More sophisticated and potentially more accurate methods utilizing 
computer models of basin hydrology and runoff quality are also 
available. These are expensive methods that require considerable input 
data to specify the configuration of the basin being modeled as well as 
to characterize the quantity and quality of the runoff. Available 
models have different limitations and assumptions and must be 
calibrated prior to model use for predicting future conditions. During 
calibration, model coefficients are adjusted until the quantity and 
quality of runoff observed during a previous rainfall event are 
successfully calculated. 
As a test case in Puget Sound the WQA method was used to estimate 
nonpoint source inputs into two embayments: 1) Elliott Bay, receiving 
runoff from the highly urbanized Seattle and Duwamish Waterway/Kent 
Valley areas; and 2) Skagit Bay, receiving runoff from the 
predominantly rural Skagit River Valley. The loadings calculated with 
WQA were then compared to loadings computed using flow and 
concentration data to examine the applicability of the method to 
quantifying nonpoint source loading to Puget Sound embayments. 
In general, the WQA procedure greatly underestimated the quantity of 
contaminant loading into the two embayments. The reason for the 
discrepancy lies in the selection of equation coefficients for the WQA 
model. The contaminant loading estimates obtained using WQA could have 
been forced to correspond to loadings computed using flow and 
concentration data by adjusting the coefficient values used (i.e., by 
calibrating the WQA coefficients to known conditions). However, 
coefficient adjustment would not be possible for embayments where 
nonpoint source loading could not be calculated by an independent 
method; coefficients determined for one embayment would not necessarily 
apply to another. Furthermore, the need to "calibrate" the WQA 
coefficients would defeat the purpose of identifying a simple, 
generalized method for quantifying nonpoint contaminant loading. 
This simple evaluation confirmed the difficulty of quantifying, with 
accuracy, contaminant loading associated with nonpoint sources. The 
overall usefulness of the WQA model or similar screening methods lies 
in applications where it is desired to make comparisons among 
embayments as to the relative significance of nonpoint sources to 
contaminant loading or to compare the relative importance of specific 
nonpoint sources to the overall loading within a given embayment. To 
obtain accurate estimates of nonpoint source contributions to Puget 
Sound detailed monitoring studies and modelling appear to be required. 
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