44 
appendix C 
A Comparison of Methods 
for Estimating K d 
The light attenuation coefficient (K d ) is used to assess the Chesapeake Bay water 
clarity criteria, measured as percent light-through-water (PLW), using the following 
equation: 
PLW = 100*exp (K d Z) 
where ‘exp’ is the base of the natural logarithms and Z’ equals the criteria applica¬ 
tion depth. 
K^j is measured in situ at DATAFLOW calibration stations using LICOR and then 
related to other measured parameters—turbidity, chlorophyll fluorescence, and 
salinity—to generate a calibration curve that enables the estimation of Kj at cruisetrack 
points. The spatially intensive nature of DATAFLOW data support the interpolative 
analysis used to produce the cumulative frequency diagram applied in criteria assess¬ 
ment. However, can be interpolated in two ways. In one method. Kj is calculated at 
each cruisetrack point using the three simultaneously measured parameters, and then 
it is interpolated. This method is described in Chapter 7 of the 2007 Ambient Water 
Quality Criteria for Dissolved Oxygen , Water Clarity and Chlorophyll afar the Chesa¬ 
peake Bay and its Tidal Tributaries - 2007 Addendum (U.S. EPA 2007). The other 
method calls for first interpolating turbidity, chlorophyll a , and salinity and then using 
the resulting estimates of these parameters to calculate Kj. This method was used by 
both Virginia Institute of Marine Science (VIMS) and Maryland Department of 
Natural Resources (MD DNR) for the 2008 water clarity assessment. 
The two methods were compared to determine if they produce similar results. 
Between 100-200 “validation” points were randomly selected and removed from 
three James River DATAFLOW cruisetracks (Figures C-l, C-2 and C-3). The 
remaining cruisetrack points were then analyzed using the two methods (i.e., calcu¬ 
lating K d from its correlated parameters prior to interpolation versus calculating K d 
after interpolating its correlated parameters). K d was calculated at each validation 
point using the turbidity, chlorophyll, and salinity measured at that point. This value 
was then compared to the estimated K d values generated from the two methods. 
The following equation (see Chapter IV, Table IV-2 of this document) was used to 
calculate K d : 
K d = 1.19267 + 0.2956*Turbidity (1/L5) - 0.05616*Salinity + 0.0002746* 
Chlorophyll a 
appendix c 
A Comparison of Methods for Estimating 
