Forests provide multiple ecosystem services, including stable supplies of clean water. Management strategies that provide some mitigation or adaptation to a warmer climate with more extreme precipitation events are needed to maintain hydrologic ecosystem services. We compared the effects of a forest management treatment on water yield in two small watersheds using a distributed ecohydrological model. Beetree Creek is a 1414 ha forested (> 96%) watershed in the Appalachian Mountains of western North Carolina that collects in a secondary drinking water reservoir for Asheville. In this rural region, land use is expected to remain relatively stable. In contrast, developed land use is expected to approach 50% in the Reedy Fork watershed by 2060, which is presently 50% forested. Reedy Fork is a 5335 ha watershed that feeds into a primary drinking water source for the City of Greensboro, NC. In these two watersheds, we examined the effects of two drought treatments: a) a 20% reduction in each precipitation event and b) a long-term summer drought, where we excluded all precipitation June-August. Our thinning treatment included a 50% reduction in leaf area, leaving a 30 m stream buffer consistent with current best management practices.
Forest management increased water yield during both drought treatments, and the effects were greater in the rural forested watershed when compared to the suburban, mixed land use watershed. Under the decreased precipitation scenario, annual water yield at Beetree Creek was reduced 27-47% over a five-year period. Reductions were only 9-21% when the thinning treatment was applied. Under the summer drought scenario, annual water yield at Beetree Creek was reduced 12-53% without thinning treatments or 2-13% with thinning treatments. At Reedy Fork, precipitation effects were slightly greater. The decreased precipitation scenario resulted in a 38-40% decrease in annual water yield, and the summer drought scenario reduced annual water yield 35-63% over five years. Thinning treatments had a mititgative effect, although not as great as at Beetree Creek. Annual water yield was reduced by between 23-28% in the decreased precipitation treatment, and 9-47% in the summer drought treatment.
Our study suggests that thinning treatments or maintaining low-density forests might increase municipal water supplies. Therefore, maintaining multipurpose forest reserves in small water supply watersheds might provide some flexibility for future climate while providing additional benefits to water quality, income from timber products, as well as wildlife habitat and recreation.