Surface water runoff response to forest management: Low-intensity forest restoration does not increase surface water yields

Abstract

Ecohydrology of the Western United States faces two interconnected, issues: a decline in snowpack reducing surface water availability and timing, and an increase in the frequency and severity of forest fires. Prior research generally agrees that large severe fires and clearcut timber harvest increase surface water runoff due to reductions in interception, infiltration, and evapotranspiration. It has thus been postulated that forest restoration with ecologically guided stand density reduction and prescribed underburning in dry forests to reduce fire risk, will also increasing surface water runoff. This study evaluates this proposition in two subbasins of the Ashland Creek watershed in southwest Oregon where forest restoration and fuel mitigation treatments, including commercial density management, surface and ladder fuel reduction, and prescribed burning, were implemented from 2012 to 2019. Despite cumulatively treating roughly 15% and 20% of the area in two basins, canopy cover at the watershed scale was decreased just 3% and 4%,. We found that in the post treatment period, the West and East basins experienced an average annual water yield decline of 26% and 24% respectively with 66% (West) and 72% (East) of the changes in water yield attributed to annual variations in precipitation. Our results demonstrate that climatic drivers may overwhelm anticipated increased surface water runoff from forest restoration treatments at this scale and intensity.