3. The Need For A
Factor
3.2.4
Masking
When monitoring water quality to determine the effectiveness of a BMP in
reducing nutrient loads, it is important to be able to isolate the effect of that BMP
of
on water quality. In practice, this is a very difficult
(or
process because many activities can occur in the watershed
the
effect of the practice. For example, a
outside the BMP project, but within
the monitored watershed, may increase-his swine production while downstream
monitoring is being conducted. This lack of control over external loading factors
can introduce substantial uncertainty into BMP effectiveness calculations.
BMP Assemblages
3.2.5
are implemented on a farm as part of a "Resource
Often, multiple
Management System." The implementation process may be staggered over a
period of months or years. When monitoring is conducted to evaluate
effectiveness, it becomes difficult, if not impossible, to ascribe load reductions to
individual
Reduction effectiveness values for a particular assemblage of
practices are also difficult to use because the particular combination of practices
is often tailored to the needs of the farm and will be different from the combination
of practices on other farms.
I
3.2.6
Groundwater Loads
Research from the Chesapeake Bay Program (Camacho, 1992) has indicated
that increase infiltration may route a substantial portion of the
that in-field
nutrient load into the groundwater. Effectiveness values based only on surface
water monitoring do not account for this process. Where avaiiable, we have
incorporated effectiveness values that represent the net effect of changes in both
surface and groundwater loads.
3.2.7
BMP Longevity
in-field
are typically assigned a "life span" indicating the time period for
which the BMP is expected to function effectively, given proper maintenance.
There is little
regarding the extent to which BMP effectiveness
changes over the life span
If the effectiveness of a practice
decreases over time, then a cost-effectiveness value based on data from a newly
installed practice may overestimate the actual cost-effectiveness of the practice.
3.2.8
Short-Term Efficiencies
Some of the BMP effectiveness studies used as in this analysis focused on
term efficiencies from single rainfall events. Extrapolation of these efficiencies to
annual or long-term efficiencies is questionable due to annual hydrologic, crop,
and farm activity changes (Camacho, 1992).
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