Potomac Watershed Model

Objectives

The USGS has responsibility for the following objectives:

Compile necessary spatial and temporal databases for simulation of Potomac watershed processes (hydrological, nutrient cycling, and sediment transport) using HSPF, in cooperation with ICPRB and Chesapeake Bay Program staff.

Create watershed segmentation, river reach segmentation, and associated control files for HSPF simulation of the Potomac River Basin, following the framework developed by Chesapeake Bay Program for Phase 5 of the CBWM.

Develop and implement innovative calibration procedures, such as inverse modeling and analysis of scaled model sensitivities, to improve HSPF model calibration and provide additional insight into important controls on nutrient and sediment transport and processing within the Potomac River Basin.

Calibrate an HSPF model for the Potomac River Basin. Through collaboration with Chesapeake Bay Program, this model will be nested within the Phase 5 CBWM.

Prepare reports on subjects that might include (but are not limited to) the following: calibration of the Potomac Watershed Model; analysis of calibration strategies; Potomac Watershed Model uncertainty; analysis of Potomac Watershed Model results; implications for present and future monitoring and other data collection activities.

Statement of Problem

The Potomac River Basin encompasses 38,000 square kilometers (14,670 sq. miles) in four states and the District of Columbia. As a hydrologic unit, it includes a complex assemblage of topography, from the Appalachian Plateau to the Coastal Plain, and land uses, including major agricultural, forested and urban and suburban areas. Surface waters of the Potomac River and its tributaries are the subject of investigation by a number of state and federal agencies; in particular, ICPRB, MDE, and Chesapeake Bay Program are interested in quantifying nutrient sources and loadings within the Potomac River Basin as part of regulatory and voluntary efforts needed to restore or protect water quality.

The quality of streams and ground water in the Potomac River Basin is affected by a number of natural and human processes. Major types of chemical compounds found in waters in the basin include nutrients (predominately nitrogen and phosphorus), trace metals, pesticides, chlorinated industrial compounds, and volatile organic compounds (Ator, Blomquist, and others, 1998). Nutrients are of particular interest to environmental managers within the basin. Although the nutrients nitrogen and phosphorus occur naturally and are essential for plant and animal growth, excessive nutrients in water can adversely affect human health and the environment.

The Potomac River Basin is a USGS National Water Quality Assessment Program (NAWQA) Study Unit. As such, a significant body of data and scientific understanding exists, and continues to be developed, for the basin. Major NAWQA findings that emerged during the last intensive study phase (1992-95) indicated that elevated concentrations of nitrogen and phosphorus in surface and ground water in the Potomac Basin often result from human activities such as manure and fertilizer application (Ator, Blomquist, and others, 1998).

MDE, in conjunction with the USGS, ICPRB, and Chesapeake Bay Program, has determined that a watershed model is needed to address the questions stated above and to assess the effects of point and nonpoint nutrient and sediment sources on water quality in the Potomac River and its tributaries. The modeling effort proposed here will involve direct collaboration with Chesapeake Bay Program efforts to develop Phase 5 of the CBWM, using HSPF.

Strategy and Approach

The hydrological and water-quality model HSPF (Bicknell, Imhoff, and others, 1996) will he used to simulate the runoff of water and transport of suspended sediment and nutrients (nitrogen and phosphorus) within the Potomac River Basin. HSPF is a deterministic lumped-parameter time series model that evolved out of the Stanford Watershed Model (Crawford and Linsley, 1966) and the USEPA Agricultural Runoff Management, or ARM (Donigian and Linsley, 1978), and Nonpoint Source, or NPS (Donigian and Crawford, 1979), models. The model requires input information including land use/cover, source (of N and P) data, stream reach characteristics, and time series of precipitation and potential evapotranspiration; additional information may be useful in estimating model parameters. Time series (observational) data, such as streamflow and water-quality, are necessary to calibrate the model.

The proposed study will involve the following tasks:

• Data Compilation: compilation of existing input data (e.g., land use/cover, sources of N and P, and meteorological data); development of model segmentation, model network, and construction of UCI (User's Control Input) files; processing of time-series data to create input WDM (Watershed Data Management) files; compilation of ancillary data and observational data (for model calibration).
• Development of Model Calibration Strategy: implementation of existing software for general inversion and calibration of multi-parameter hydrological models.
• Model Calibration: calibration of hydrological model; calibration of water- quality model (suspended sediment and nutrients, N and P, and their speciation).
• Analysis of Model Results: results of model calibration, examination of model output, and consideration of specific study questions.
• Delivery of Results and Final Reports: dissemination of data sets and model input and output files and preparation of final reports analyzing the model results.

These tasks will be accomplished in collaboration with Chesapeake Bay Program efforts to refine the CBWM. Output from the calibrated model may be used as input for an estuarine hydrodynamic and water-quality model of the tidal Potomac River. Also, the model may provide a starting point for examination of model scenarios, using software such as USGS's GENSCN.