Distributed Hydrology Soil Vegetation Model (DHSVM) and Sediment Discharge Validation in a small, Pacific Northwestern Watershed

The Distributed Soil Vegetation Model (DHSVM) and its sediment production prediction capabilities are evaluated in a small, gaged, timber-production watershed in northwestern California. McReady Creek drains a 5 km2 basin dominated by coastal redwood (Sequoia sempervirens). Forest soils are fine grained and poorly consolidated marine sediments approximately 40 kya. Stream and sediment discharge data collected by Humboldt State University and Humboldt Redwoods Company (HRC) from 2003 to present were used to calibrate and validate DHSVM. Model parameters like forest stand (over- and understory height, fractional coverage, monthly LAI, etc) and soil characteristics (porosity, bulk density, field capacity, etc) were measured via in-field investigations or gathered by HRC through timber cruising activities. All data was gridded to 10 meters for the analysis. Forest stand data was manipulated throughout the model run to reflect the operational nature of the watershed. Modeled stream discharge will be evaluated against observed discharge on an event by event basis, as well as against weekly and monthly totals. Modeled sediment discharge will be evaluated on a similar basis, including total annual sediment discharge. Sources of potential incoherence between modeled and observed data may be road density (6.7 km / km2) and road/stream connectivity or the presence of legacy forest practices that still produce sediment within the basin. Further work will involve field investigations that will clarify road/stream connectivity and legacy sediment production zones.