14 Oct 2020

The Demonstration and Validation of a Linked Watershed-Riverine Modeling System for DoD Installations – Patuxent Watershed, Maryland

Abstract: This work evaluated a linked watershed and riverine modeling system for the Patuxent River Watershed, Maryland against observed field data and model output from a watershed model. The performance objectives were computed for streamflow, sediment, total phosphorus, orthophosphorus, total nitrogen, ammonium, and nitrate using daily and monthly average model predictions and measured data. Hydrological Simulation Program – Fortran (HSPF) was used to compute runoff, sediment, and nutrient loadings, whereas the Hydrologic Engineer Center – River Analysis Sys-tem (HEC-RAS) was used to evaluate in-stream flow, channel sedimentation, and the fate/transport of nutrients. Model results were successful for calibration, validation, and management scenario analysis. Contaminants were not simulated for this watershed due to a lack of observed data to compare against. The study identified two implementation issues. First, while the Patuxent River did not experience dry bed conditions, where a stream may be intermittent, one can incorporate a very narrow slot at the low point in the cross-section to numerically keep the channel wet during very low flows. Second, to set up the linked model, there needs to be more observed water quality data to better constrain the HSPF output being used as boundary conditions to the HEC-RAS model.