22 
Table IV-1. Revisions to Water Clarity Acres Attainment Assessment Methodology. 
Procedure 
2007 Addendum 
2008 Addendum 
Assessment calculation 
U.S. EPA 2007, p54. “Assessment of 
attaining a segment’s water clarity 
restoration acreage should be based on 
a calculation of the arithmetic mean of 
the year-by-year means of a month-by- 
month accounting of water clarity 
acres over the three year SAV growing 
season assessment period.” 
Water clarity acres for the segment are 
calculated by the taking the annual 
mean of the monthly acreage within 
the SAV growing season. Single best 
year assessments are compared with 
segment SAVAVater Clarity restoration 
acreage goals. 
Assessment calculation and 
interpolation 
U.S. EPA 2007, p54-55. “Calculation of 
water clarity acres should be based on 
spatially intensive shallow-water moni¬ 
toring turbidity data converted to K d . 
interpolated as described in Chapter 2 
and then compared to the corresponding 
K d threshold assigned to each interpo¬ 
lator grid cell.” 
U.S. EPA 2007, p80, “The very dense 
in situ measurements of turbidity from 
each sampling cruise track are first 
converted to K d . The natural log of the 
converted K d values are then interpo¬ 
lated using a standardized ordinary 
kriging procedure with ARC/GIS into 
a 25-meter square grid over the 
segments entire surface area. Once 
interpolated, the resultant interpolated 
K d values are transformed back.” 
Calculation of water clarity acres 
should be based on spatially intensive 
shallow-water monitoring data for 
turbidity, chlorophyll a and salinity in 
order to convert results to K d . Within 
each segment, the individually interpo¬ 
lated chlorophyll, turbidity, and 
salinity layer grid results are input into 
the appropriate equation on a matching 
25-m 2 cell-by-cell basis. The result of 
this cell-specific calculation of K d is 
based on region-specific multiple 
regression model equations (Table IV- 
2. Appendix D). The result is a new 
grid representing the K d surface. The 
K d grid is compared to the appropriate 
K d threshold on a cell-by-cell basis to 
create the attainment grid. The attain¬ 
ment grid results are stored in a 
database and used to calculate water 
clarity acres by initially converting cell 
counts of attainment into acreage of 
attainment inside and outside of 
current mapped SAV areas for each 
segment. As previously defined, attain¬ 
ment evaluations account for any SAV 
no grow zones by removing them 
before conducting final calculations for 
the segment (U.S. EPA 2007). Water 
clarity acres for each segment are then 
calculated by taking the annual mean 
of the monthly acreages. 
chapter iv • 
Refinements to Procedures for Assessing Chesapeake Bay Water Clarity and SAV Criteria 
