A-9 
Reasoning Behind the CFD Approach 
The CFD assessment methodology evolved from a need to allow for variability in 
water quality parameters due to unusual events. For the water quality parameter to 
be assessed, a threshold criterion is established for which it is determined that water 
quality that exceeds this threshold is in a degraded state (For simplicity, we will 
speak of exceeding the threshold as representing degradation, even though for some 
water quality constituents such as dissolved oxygen, it is falling below a threshold 
that constitutes degradation). Because all water quality parameters are inherently 
variable in space and time, it is unlikely that a healthy bay will remain below the 
threshold in all places at all times. In the spatial dimension, there will be small 
regions that persistently exceed the threshold due to poor flushing or other natural 
conditions. It is recognized by CBP that these small regions of degraded condition 
should not lead to a degraded assessment for the segment surrounding this small 
region. Similar logic applies in the temporal dimension. For a short period of time, 
water quality in a large proportion of a segment may exceed the threshold, but if this 
condition is short lived and the segment quickly returns to a healthy state, this does 
not represent an impairment of the designated use of the segment. Recognition that 
ephemeral exceedances of the threshold in both time and space do not represent 
persistent impairment of the segment leads to an assessment methodology that will 
allow these conditions to be classed as acceptable while conditions of persistent and 
wide spread impaired condition will be flagged as unacceptable. The assessment 
methodology should first ask how much of the segment (for simplicity, a spatial 
assessment unit is called a segment, but more detail is given on spatial assessment 
units in Section 2) is not in compliance with the criteria (percent of space) for every 
point in time. In a second step the process should ask how often (percent of time) is 
a segment out of compliance by more than a fixed percent of space. The results from 
these queries can be presented in graphical form where percent of time is plotted 
against percent of space (Figure 2.1). It is arbitrary to treat space first and time 
second. A similar diagram could be obtained by first computing percent noncompli¬ 
ance in time and then considering the cumulative distribution of percent time over 
space. 
If a segment is generally in compliance with the criterion, then one expects a high 
frequency of dates where the percent out of compliance is low. In this case, the CFD 
should descend rapidly from the upper left corner and pass not too far from the lower 
left comer and then proceed to the lower right comer. The trace in Figure 2.1 shows 
the typical hyperbolic shape of the CFD. The closer the CFD passes to the origin 
(lower left corner), the better the compliance of the segment being assessed. As the 
CFD moves away from the origin, a higher frequency of large percents of space out 
of compliance is indicated. 
Formulating an Estimate of the CFD 
The algorithm developed by CBP for estimating the CFD is most easily described as 
a series of steps. These steps are given in bullet form to provide a frame work for the 
overall approach. The quickly defined framework is followed by a simple example. 
This in turn is followed by more detailed discussion of each step. 
appendix a 
The Cumulative Frequency Diagram Method for Determining Water Quality Attainment 
