ADDRESSING MAGNITUDE, DURATION, 
FREQUENCY, SPACE AND TIME 
( 
To define and measure criteria attainment, a number of factors are taken into 
account. According to a recent National Research Council (2001) review, estab¬ 
lishing the “magnitude, duration and frequency” of a condition is crucial for 
successful development and application of state water quality standards. Equally 
important is the spatial extent of a condition, and the spatial and temporal dimen¬ 
sions of attainment assessment must be defined. 
Magnitude refers to how much of the pollutant—or a given quantifiable measure of 
condition—can be allowed while still achieving the designated uses. Magnitude is 
assessed through a direct comparison of ambient concentrations with the appropriate 
Chesapeake Bay criterion value. The magnitude of nonattainment of a criterion value 
also provides information useful to making management decisions on taking correc¬ 
tive actions. 
Attainment of all three Chesapeake Bay criteria should be assessed by Chesapeake 
Bay segment (Figure VI-1; Table VI-4), separately for each designated use habitat. 
Therefore, each designated use habitat in an individual Chesapeake Bay Program 
segment is considered a spatial assessment unit. This is consistent with the scale of 
data aggregation and reporting for Chesapeake Bay tidal-water quality monitoring 
and the physical scale of the designated use areas. 
Criteria attainment should be presented in terms of spatial extent, i.e., the percentage 
of the volume (dissolved oxygen) or surface area (water clarity, chlorophyll a) of the 
particular designated use habitat in each Chesapeake Bay Program segment that 
meets or exceeds the applicable criteria. Measuring spatial extent will be enabled 
through the use of spatial interpolation methods, which are described later in this 
chapter. Such ‘interpolators’ work by dividing a water body into a three-dimensional 
grid, with cell size depending on data density and the application’s resolution 
requirements, among other factors. 
Duration is defined as the period over which exposure to the constituent of concern 
is to be averaged within the assessment period (see below) to prevent detrimental 
effects. Duration can also be thought of as the allowable time of exposure before 
effects occur. For example, the open-water dissolved oxygen criteria includes a crite¬ 
rion with a magnitude of 5 mg liter" 1 evaluated as a 30-day mean; another criterion 
has a magnitude of 4 mg liter" 1 evaluated as a 7-day mean. 
The dissolved oxygen, water clarity and chlorophyll a criteria are season-specific, 
and attainment should be measured only over the applicable season. For example, 
attainment of the dissolved oxygen criteria for the migratory fish spawning and 
nursery designated use should be assessed and reported for the period of February 1 
through May 31; attainment of the open-water fish and shellfish designated use 
chapter vi 
Recommended Implementation Procedures 
