Simulated Field Conditions/Sampling Techniques
Some of the BMP effectiveness studies used in this report were carried out in
systems. For example, experimental data are frequently
highly
obtained from highly managed demonstration systems that frequently have
rigorous design and control requirements (e.g., artificial rainfall, intensive site
preparation, carefully controlled fertilizer application). Extrapolating such results
to "the real world" requires a significant amount of subjective judgment.
a small-field scale, and does not
Furthermore, research is typically
account for important "landscape-scale" factors, such as the location
a field in
a watershed, the size of the area treated relative to the watershed size, adjacent
land cover, or the distance to receiving waters.
Modeling Results
For certain practices, this study relies on loading factors and effectiveness data
from the Chesapeake Bay Watershed Model (Appendix 2). We have not
investigated in detail the assumptions and validity of the Chesapeake modeling
process. However, the Chesapeake Bay Program has accepted the results of the
model as the basis for developing a
source nutrient management plan in
the Chesapeake basin. Nevertheless, it is important to note that modeling results
are subject to uncertainty related to both the model's mechanisms and the input
parameters. Where we have used the modeling results for cost-effectiveness
calculations. we believe that they represent the best data available.
3.3
Economic Uncertainties
3.3.1
Decreasing Returns
As more money is spent on
in the Tar-Pamlico basin, we may begin to see
decreasing returns on invested dollars. Early in the program, the State may
realize a high cost-effectiveness because the practices are presumably being
used on farms with the worst nutrient loading problems. However, once the worst
cases are addressed, it may become increasingly expensive to reduce a given
to estimate at what point decreasing returns will
level of nutrient load. It is
become a factor. However, they should be considered when developing a safety
factor.
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