvest resulting in higher risks of flooding from large rainfall events. Three logging scenarios including logging in areas with low P(Ai = HS), high P(Ai = HS), and in the entire watershed area, showed that logging areas had a positive relationship with water yield and that more extreme peak flow was likely to occur after clearcutting in high P(Ai = HS) areas (high elevation) under rainfall driven events or logging in low P(Ai = HS) areas (medium elevation) during snowmelt dominated events. Clearcutting on south-facing slopes at medium elevation strongly increased the average melt rate leading to potential flooding if a snowpack was present and temperatures increased. Scenario analysis suggests that partial cutting in areas with low P(Ai = HS) would reduce peak flow amount by as much as 40%. However, for watersheds to be logged entirely, choosing proper harvest locations such as clearcutting in high P(Ai = HS) areas having large snow events may effectively reduce extreme peak flow from spring snowmelt. Our findings inform forest management decisions favoring partial cutting over clearcutting, and how to avoid concentrating harvest practices in HSAs by adapting spatial distribution and timing of harvesting to climate conditions to minimize impacts on the water cycle.