2. Development of BMP Cost-Effectiveness Values
Effectiveness data for vegetated filter strips and terracing are available from
numerous studies; however, only a portion of the studies are applicable to
conditions in the Tar-Pamlico basin. Often, these studies reported effectiveness
and subsurface nutrient load. To convert
as the percent reduction
loading
these percentages to mass reductions (kilograms), we used the
factors
the Chesapeake Bay Watershed Model for ecoregions 63 and 65.
The box and whisker plots of effectiveness (and cost-effectiveness) that we
present for certain practices are the result of multiplying an estimated
effectiveness percentage by the range
loading factors.
the
range of cost-effectiveness values is indicative..of the spatial variability in loading
across the 23 watersheds, but not of the variability associated with BMP cost or
effectiveness. Appendix 4 lists the loading factors for the Chesapeake subbasins
in ecoregions 63 and 65. Note that the difference in spread between `ecoregions
seen in the box and whisker plots is primarily due to the different number of
loading factors used for each ecoregion. For ecoregion 63, N = 16; for ecoregion
65, N = 7.
For the remaining major cost-share practices (cropland conversion, grass
waterways, diversions, and stripcropping), the available data were either
insufficient or too widely scattered to make justifiable estimates of nutrient
reduction effectiveness. For these practices, only cost data are presented.
are not
It is important to note that BMP effectiveness values for
necessarily additive. For example, a practice installed on conventional
may result in a
percent net nutrient load decrease. However, the same
practice installed on conservation
will not necessarily yield
percent
further reduction in the runoff nutrient load.
2.2.3
Cost-Effectiveness Calculation Methods
The approach that we have used to calculate the cost-effectiveness of
agricultural management practices is similar in some ways to the original
basin approach described in Appendix 5. The total money spent on management
practices is divided by the total mass reductions in order to calculate an
effectiveness ratio of the form $/kilogram of nutrient load reduced. However, our
approach is also different in several ways:
We have used the results of
monitoring and modeling studies to
estimate BMP effectiveness. These data did not exist when the
study was conducted.
While the
study presented a cost-effectiveness value for
management practices in general, we have calculated cost-effectiveness for
several practice types (where the data permit).
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