The shape of the curve also indicates the spatial and temporal pattern of criteria 
exceedance. Figure VI-8 illustrates three potentially observable CFD plots. Curve (a) 
indicates a situation in which the water quality criteria are chronically exceeded in a 
relatively small amount of a given segment. Managers could use this information to 
target segments for further monitoring and assessment and to identify chronic prob¬ 
lems and tailor management plans to address them. Curve (b) illustrates a situation 
where criteria are exceeded on a broad spatial scale, but relatively infrequently. Such 
broad-scale acute problems should be evaluated individually. If the frequency and 
duration of broad-scale criteria exceedances were low enough, ecological impacts 
could be limited. On the other hand, some short-term exceedances can have signifi¬ 
cant ecological effects. Curves (a) and (b) reflect a similar degree of overall criteria 
exceedance; however, the exceedance of curve (a) is primarily temporal, and the 
exceedance of curve (b) is primarily spatial. Curve (c) reflects broad-scale criteria 
exceedance in both space and time. The shape of the curves should be used for diag¬ 
nostic purposes only. Decisions regarding full attainment should be based on the 
overall amount of criteria exceedance indicated by the area under the curve. 
As discussed above, it is possible that some spatial and temporal criteria exceedances 
could be observed, without necessarily having significant effects on ecological 
health or on the designated use of a portion of the Chesapeake Bay. Such 
exceedances are referred to as ‘allowable exceedances.’ Such exceedances have been 
Figure VI-8. Use of cumulative frequency distribution to characterize patterns of water 
quality criteria exceedance. Curve (a) indicates that criteria are chronically exceeded in a 
relatively small portion of the spatial unit. Curve (b) indicates that criteria are exceeded 
over a large portion of the spatial unit on a relatively infrequent basis. Curve (c) indicates 
that criteria are exceeded over large portions of space and time. 
chapter vi 
Recommended Implementation Procedures 
