62 
The next step is to calculate a K d value for each cell and compare this value to a 
defined threshold. 
STEP 3. ATTAINMENT THRESHOLDS 
The following equation defines the relationship between the light attenuation coeffi¬ 
cient (K d ), PLW is the percent light through water, e is the base of the natural 
logarithms, K d is the value of the light attenuation coefficient, and Z is depth 
Equation 3: PLW = 100*exp ( K d Z) 
This equation can be used to determine the attainment thresholds at different depths 
and PLW’s (Table E-2). 
Table E-2. K d thresholds. 
Zones 
PLW 
Segment 
0-lm 
l-2m 
0-.5m* 
0.22 
Polyhaline, Mesohaline 
1.51 
0.76 
3.03 
0.13 
Oligohaline, Tidal Fresh 
2.04 
1.02 
4.08 
Each cell in the K d grid is evaluated against the appropriate ‘segment x zone’ 
threshold. For each segment, two comparisons are performed, one for each depth 
zone. For example, for Piankatank mesohaline segment, each cell must be less than 
or equal to the 1.51 threshold for zones where depth is 0-lm, and less than or equal 
to 0.76 where depth is 1-2 m. These two comparisons are merged to form an attain¬ 
ment grid (Figure E-4). Each cell in the attainment grid gets a value of one or zero, 
one if it meets the appropriate threshold and zero if it does not meet the appropriate 
threshold. Also, any designated Chesapeake Bay exclusion zones are removed from 
further analysis (Figure E-5). 
It is important to identify where this attainment is occurring in relation to other envi¬ 
ronmental factors. A code system is used to identify the presence/absence of historic 
and current SAV, and the depth zone for each cell in the grid. To determine the code 
for each cell in the grid: bathymetry, historic SAV, and current SAV are overlayed 
(Figure E-6). The resulting grid contains a representative, 3-digit code for each cell. 
The first digit indicates which bathymetric zone the cell is in, the 2nd digit desig¬ 
nates whether historic SAV is present or absent, and the last digit indicates whether 
current SAV is present or absent (Figure E-7). Finally, the attainment grid and zone 
codes are combined and the results are exported in table format to an ACCESS data¬ 
base for further analysis (Figure E-8). This method groups the attainment data by 
appendix e 
Chesapeake Bay Water Clarity Assessment Framework 
