Virginia Planning District Commission 

 Cooperative Agreement has three major 

 components (Hartigan 1980). The 

 first component will be a comprehen- 

 sive analysis of the hydrologic and 

 nonpoint source nutrient loading data 

 collected by the investigators for 

 the Pequea, Chester, Patuxent, 

 Occoquan and Ware sites. This 50 to 

 60 station-years of data will then be 

 used to develop, using continuous 

 simulation model calibration tech- 

 niques, transferable land use, non- 

 point pollution relationships for 

 application to the entire Chesapeake 

 Bay drainage basin (approximately 

 64,000 square miles). 



The second component will be the 

 actual calibration and verification 

 of the HSPF model to the entire 

 drainage basin. This basinwide model 

 will be segmented to provide suffi- 

 cient loading information to account 

 for point and nonpoint sources of 

 nutrients at the fall lines (Susque- 

 hanna, Potomac and James) yet not so 

 detailed that computer costs for 

 long-term simulations would be pro- 

 hibitive. 



The third component of the pro- 

 posed effort involves the modeling 

 production run phase. The verified 

 basinwide model will be run to pro- 

 duce time series output. This output 

 will then be analyzed to generate 

 loadings, based on existing (1980) 

 and future (year 2000) land use pat- 

 terns for the entire bay. These 

 loading data are an essential input 

 to a bay-wide water quality model of 

 the tidal Chesapeake. 



The bay-wide model covering the 

 bay proper to the head of tide will 

 be employed to identify water quality 

 problem areas based upon fall line 

 loading information. The work will 

 be performed by the Virginia Insti- 

 tute of Marine Scince (VIMS). The 

 approach will be to adapt an existing 



water quality model to the entire 

 tidal portion of the Chesapeake Bay. 

 The model will not only address the 

 effects of a particular loading 

 scenario, but will be used in an 

 iterative fashion to determine neces- 

 sary fall line loadings given a par- 

 ticular bay water quality condition. 



The model is a two-dimensional, 

 depth averaged, finite element, real 

 time, hydrodynamic , water quality 

 model developed by H.S. Chen at the 

 Virginia Institute of Marine 

 Sciences. The hydrodynamic portion of 

 the model incorporates hydrologic as 

 well as meteorological and astronomi- 

 cal effects. The water quality com- 

 ponent addresses the following 

 constituents: phytoplankton, organic 

 nitrogen, ammonia nitrogen, nitrite- 

 nitrate nitrogen, organic phosphorus, 

 inorganic phosphorus, carbonaceous 

 biochemical oxygen demand and dis- 

 solved oxygen deficit. In general, 

 the model simulates primary pro- 

 duction and its resultant affect on 

 dissolved oxygen concentrations with 

 constants for zooplankton grazing 

 and sediment to water column fluxes 

 of nutrients . 



The limiting constraint in using 

 a depth-averaged model is the loss of 

 vertical resolution. This problem is 

 negligible when mixing causes surface 

 to bottom exchange of water mass re- 

 sulting in a uniform concentration 

 throughout the water column. How- 

 ever, when pressure gradients are 

 strong and stratification is exhibit- 

 ed, dramatic changes in surface to 

 bottom concentrations are possible. 

 It is the latter case in which a 

 better resolution of the problem is 

 needed. 



The problem becomes one of test- 

 ing the validity of depth averaging 

 in areas delineated as having a dis- 

 solved oxygen problem. In order to 

 accomplish this, a three-dimensional 



197 



