Draft Appendix H – Chesapeake Bay TMDL for Nutrients and Sediment

H-1 September 24, 2010

APPENDIX H. Criteria Assessment Procedures using Model Scenario Output with 

Bay Monitoring Data

Scenarios representing different nutrient and sediment loading conditions were run using the 

Chesapeake Bay Phase 5.3 Watershed Model and the resultant model scenario output was fed as 

input into the Chesapeake Bay Water Quality Model to evaluate the response of critical water 

quality parameters, specifically dissolved oxygen, water clarity, underwater bay grasses and 

chlorophyll a. To determine whether the loading scenarios met the applicable Bay jurisdictions‟ 

Chesapeake Bay water quality standards, the Bay Water Quality Model‟s simulated water quality 

response for each variable was used to increase/decrease the corresponding observed monitoring 

values collected during the same 1991-2000 hydrological period. In other words, the Bay Water 

Quality model was used to estimate the change in Bay water quality that would result from 

various loading scenarios. The model-simulated change in water quality was then applied to the 

actual Chesapeake Bay water quality monitoring data. Figure H-1 provides an example of the 

relationship between the calibration (cal) and scenario (E3) Bay Water Quality Model outputs 

described above, as well as their relationship to hypothetical monitoring observations (Data) over 

the same ten-year period. 

In its simplest terms the following steps were taken to apply the Bay Water Quality Model 

outputs to predict Bay water quality:

1. Calibrate the Bay Water Quality Model to actual monitoring data.

2. Run a Bay Water Quality Model simulation for a given “loading scenario” (usually a 

management scenario resulting in lower loads relative to the calibration scenario) through 

the Bay Watershed and Bay Water Quality models. 

3. Determine the simulated change in water quality from the calibration scenario to the 

given loading scenario.

4. Apply the change in water quality as predicted by the Bay Water Quality Model to the 

actual historical water quality monitoring data, and evaluate attainment based on this 

“scenario modified” data set.
Draft Appendix H – Chesapeake Bay TMDL for Nutrients and Sediment

H-2 September 24, 2010

Frequency of DO, CB5, Summer

0

1

2

3

4

5

6

7

8

9

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

percent less than

mg/l

Data

Model (cal)

Model (3E)

Source: Linker et al. 2002

Figure H-1. Frequency distribution of hypothetical observed data (blue), model calibration (solid red) and 

model scenario (dashed) for a designated use.

In following these steps, the scenario assessment process uses both model simulated outputs and 

observed water quality monitoring data. 

For a more detailed description of the model calibration process (Step 1 above), as well as the 

process of constructing management scenarios to simulate reduced loads to the Bay Water 

Quality Model (step 2 above), see Section 5. More detailed descriptions of Steps 3 and 4 are 

summarized below.

In order to determine the expected effect of reduced pollutant loads on a water quality parameter 

such as dissolved oxygen or chlorophyll a (Step 3 above), the simulated parameter 

concentrations from the Bay Water Quality Model‟s calibration scenario are compared to the 

parameter concentrations from a given load reduction scenario. This is accomplished by relating 

each month‟s worth of values from the calibration scenario for a given location to the same 

month‟s worth of values from the load reduction scenario at the same location. The resulting 

“linear regression” equation represents the degree of change (in dissolved oxygen or chlorophyll 

a concentration) from the calibration scenario to the load reduction scenario. In Figure H-2, a 

dissolved oxygen concentration of 2 mg/L (x) in the calibration scenario becomes 3.6 mg/L (y) 

in the load reduction scenario.
Draft Appendix H – Chesapeake Bay TMDL for Nutrients and Sediment

H-3 September 24, 2010

Figure H-2. Hypothetical example of a linear regression between model calibration (x axis) and scenario (y 

axis) data. 

Regressions are generated for all Bay Water Quality Model cells that match up with the longterm Chesapeake Bay mainstem and tidal tributary water quality monitoring stations and vertical 

sampling locations through the water column. The regressions are generated using all Bay Water 

Quality Model simulated values (hourly for dissolved oxygen; daily for chlorophyll a) for the 

month when the historical monitoring observation occurred. The result is a unique equation for 

each monitoring location and month (Figure H-3).

Figure H-3. Individual regression equation generated for each monitoring station location and month
Draft Appendix H – Chesapeake Bay TMDL for Nutrients and Sediment

H-4 September 24, 2010

Once the relationship between the calibration and a given loading scenario is established, this 

relationship is used to generate a „scenario-modified‟ value for each observation in the historical 

monitoring dataset spanning 1991-2000 (step 4 above). These „scenario-modified‟ values 

represent an estimate of the concentration that would have been observed under the conditions of 

nutrient and sediment management represented by the scenario. In this manner, each observed 

concentration for dissolved oxygen or chlorophyll a in the 1991-2000 dataset is replaced with a 

„scenario-modified‟ concentration for the same sampling location and date.

Figure H-4 illustrates the modification of hypothetical historical monitoring data using a 

regression generated with the described procedure. The result is shown on a frequency plot so 

that changes in the prediction of attainment can be seen. The perpendicular blue lines in the 

lower left portion of the graph illustrate the predicted change in dissolved oxygen from the 

hypothetical historical monitoring data (solid line) to the E3 scenario (dashed line). In this case, 

the incidence of dissolved oxygen concentrations less than 2.0 mg/L is predicted to decrease 

from 20% to 10%. Frequency of DO, CB5, Summer

0

1

2

3

4

5

6

7

8

9

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

percent less than

mg/l

Data

Data (3E)

Source: Linker et al. 2002

Figure H-4. Frequency distribution of hypothetical summer DO concentrations, as observed (solid blue line) 

and as simulated using a regression equation generated from water quality model scenarios.

For a full discussion of this procedure, see A Comparison of Chesapeake Bay Estuary Model 

Calibration With 1985-1994 Observed Data and Method of Application to Water Quality 

Criteria (Linker et al. 2002).

References

Linker, L., G. Shenk, P. Wang, C. Cerco, A. Butt, P. Tango, and R. Savage. 2002. A Comparison 

of Chesapeake Bay Estuary Model Calibration with 1985–1994 Observed Data and Method of 

Application to Water Quality Criteria. Chesapeake Bay Program Modeling Subcommittee 

Report. U.S. Environmental Protection Agency, Chesapeake Bay Program Office, Annapolis, 

MD.