Chesapeake Bay Watershed Essay

Case 2: Chesapeake Bay watershed

System Complexity, Data, and Impairment Description

The Chesapeake Bay watershed is an example of a complex modeling system that consists of a large watershed (165,759 km2) with flow-regulated rivers, large urban centers and expansive rural areas, and an estuary (US EPA, 2010). The Chesapeake Bay TMDL allocates total nitrogen, total phosphorus, and sediment loads to reduce the size of the oxygen-depleted zones in the bay to restore its living resources. As such, estimating nutrient loads from all sources (air and land) and assessing their effect on the bay’s aquatic life requires the use of an integrated modeling framework. Because of the areal extent and system complexity of the watershed and the estuary, the Chesapeake Bay TMDL is one of the most complex TMDLs conducted in the United States.

Models

We present herein a brief description of each model.

• Air-shed model (Community Multi-Scale Air Quality – CMAQ) simulates atmospheric deposition predictions for nutrients and other constituents. CMAQ runs on a fine grid in the Chesapeake region;

• Watershed loading model is a modified version of the -Hydrological Simulation Program—FORTRAN (HSPF) known as the Phase 5.3 Community Watershed Model. The loading model predicts sediment and nutrient loadings that come from watersheds to the estuary (Linker et al., 2000);

• Estuary model--Water Quality Model and Sediment Transport Model (WQSTM) consists of three linked models which are a hydrodynamic model (Johnson et al., 1993), a eutrophication model (Cerco and Cole, 1993), and a sediment diagenesis model (DiToro, 2001). WQSTM is a three dimensional model of the tidal bay which represents transport processes, eutrophication processes, and living resources such as submerged aquatic vegetation and benthos.

TMDL Modeling Results and