The objective of this project was to evaluate a linked watershed and riverine modeling system for both the Patuxent River Watershed and the Calleguas Creek Watershed. The model evaluation consisted of calibrating the models for the period of record from 1991 to 2000 (HSPF-only) and 1992 to 2000 (linked models), and validating the models for 2001 to 2005 (HSPF-only) and 2001 to 2005 (linked models). The performance objectives (NSE, PBIAS, and RSR) were computed for streamflow, sediment, total phosphorus, orthophosphorus, total nitrogen, ammonium, and nitrate using both daily and monthly average model predictions and measured data. The system helps land managers assess outcomes resulting from military activities; the system also supports installation sustainability through informed watershed management of water, water quality, contaminants, and land-use impacts.
The modeling system was developed and enhanced from existing watershed and riverine models. Hydrological Simulation Program-Fortran (HSPF) was used to compute water flow, soil erosion/sedimentation, nutrients, and contaminant loadings, whereas the Hydrologic Engineering Centers-River Analysis System (HEC-RAS) was used to evaluate instream water quality and aquatic ecosystem responses from watersheds both within and outside an installation.
For the Patuxent Watershed, model results were deemed 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. HSPF was used to compute runoff, sediment, and nutrient loadings, whereas HEC-RAS was used to evaluate in-stream flow, channel sedimentation, and the fate/transport of nutrients.
For the Calleguas Creek Watershed, the watershed only and linked modeling system performance was evaluated using both quantitative and qualitative measures. The quantitative measures used were performance evaluation statistics, and the qualitative measures used were visual comparisons of simulated and observed time series. There were sufficient daily streamflow records to compute the required statistics for the three time periods mentioned above. For sediment, contaminants, and nutrients, the availability of samples was sparse relative to the time period over which sampling was completed (approximately one sample per month for most parameters); thus, in some cases, only qualitative measures were evaluated.
In regard to implementation issues from the Patuxent River Watershed Demonstration Study, there were a couple of items that were identified. The first was investigating the methods for dealing with dry bed conditions in the HEC-RAS model. While the Patuxent River did not experience dry bed conditions, the research team does want to discuss one method used by hydraulic engineers. In cases where a stream may be intermittent, one can incorporate a very narrow slot at the low point in the cross-section in order to numerically keep the channel wet during very low flows. The volume of water in the slot is negligible, but numerically the channel never goes dry while practically it does.
The second issue is that if one wishes to set up the linked model, there needs to be more observed water quality data (frequency and nutrient species) in order to better constrain the HSPF output being used as boundary conditions to the HEC-RAS model. Across demonstration studies done as part of this project, that was a continuing data gap that would need to be closed in order to make full use of this system.
A number of challenges were encountered at this demonstration site, including unique hydrologic characteristics of the Calleguas Creek Watershed that led to model instabilities, a relatively large number of model segments, a relatively long simulation period, observed data limitations, and a relatively high number of constituents modeled. These challenges presented opportunities for applying lessons learned when using the linked watershed-riverine models in the future.