Coverage Through Time
Monitoring for multiple years before and after BMP implementation is essential
for successful documentation of a change in water quality at the subwatershed or
watershed level. Water quality and land use monitoring prior to BMP implemen-
tation is required to establish baseline data for statistical comparisons with
BMP data. Consistent sampling frequency and sample collection procedures must
be maintained across seasons and years.
Year-to-year variability is so large that at least two to three years each of pre- and
post- BMP water quality monitoring are required to indicate the improvement in
water quality is consistent. Expected changes that remain consistent over time
improve the relationship between land treatment or land use and water quality.
Short-term monitoring is seldom effective because climatic and hydrologic
variability can mask water quality changes. However, for small watersheds
affected by a few relatively large pollutant sources, the required monitoring period
may be shorter. Longer monitoring time periods are required for watersheds in
which water quality changes occur gradually. For example, there may be a lag time
for water quality changes to be observed in response to land treatment in large
watersheds and lakes. This lag time may be due to a buffering effect of long
hydraulic residence times and recycling of pollutants.
Experimental Design for
A good experimental design for water quality and land treatment monitoring is
Linking Water Quality
essential to document a strong relationship between land treatment and water
quality changes. Common designs include: the paired watershed design, up-
and Land Treatment
stream-downstream sites monitored before and after land treatment implementa-
tion, or multiple watershed monitoring.
The paired watershed design is the best for documenting BMP effectiveness in the
shortest number of years (at least 3-5 years). This design involves the monitoring
of two or more similar subwatersheds before and after implementation of
in one of the watersheds. The paired drainages should have similar precipitation
runoff patterns.
Matching Land
Land treatment data must be collected on a hydrologic or drainage basis such that
the land area being tracked corresponds to the drainage area served by the water
Treatment and Water
quality monitoring station. Being able to match water quality data with land
Quality Data on a
treatment data increases the likelihood of being able to attribute water quality
Drainage Scale
changes to
The more direct the linkage, the stronger the evidence for the
direct effects from land treatment/land use changes on water quality. The land
treatment and water quality data bases must be collected and summarized to the
spatial scale desired.
The linkage of land treatment to water quality impacts can be made at the farm
field, subwatershed, watershed, or project level. The scale of monitoring is a
function of the monitoring objective. In general, the larger the drainage area, the
harder it is to identify and quantify the linkage. Subwatershed monitoring is the
most effective for demonstrating water quality improvements from a system of
Water quality changes are more likely to be observed at the subwatershed
level compared to a larger watershed level. Confounding effects of external
factors, other pollutant sources, and scattered BMP implementation are mini-
mized at the subwatershed level. However, it is important to locate a monitoring
station at the watershed outlet if changes at the watershed level are to be
documented
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