Wildfires are an increasingly common occurrence in the Western United States. Wildfire mitigation measures 鈥� or 鈥渇orest treatments鈥� 鈥揳re being utilized to reduce fire severity and protect valuable resources. By reducing forest density, treatments have the potential to reduce evapotranspiration (ET), potentially leaving excess water available for runoff. Past work has verified that large reduction in forest density can increase runoff. However, there is significant debate surrounding moderate reduction in forest density and the impact on runoff. Past results are highly variable, finding increased, reduced, or no change in runoff depending on the study. To improve our understanding this research asks, how do forest treatments impact water yield at the Sagehen Watershed? At what spatial scale is the impact quantifiable? Located in the Sierra Nevada Mountains of California, Sagehen offers a unique and ideal location of study because 1) multiple types of forest treatment were conducted across the watershed, and 2) nine streamflow gauges were distributed throughout the basin to capture spatially localized changes at sub-basin scale. Results show that since 1954, precipitation accounts for 93% of water yield variability and forest treatment starting in 2014 did not alter this. For the past 20 years, ET remained relatively constant especially in the context of highly variable precipitation. Year-to-year changes in basin and sub-basin scale ET never exceeded 15%, while year-to-year changes in precipitation exceeded 100%. Treatments had minimal impact on forest structure, and as far as it relates to the water budget, no impact on water yield was observed at either the basin or sub-basin scale. The largest treatment plan for a sub-basin, 56% of total area, did result in a 15% change in sub-basin ET. However, this decrease in ET did not translate to an observable increase in water yield. Overall, this study helps inform water resource management decisions and natural resource protection measures as we adapt to climate resilient forests and water supplies.
Graduate Student, Colorado School of Mines