Changes in land use should be recorded to help isolate the water quality changes associated
with the NPS controls from water quality changes due to other land use factors. Land use modifications
that affect water quality include acres converted from row crops to pasture (permanently or based on
rotation), set-aside acres, changes in the number of animals or animal units per acre, closure of animal
operations, changes in impervious land areas, implementation of soil and water conservation practices
not being recorded as part of the project, and changes in non-agricultural land uses.
Matching of Land Treatment and Water Quality Data on a Spatial (Drainage) Scale
Land treatment data must be collected on a hydrologic or drainage basis so that the land area being
tracked corresponds to the drainage area served by each water quality monitoring station. Water
quality and land treatment data must be matched if water quality changes are to be attributed to BMP
Linkage of land treatment and water quality impacts can be made at different spatial scales
(such as farm field, subwatershed, or watershed). Spatial scale should be determined based on project
goals and desired interpretations. In general, the larger the drainage area, the harder it is to identify and
quantify a water quality - land treatment linkage. Water quality changes are more likely to be observed
at the subwatershed than watershed level. Confounding effects of external factors, other pollutant
sources, and scattered BMP implementation are minimized at the subwatershed level. If the goal is to
document changes at the entire watershed level, a monitoring station must be located at the watershed
Matching of Land Treatment and Water Quality Data on a Temporal Scale
quality and land treatment data should be collected during the same time periods so both data sets are
temporally related. Actual implementation of land treatment needs to be recorded at least seasonally or
annually. Land treatment data (such as timing of manure or commercial fertilizer applications,
construction of a lagoon storage structure, or a dairy closure) should be collected more frequently than
annually or seasonally if the effect on water quality is more short-term or has a large, immediate impact.
Water quality samples are usually collected weekly or biweekly. These data do not have to be
summarized on the same time scale as the land treatment data; land treatment data can be added to the
trend analysis as repeating explanatory variables. Alternatively, water quality data can be aggregated to
the same time scale as the land treatment data for analysis. Data aggregation is particularly useful for
plotting and explanatory data analysis.
Matching Monitored Pollutants with Pollutants Addressed by Land Treatment.
Pollutants monitored at water quality stations must correspond to pollutant(s) being treated by the BMP
Monitoring Explanatory Variables. Accounting for all major sources of variability in water
quality and land treatment data increases the likelihood of isolating water quality trends resulting from