CHESAPEAKE BAY WATER QUALITY MODEL 
The complex movement of water within the Chesapeake Bay, particularly the density 
driven vertical estuarine stratification, is simulated with a Chesapeake Bay hydrody¬ 
namic model of more than 13,000 cells (Wang and Johnson 2000). 
Three-dimensional equations of the intertidal physical system, including equations 
of continuity, momentum, salt balance and heat balance, are employed to provide the 
correct simulation of the movement, or the barriers to movement, of the water quality 
constituents of dissolved oxygen, water clarity and chlorophyll a. Correct formula¬ 
tion of vertical mixing, including the simulation of vertical eddy diffusion 
coefficients in the hydrodynamic model is particularly important for the dissolved 
oxygen criteria as the principal barrier to vertical movement of dissolved oxygen 
from surface waters to the deep water is the pycnocline simulated by the hydrody¬ 
namic model. 
The water quality model is linked to the hydrodynamic model and uses complex 
nonlinear equations describing 26 state variables relevant to the simulation of 
dissolved oxygen, water clarity and chlorophyll a (Cerco 1993, 1995a, 1995b, 2000; 
Thomann et al. 1994; Cerco and Meyers 2000). Dissolved oxygen is simulated as the 
mass balance calculation of reaeration at the surface, respiration of algae, benthos 
and underwater bay grasses; photosynthesis of algae, benthic algae and underwater 
bay grasses; and the diagenesis, or decay of organics, by microbial processes in the 
water column and sediment. This mass balance calculation is made for each model 
cell and for associated sediment cells at each hourly time step, providing an estimate 
of dissolved oxygen from nutrient loads from the watershed and airshed to the waters 
of the 35 major segments of the Chesapeake Bay and its tidal tributaries. Chlorophyll 
a is estimated based on Monod calculations of algal growth given resource 
constraints of light, nitrogen, phosphorous or silica. Water clarity is estimated from 
the daily input loads of sediment from the watershed and shoreline acted on by 
regionally-calibrated settling rates, as well as estimated advection due to hydrody¬ 
namics. Coupled with the water quality model are simulations of settling to the 
sediment of organic material and its subsequent decay and flux of inorganic nutri¬ 
ents from the sediment (Di Toro 2001) as well as a coupled simulation of underwater 
bay grasses in shallow waters (Cerco and Moore 2001). 
INTEGRATION OF MONITORING AND MODELING 
FOR CRITERIA ASSESSMENT 
The load allocation process requires that specific water quality conditions be met 
over critical time periods within designated use areas. These areas are given either a 
‘pass’ or ‘fail’ status. While the Chesapeake Bay water quality model can estimate 
changes in water quality due to changes in input loads with reasonable accuracy, an 
exact match of the simulated and observed data is impossible. The following method 
was developed to make the best use of the strengths of the Chesapeake Bay water 
chapter vi 
Recommended Implementation Procedures 
