0 10 20 30 40 50 60 70 80 90 100 
Percentage of Area/Volume Exceeding the Criteria 
Figure VI-18. Cumulative frequency distribution curve in the shape of a hyperbolic curve 
that represents approximately 10 percent allowable exceedances equally distributed 
between time and space. 
in recent years. Separate reference curves were developed for low salinity—tidal- 
fresh and oligohaline-and higher salinity-mesohaline and polyhaline-zones. The 
supporting analyses for deriving the water clarity criteria reference curves are 
provided in Appendix H. 
Once the reference Chesapeake Bay Program segments were identified, the water 
clarity data (as measured by Secchi depth) for those segments were extracted from 
the Chesapeake Bay water quality monitoring program data base. Percent light- 
through-water (PLW) is the operational parameter used for assessing attainment of 
the water clarity criteria. PLW = 100exp(-K d Z), where Z is the target restoration 
depth and K d , the coefficient of extinction, is estimated as K d = 1.45/Secchi depth 
(see Chapter III for details). K d values calculated from the Secchi depth data were 
averaged by month for each station. The monthly data were then spatially interpo¬ 
lated bay wide for each month in the underwater bay grass growing season from 1985 
through 1994 to match the Chesapeake Bay water quality model hydrologic simu¬ 
lation period. PLW was calculated for each interpolation cell using the interpolated 
K d value and the defined segment-specific restoration depth. The PLW values were 
compared to the criterion value appropriate to the Chesapeake Bay Program 
segment’s salinity zone, and the percent of the shallow-water area (< 2 meters) 
failing the criterion in each segment was calculated for each month. The monthly 
chapter vi • Recommended Implementation Procedures 
