7 
space. The results of these queries can be presented in graphical form with percent 
of time plotted against percent of space. 
Figure II-2 illustrates a typical CFD based on 12 measures of spatial extent of criteria 
exceedance over time. In general, if a segment is in attainment with the criterion, 
then one expects a high frequency of dates for which the percent out of attainment 
is low. In this case, the CFD should descend rapidly from the upper left comer, pass 
not far from the lower left corner, and then proceed to the lower right comer. The line 
in Figure II-2 shows the typical hyperbolic shape commonly observed using the CFD 
to assess water quality criteria in the Chesapeake Bay. The closer the CFD curve 
comes to the origin (lower left comer), the better the attainment of the assessed 
segment. A curve that is far from the origin indicates that a larger percent of space 
in the segment is out of attainment and the probability of use impairment increases. 
The CFD methodology offers many advantages over other criteria assessment 
approaches. Through interpolation, it provides a method for using data collected in 
areas surrounding the area of interest (the spatial assessment unit). This factor is 
important since the sample size of observations within a spatial assessment unit may 
not be sufficient to determine the area (or volume) of exceedance within the unit 
accurately. The method also weights the data collected from a given location 
according to the amount of area (or volume) that the location represents. This capa¬ 
bility is important because data may be collected from locations that do not represent 
Figure 11-2. A water quality criteria attainment assessment cumulative frequency diagram 
(CFD) based on 12 measures of the spatial extent of criteria exceedance over time. 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
