Evaluation of forest road scenarios using field measurements and DHSVM modelling of the South Fork of Caspar Creek

This report presents an evaluation of forest road effects following the Phase III forest harvest (2017-2019) within the South Fork of Caspar Creek, Jackson Demonstration State Forest, California. A two-step approach was utilized: (1) road field measurements; and (2) use of the Distributed Hydrology Soil Vegetation Model (DHSVM) to examine road scenarios at the watershed scale. Storm runoff, turbidity, and suspended sediment measurements were collected for select forest roads in South Fork Caspar Creek during the winter of 2018-2019. Statistical relationships between road runoff, turbidity, and road dimensions to predict suspended sediment loads (SSL) were evaluated. DHSVM was calibrated and validated to two South Fork Caspar Creek gauging station records and road measurements for hydrologic years 2015-2019. DHSVM was used to extrapolate the measured road runoff and SSL relationships to evaluate the cumulative watershed effect. DHSVM also enabled different simulations of road network and drainage scenarios to evaluate the influence of different road practices. Quantification of uncertainty in DHSVM utilized a range of parameter inputs that provided a range of behavioral models, models that provide good fit to measured streamflow based on two likelihood function thresholds.