A-10 
Figure 2.1. Illustration of CFD for 12 dates. 
The steps: 
1. Collect data from a spatial network of locations on a series of dates in a three 
year assessment period . 
2. For each date, interpolate the data for the entire system (e.g. mainstem bay) to 
obtain estimates of water quality in a grid of interpolation cells. 
3. For each interpolation cell assess whether or not the criterion is exceeded. 
4. For each assessment unit (e.g. segment), compute the percentage of interpolator 
cells that exceed the criterion as an estimate of the percent of area that exceeds 
the criterion. 
5. Rank the percent of area estimates for the set of all sample days in the assess¬ 
ment period from largest to smallest and sequentially assign to these ranked 
percents a value that estimates percent of time. 
6. Plot the paired percent of time and percent of area data on a graph with percent 
of area on the abscissa and percent of time on the ordinate. The resulting curve 
is the Cumulative Frequency Diagram. 
7. Compare the CFD from a segment being assessed to a reference CFD. If at any 
point the assessment CFD exceeds the reference CFD, that is, a given level of 
spatial noncompliance occurs more often than is allowed, then the segment is 
listed as failing to meet it’s designated use. 
appendix a 
The Cumulative Frequency Diagram Method for Determining Water Quality Attainment 
