Effect of Forest Management Practices on Water Balance Across a Water–Energy Gradient in the Upper Kings River Basin, USA

Abstract

Headwater watersheds and forests play a crucial role in ensuring water security for the western United States. Reducing forest biomass from the current overgrown forests can mitigate the severity and impact of wildfires and offer additional competing ecohydrological benefits. A reduction in canopy interception and transpiration following forest treatments can lead to an increase in available water for the remaining trees and runoff. However, the impact of forest management on water balance can be highly variable due to differences in climate, topography, location and vegetation. In this study, we used the Soil Water Assessment Tool Plus model to investigate how decisions regarding location, intensity and scale of forest treatments can affect both evapotranspiration and streamflow in a large watershed such as the upper Kings River Basin (3998 km²). The model was parameterized using a multiobjective calibration of streamflow, snow water equivalent and evapotranspiration. Various forest treatment scenarios were simulated across different years and regions in the landscape. Modelling results show that during dry years, streamflow gains from biomass reduction are primarily originated from energy-limited regions (i.e., 82% of total streamflow increase in the first year). In water-limited regions, the water is prioritized for sustaining remaining trees, improving forest health and recharging subsurface storage, rather than increasing streamflow. During wet years, the contribution to streamflow from biomass reduction comes from both energy- and water-limited areas. These findings emphasize the importance of evaluating forest treatments on a larger scale. The competing benefits for forests and downstream users are driven by the energy and water limitations of the vegetation targeted by forest treatments, as well as the climate variability that modulates the water availability and forest recovery time.