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Impact of forest treatment on water yield in a Sierra Nevada watershed
Boden, Kate S.
Boden, Kate S.
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2022
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Abstract
The future of the Western U.S. is threatened on two fronts: increase in wildfire intensity and decrease in water availability. By reducing forest density and thus fuel loads, wildfire mitigation measures – or “forest treatments” – promise reduced fire severity, protection for wildlife and biodiversity, and increased water yield via reduction in evapotranspiration (ET). While the benefits of forest treatments for fire management and biodiversity are well studied, their impact on ET and water yield remain largely unknown, and existing literature shows conflicting results. Past work has verified that large reduction in forest density increases runoff, but results from studies on smaller scale reductions are variable, finding increased, reduced, or no change in runoff depending on the study. The present research aims to fill this gap by quantifying the impact of forest treatments on water yield, using the Sagehen experimental watershed in the Sierra Nevada of California as a case study. A range of forest treatments have been implemented across the watershed since 2014. Using remotely sensed imagery, LiDAR, and long-term streamflow monitoring across nine sub-basins of the watershed, we analyze changes to forest density, ET, and runoff at varying spatial scales. For both pre-treatment and post-treatment periods, we find that variability in water yield is almost entirely predicted by precipitation (R^2 = 0.93) and does not rely on ET. Despite large variability in precipitation (>100%), year-to-year change in basin and sub-basin scale ET remains nearly constant (>15%). The most extensive treatments, up to 56% of total sub-basin area, led to a maximum of 15% reduction in sub-basin ET. This localized reduction in ET did not translate into a quantifiable impact on annual water yield at either the basin or sub-basin scale. Thus, even the most extensive forest treatments at Sagehen were insufficient to increase water yield at both basin and sub-basin scale, especially in the context of highly variable precipitation. Though forest management is key to addressing the major threat of wildfires, our results suggest that corresponding increases in water availability are less certain.
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