application incentive payments only for the years in which the payments are
given. This will most likely underestimate the effectiveness of land application
incentive payments
3.2
in
BMP Effectiveness Estimates
3.2.1
Loading Factor Variability
The spread in cost-effectiveness values seen in the box-and whisker plots is a
result of the variability in nutrient loading factors across the 23 Chesapeake
subbasins in ecoregions 63 and 65. (See section 2.2.2 for a discussion of the
calculation process). However, the variability in loading factors seen in the 23
Chesapeake subbasins is not necessarily indicative of the loading variability in
the Tar-Pamlico basin. The variability in the Tar-Pamiico could be either smaller
or greater. Furthermore, the central tendency of the loading factors between the
two basins also may not be similar. A conservative estimate would be to assume
that the box-and-whisker plots represent the minimum variability in practice
effectiveness.
3.2.2
Unit Cost and Percent Effectiveness Variability
Site-specific factors can cause the costs for installing a practice/to vary
substantially. In our calculations of cost-effectiveness, we have used an average
cost value for practices installed in the Tar-Pamlico basin. Similarly, we have
used a single value for percent effectiveness, not a range. Consequently, the
variability in unit cost and effectiveness is not incorporated into the
whiskerplots of cost-effectiveness. Despite the fact that this variability is difficult
to quantify, it still greatly affects the variability of cost-effectiveness estimates and
therefore must be accounted for in the safety factor.
3.2.3
Site-Specific Variability
A
of site-specific factors can affect both the cost and effectiveness of a
`land management practice. Among these, soil type and slope are especially
important. Steep slopes or highly erodible land can increase the amount of soil
and/or nutrients lost from a site. In addition, the cost of constructing a BMP can
be greatly increased if site
require labor-intensive activities such as
land clearing or grading. Within the Tar-Pamlico basin, slopes and soil types vary
substantially between the Piedmont and Coastal Plain. In addition, some of the
BMP effectiveness data used in this report are taken from studies conducted in
the Chesapeake Bay basin. Where possible, we used only results from
ecoregions present in both basins. Nevertheless, conditions within a given
ecoregion may also vary substantially.
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