AQUA TERRA Consultants
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Model Development and Application Assistance for the Chesapeake Bay Watershed Model


For more than a decade, AQUA TERRAs principals and staff provided watershed model development and application assistance to the EPA Chesapeake Bay Program Office and its consultants. Seven contracts with the firms and agencies listed above have resulted in AQUA TERRA providing nearly continual services that support long-term comprehensive watershed management.

Starting in 1985 the EPA Chesapeake Bay Program (CBP) has sponsored a series of projects to convert the proprietary HSP-NPS model to the public domain HSPF model, make it operational on the CBP computer system, and implement a number of model refinements so that the resulting 'Watershed Model' could be used effectively to estimate nutrient loadings to the Bay and to evaluate the impacts of agricultural Best Management Practices (BMPs). The effort to date has been segmented into four phases. Phase I was designed to improve the nonpoint loading representation, refine/re-evaluate the data input to the Model, and perform a preliminary re-calibration to available water quality data for the 1984-85 period. This period was used as the basis for the 40% nutrient loading reduction goal defined by the 1987 Chesapeake Bay Agreement.

Phase II was designed to focus on a better representation of the effects of agricultural BMPs, including application of the detailed agri-chemical modules (AGCHEM) of HSPF to allow a more deterministic, process-oriented approach to BMP analysis and evaluation. AGCHEM provides a nutrient balance approach to modeling cropland areas so that sensitivity to nutrient inputs can be represented. In addition, HSPF code enhancements were performed to allow specific consideration of sediment-nutrient and bed interactions within the stream channel. The Phase II Watershed Model was then applied to the Bay drainage area to include sediment erosion, sediment transport, and associated nutrients, in addition to the current modeled water quality constituents, to provide input to the Bay model.

To support the Phase I/Phase II effort, AQUA TERRA completed model calibrations for hydrology, nonpoint loadings, and instream water quality for numerous sites throughout the Bay drainage for the 1984 through 1987 period. In addition, model estimates of fall line loads for various nutrient forms were compared with regression estimates used in the calibration of the Bay Model. The results indicated that the Watershed Model was a valid representation of nutrient loadings to the Bay, and provided a framework for assessing the impacts of land use changes and alternative nutrient and agricultural management practices. Further refinements and recommendations were provided to improve model simulations and the models use as a tool to assist in the 1991 re-evaluation of the 40% nutrient reduction goal of the Bay Agreement.

Refinements and calibration of the Chesapeake Bay Watershed Model completed in 1991 under the Phase II efforts identified several additional recommended improvements and refinements to enhance the utility of the Watershed Model; the recommendations led to the Phase III effort which began in 1993. Several model segments were subdivided, resulting in the addition of approximately 15 new model segments. The meteorologic, precipitation, evaporation, and observed flow data bases were updated from the original 1984-87 period to include 1988-91. An enhanced land use dataset was developed, based on EPA's EMAP database. All of the new data were incorporated into the model, and the hydrology calibration was adjusted to account for the new land use and meteorologic data. Finally, improvements to the AGCHEM simulation, particularly nutrient uptake by plants, were researched in order to better represent impacts of agricultural practices and nutrient management scenarios.

AQUA TERRA supported the Phase III work by performing the following tasks: 1) assisting with the data base update, including diversions, point source inputs, atmospheric deposition inputs, and observed water quality data; 2) refinement, incorporation and testing of new AGCHEM methods for plant uptake of nutrients; and 3) recalibration of the model. The data base update also included development of new FTABLE's and a refinement of the precipitation data base to better represent the new and modified model segmentation.

To extend the nutrient balance approach to non-agricultural areas, Phase IV efforts assessed the nutrient balances and needed procedures to implement the approach for all non-agricultural land uses in the CBP Watershed Model, including forest, pasture, and urban land. The results provided a sound scientific basis for assessing atmospheric contributions and potential impacts of nutrient reduction alternatives for all land categories in the model. AQUA TERRA reviewed and summarized the available data and experience with nutrient balances for non-agricultural areas; demonstrated application of the nutrient balance modeling procedures on a test model segment; and supported the CBP modeling staff in applying the procedures throughout the basin drainage area.

In 2008 AQUA TERRA provided its most recent contribution to the Chesapeake Bay Watershed Model in the form of precipitation and air temperature time series data that enabled representation and evaluation of potential climate change effects in tandem with the Phase V model.

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