DIAGNOSING THE MAGNITUDE OF 
CRITERIA EXCEEDANCE 
The CFD is a useful tool for evaluating water quality criteria attainment, but it is 
based on pass/fail principles and provides no information on the magnitude of 
criteria exceedance, which would interest managers, because it indicates how much 
effort is needed to correct any impairment. To fill this need and provide supporting 
information for the CFD, it is recommended that interpolator plots be generated for 
each monitoring event conducted during an assessment period. Viewed either 
individually or as a movie, interpolator plots will shed light on the magnitude of 
exceedance during the assessment period. 
Two types of interpolator plots are useful for this purpose. The first is the basic inter¬ 
polator plot of the criteria parameter (i.e., concentration for dissolved oxygen and 
chlorophyll a, and percent light-through-water for water clarity; Figure VI-12). Such 
Chlorophyll a 
Relative to Possible Criteria 
0-3 
3-0 
6-10 
10-30 
30-44 
45-53 
54-62 
Figure VI-12. Example plot of chlorophyll a concentration (/jg liter 1 ) estimates 
generated through spatial interpolation for purposes of evaluating the magnitude 
of criteria exceedance. 
chapter vi 
Recommended Implementation Procedures 
