Addendum to Caspar Creek Experimental Watersheds Experiment Three Study Plan: The influence of forest stand density reduction on watershed processes in the South Fork

Since their establishment in 1962, the Caspar Creek Experimental Watersheds have provided foresters, land managers, researchers, and citizens with information that has influenced forest management in northwestern California. The primary goal in establishing Caspar Creek was to understand how harvesting timber could affect streamflow and suspended sediment concentrations (SSC). The first experiment (1962-1985) was a classic paired-watershed study undertaken before the implementation of the modern California Forest Practice Rules (FPRs): roads were constructed in the South Fork and 60-70% of the stand volume was selectively cut and tractor yarded, while the North Fork served as a control (Rice et al., 1979). The second experiment (1985-present) was designed to address cumulative watershed effects using the modern FPRs by investigating how clearcutting sub-watersheds in the North Fork influenced downstream streamflow and SSC (Ziemer, 1998; Lewis et al., 2001). Streamflow and SSC measurements began in 2000 at a network of sub-watersheds in the South Fork in anticipation of a third experiment that would investigate the impact of harvesting under updated California FPRs.

The U.S. Forest Service established numerous paired watershed experiments across the United States in the mid-1900s with the goal of understanding how forest practices affect streamflow and sediment production. These paired watershed studies, located in a wide range of ecosystems, found similar water yield results regardless of topography or vegetation type: removing >20% of basal area from a stand resulted in increased streamflow (Hibbert, 1967; Stednick, 1996). The effects of vegetation removal were found to decrease as regrowth occurred. Increases in water yield were caused by decreases in evapotranspiration and interception loss that occur when removing vegetation from a watershed. 

In contrast to water yield results, there is still not agreement on the magnitude, persistence, and mechanisms responsible for peak flow changes, despite decades of data from paired watershed studies (Grant et al., 2003). Similarly, while there is general consensus that sediment generation has decreased in experimental watersheds with improved forest practices, the degree that remaining effects from timber operations should be reduced continues to be debated (Loehle et al., 2014). While paired watershed studies have answered key questions on cause and effect, the question of equifinality (the idea that a given end state can be achieved by many potential means) still remains. The third experiment will broaden our process-based understanding of forest hydrology by investigating how contemporary forest management, including harvests, affect water movement and storage, peak flows, and sediment production in the South Fork Caspar Creek.