attainment assessment process applied to observed data is applied to integrated 
modeling/monitoring ‘scenario’ data to determine likely criteria attainment under 
management loading scenarios. 
The watershed and airshed models are loading models. As such, they provide an esti¬ 
mate of management actions through air controls, agricultural best management 
practices, or point source controls which will reduce nutrient or sediment loads to 
the Chesapeake Bay tidal waters. The advantage of using loading models is that the 
full simulation through different hydrologies of wet, dry and average periods can be 
simulated on existing or hypothetical land use patterns. All of the Chesapeake Bay 
Program models used in this system simulate the 10-year period from 1985 to 1994 
(Linker and Shenk 2000). 
CHESAPEAKE BAY WATERSHED MODEL 
The Chesapeake Bay Watershed Model is designed to simulate nutrient and sediment 
loads delivered to the Chesapeake Bay under different management scenarios 
(Donigian et al. 1994; Linker et al.1996; Linker 1996). The simulation is an overall 
mass balance of nitrogen and phosphorus in the basin, so the ultimate fate of the 
input nutrients is incorporation into crop or forest plant material, incorporation into 
soil, or loss through river runoff. 
The Chesapeake Bay Watershed Model has been in continuous operation in the 
Chesapeake Bay Program since 1982 and has had many upgrades and refinements. 
The current version of the Watershed Model, Phase 4.3, is a comprehensive package 
for the simulation of watershed hydrology, nutrient and sediment export from 
pervious and impervious land uses and the transport of these loads in rivers and 
reservoirs. The model is based on a modular set of computer codes called Hydro- 
logic Simulation Program—Fortran (HSPF). A slightly modified version of HSPF 
release 11.1 (Bicknell et al. 1996) is applied in the watershed simulation. Version 11 
is a widely-used public-domain model supported by the U.S. EPA, U.S. Geological 
Survey and U.S. Army Corps of Engineers (Shenk et al. 1998). 
The Watershed Model allows for the integrated simulation of land and soil contam¬ 
inant runoff processes with in-stream hydraulic and sediment-chemical interactions. 
The model takes into account watershed land uses and the application of fertilizers 
and animal manure; loads from point sources, atmospheric deposition and onsite 
wastewater management systems; and best management practice reduction factors 
and delivery factors. Land uses, including cropland, pasture, urban areas and forests, 
are simulated on an hourly time-step. 
Fourteen calendar years (1984—1997) of varying hydrology are simulated by the 
Watershed Model, although only 10 of those years (1985-1994) are used in this 
study because of the more limited simulation period of the Chesapeake Bay water 
quality model. Scenarios are run on a 1-hour time step and results are often aggre¬ 
gated into 10-year-average annual loads for reporting and comparisons among 
scenarios. Watershed Model results, in the form of daily flows and nutrient and sedi¬ 
ment loads, are used as input to the Chesapeake Bay water quality model. 
chapter vi 
Recommended Implementation Procedures 
