9 
The CFD methodology was first applied in the Chesapeake Bay for the most recent 
303(d) listing cycle, completed in the spring of 2006 and based on data from 2002 
through 2004. The CFDs were developed and used primarily for the dissolved 
oxygen open-water and deep-water 30-day mean criteria because insufficient data 
and data analysis techniques existed to assess the higher-frequency dissolved oxygen 
criteria components. Similarly, the water clarity criteria were not assessed based on 
the CFD because few tidal systems had sufficient shallow-water monitoring data for 
an assessment. 
In fall 2005, the Chesapeake Bay Program’s Scientific and Technical Advisory 
Committee (STAC) established a scientific panel to review and refine the CFD 
assessment methodology. Nationally recognized academic experts in spatial and 
environmental statistics made up the panel. The STAC-convened panel concluded 
that the CFD approach is both feasible and innovative, qualifies as the best available 
science, and represents an improvement over criteria assessment methods used in the 
past (STAC 2006). 
The panel also recognized, however, that the approach remains in the early stages of 
management application. It stated that the CFD approach deserves further directed 
study and analysis to evaluate the bias and imprecision that can occur due to limita¬ 
tions in available data and in current interpolation and CFD algorithms (STAC 
2006). This chapter provides guidance for criteria assessment application, summa¬ 
rizes findings from the CFD evaluations, and offers recommendations for further 
refinement of the CFD assessment methodology. Appendix A provides a complete 
copy of the scientific panel’s final report. 
DESCRIPTION AND EVALUATION OF THE CFD-BASED 
ASSESSMENT METHODOLOGY 
The methodology for estimating the CFD is most easily described as a series of eight 
steps as shown in Table II-1. These steps, described below, provide a framework for 
considering the process and are elucidated by a simple example. More detailed 
discussions of each step follow later in this chapter. 
EXAMPLE CFD-BASED CRITERIA ASSESSMENT 
To illustrate the CFD criteria assessment methodology, a simple theoretical example 
based on a small data set can prove useful. Assume a segment for which the inter¬ 
polation grid is 4 cells by 4 cells. In reality, the number of grid cells is much larger 
(hundreds to thousands), but this small grid is illustrative. Also assume that data 
were collected on five distinct dates, and that each date is representative of the appro¬ 
priate time scale (in an actual application, data would be collected over many more 
dates). The criterion threshold for this fictitious water quality parameter is 3. 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
