quality model and the Chesapeake Bay Water Quality Monitoring Program in 
assessing criteria attainment. 
The observed data is used to assess criteria attainment during a ‘base’ period corre¬ 
sponding to the years of calibration for the Chesapeake Bay water quality model, 
1985-1994. The Chesapeake Bay water quality model is used in scenario mode to 
determine the effect of changes in nutrient and sediment loads on water quality 
concentrations. A modified 1985-1994 observed data set is generated for each 
scenario using both the model and the observations. The same criteria attainment 
assessment process applied to the observed data is then applied to this scenario data 
to determine likely criteria attainment under modified loading scenarios. 
To generate the modified data set for a particular scenario (e.g., 2010 Clean Air Act), 
the EPA compared the output of the scenario to the output of the calibration on a 
point-by-point and month-by-month basis. For each point in space and time where 
an observation exists during the 1985-1994 period, a mathematical relationship 
between the model scenario and the model calibration was established by regressing 
the 30 or so daily values for the month when the observation occurred in the water 
quality model cell that contains the observation. The regression generates a unique 
equation for each point and month that transforms a calibration value to a scenario 
value. This relationship is then applied to the monitored observation as an estimate 
of what would have been observed had the Chesapeake Bay watershed been under 
the scenario management rather than the management that existed during 
1985-1994. This procedure is repeated for each monitored observation of dissolved 
oxygen, water clarity and chlorophyll a to generate an ‘observed’ data set for the 
scenario. 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). 
LITERATURE CITED 
Alden, R. W. Ill and E. S. Perry 1997. Presenting Measurements of Status: Report to the 
Chesapeake Bay Program Monitoring Subcommittee’s Data Analysis Workgroup. Chesa¬ 
peake Bay Program, Annapolis, Maryland. 
Appleton, E. 1996. Air quality modeling’s brave new world: A new generation of software 
systems is set to tackle regional and multipollutant air quality issues. Environmental Science 
and Technology 30(5):200A-204A. 
Appleton, E. L. 1995. A cross-media approach to saving the Chesapeake Bay. Environmental 
Science and Technology 29( 12):550-555. 
Bahner, L. 2001. The Chesapeake Bay and Tidal Tributary Volumetric Interpolator, VOL3D 
Version 4.0. National Oceanic and Atmospheric Administration, Chesapeake Bay Office. 
http://www.chesapeakebay.net/cims/interpolator.htm 
Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V. 
Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A. 
Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation 
chapter vi 
Recommended Implementation Procedures 
