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 program that combined land treatment and water quality monitoring in a continuous feedback loop to
document NPS control effectiveness (Gale et al., 1993; Spooner and Line, 1993).
2.2.2.2 Documenting a Cause-and-Effect Relationship
Documenting that water quality changes at a watershed scale were caused by implementation of
BMPs is difficult. Not only must a strong correlation be established, but the observed changes must be
repeatable over time and space in an experimental manner. The only major changes made in the
watershed during the evaluation period should be changes in land treatment. Observed changes in water
quality should match predicted pollutant reductions based on the estimated land treatment effectiveness.
Some projects have been able to document a strong relationship, increasing confidence that appropriate
land treatment can result in (cause) improved water quality. The stronger the relationship, the more
likely it is that a cause-and-effect relationship exists and that water quality changes are caused by
changes in land treatment rather than other factors.
An association (statistically significant correlation or relationship) between land treatment and
water quality changes is required to demonstrate a cause-and-effect relationship. As the implementation
of land treatment (specifically BMPs) occurs, improvements in water quality are observed. However,
an association by itself is not sufficient to infer a cause-and-effect relationship. Other factors not related
to BMP implementation may be causing the changes in water quality, such as changes in land use or
rainfall. If, however, the association is consistent and responsive and has a mechanistic basis, causality
may be supported (Mosteller and Tukey, 1977).
Consistency means that the relationship between the measured variables (such as total
phosphorus and acres treated with the nutrient management BMP) holds in each data set in terms of
direction and degree. A consistent, multi-year, improving trend in water quality after BMP
implementation provides evidence needed to attribute water quality improvements to land treatment.
Improvements in multiple watersheds treated with systems of BMPs provide strong evidence that water
quality improvements resulted from land treatment.
Responsiveness signifies that as one variable changes in a known, experimental manner, the
other variable changes similarly. For example, as the amount of land treatment increases, further
reduction of pollutant delivery to the water resource is documented.
Mechanistic means that the observed water quality change is that which is expected based on
the physical processes involved in the installed BMPs. For example, based on knowledge of absorption
and solubility of nutrients, greater reduction of nutrient delivery to the water resource might be predicted
as the result of implementation of the manure management BMP than a soil erosion control practice
alone.
2.2-2
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