DEVELOPING THE CUMULATIVE 
FREQUENCY DISTRIBUTION 
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The use of cumulative frequency distributions (CFDs) is recommended for assessing 
spatial and temporal water quality criteria exceedance in the Chesapeake Bay. CFDs 
offer a number of advantages over other techniques that are applied for this purpose. 
First, the use of CFDs is well established in both statistics and hydrologic science. 
CFDs have been used for much of the past century to describe variations in hydro- 
logic assessments (Haan 1977). For example, the U.S. Geological Survey has 
traditionally used CFDs to describe patterns in historical streamflow data for the 
purpose of evaluating the potential for floods or droughts (Helsel and Hirsch 1992). 
Second, the application of the CFD for evaluating water quality criteria attainment 
in the Chesapeake Bay allows for the evaluation of both spatial and temporal varia¬ 
tions in criteria exceedance. Methods currently used for the assessment of criteria 
attainment are based only on temporal variations because measurements are usually 
evaluated only at individual monitoring station locations. One of the limitations of 
this approach is that it is often difficult to determine whether an individual sampling 
location is representative, and there is always potential for bias. In a water body the 
size of the Chesapeake Bay, accounting for spatial variation can be very important 
and in that respect, the CFD approach represents a significant improvement over 
methods used in the past. 
A CFD is developed first by quantifying the spatial extent of criteria exceedance for 
every monitoring event during the assessment period. Compiling estimates of spatial 
exceedance through time accounts for both spatial and temporal variation in criteria 
exceedance. Assessments are performed within spatial units defined by the intersec¬ 
tion of Chesapeake Bay Program segments (see Figure VI-1) and the refined 
tidal-water designated uses (see U.S. EPA 2003 for specific boundaries), and 
temporal units of three-year periods. Thus, individual CFDs will be developed for 
each spatial assessment unit over three-year assessment periods. Details on the steps 
involved in developing CFDs are described below. 
STEP 1. INTERPOLATION OF WATER QUALITY MONITORING DATA 
The Chesapeake Bay Program partners collect monitoring data over a range of spatial 
scales and frequencies. Much of the water quality monitoring data collected in the 
Chesapeake Bay and its tidal tributaries is drawn from a limited number of fixed 
stations that are visited on a monthly (or more frequent) basis. Other types of data are 
collected at different spatial frequencies. For example, some chlorophyll a data are 
collected in a spatially continuous in-situ manner along the cruise tracks of moni¬ 
toring vessels. All of the different types of data are useful for assessing criteria 
attainment; however, they must be connected to a single spatial framework in order to 
provide a common basis for interpretation. Assessment of criteria attainment requires 
that conclusions be drawn for all locations within a spatial unit and not just the loca- 
chapter vi 
Recommended Implementation Procedures 
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