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 b ay grasses and chlorophyll a . To determine whether the loa ding 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 collect ed 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 appli ed 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 W ater Quality Model output s 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 p redict 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 scenari o) 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 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 mod el 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 com pared 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 scenar io 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 concent ration 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 e xample 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 long

term Chesapeake Bay mainstem and tidal tributary water quality monitoring stations and vertical sampling locations through the water column. The regressions are gen erated 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 monthDraft 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 i s established, this relationship i s used to generate a „ scenario

- modified ‟ value for each observation in the historical monitoring dataset spanning 1991

- 2000 (step 4 above) . The se „ scenario

- modified ‟ va lues 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, e ach observed

concentration for dissolved oxygen or chlorophyll a in the 19 91

- 2000 dataset i s 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. Th e 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 mon itoring 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%.

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 Chesapea ke 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. Sav a ge. 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,

M D

.