criteria, as applied to both open- and shallow-water bay grass designated uses, 
should be assessed and reported seasonally, in winter (December, January and 
February), spring (March, April and May), summer (June, July, August and 
September) and fall (October and November). Tables VI-1 and VI-2 define ‘seasons’ 
and applicable criteria for dissolved oxygen and water clarity, respectively. Numer¬ 
ical chlorophyll a criteria should be applied to the spring and summer seasons 
defined previously. 
The assessment period refers to the most recent three consecutive years for which 
relevant monitoring data are available. In circumstances where three consecutive 
years of data are not available, a minimum of three years within the most recent five 
years should be used. 
A three-year period is consistent with the water quality status assessment period 
used for over a decade by the Chesapeake Bay Program partners (e.g., Alden and 
Perry 1997). A three-year period includes some natural year-to-year variability 
largely due to climatic events, and it also addresses residual effects of one year’s 
conditions on succeeding years. Two years is not enough time to assess central 
tendency, and four or more years delay response to problems that may be detected. 
Longer periods are more appropriate for detecting trends than for characterizing 
current water quality conditions. 
A comparison of criteria attainment across one-, three- and five-year assessment 
periods confirmed the selection of three years as the appropriate temporal averaging 
period. Attainment levels were highly variable using single-year periods. The five- 
year period smoothed much of the variability and resulted in little difference 
between one assessment period and the next. 
The allowable frequency at which the criterion can be violated without a loss of the 
designated use also must be considered. Frequency is directly addressed through 
comparison of the generated cumulative frequency distribution with the applicable 
criterion reference curve. All values falling below the reference curve are considered 
biologically acceptable exceedances of the applicable Bay criteria. Through its deri¬ 
vation, the reference curve directly incorporates a biologically acceptable frequency 
of exceedances of the applicable Chesapeake Bay criteria. 
By combining these factors to measure attainment, the spatial extent of violation or 
attainment of the criterion can be determined for each designated use within each 
Chesapeake Bay Program segment at temporal increments defined by the criterion. 
As the next section describes, the frequency of these occurrences is tallied for each 
season over the assessment period. 
chapter vi • Recommended Implementation Procedures 
