Methodology and Interpretation
Percent total agriculture
The percentage of all agricultural land cover is calculated by dividing the number of all
agricultural land cover (i.e., pasture and crop) cells in the reporting unit by the total
number of land cover cells in the reporting unit minus those cells classified as water
(i.e., total land area). Agricultural practices typically employ fertilizers, pesticides
and other chemicals that may be transported to streams in water runoff. The closer agriculture
is to a stream the more likely related pollutants will enter the stream. Concentrations of
pollutants transported into streams are also more likely to be higher when agriculture is
closer to streams. Animals grazing on pastures may decrease vegetation cover, possibly leading
to increased runoff and erosional soil loss, which may result in increased stream sedimentation.
Livestock may also degrade within-stream and stream-bank ecological functions by defecating in
the streams and trampling riparian vegetation, respectively.
Quantile: Each class contains an approximately equal number (count) of features. A quantile
classification is well-suited to linearly distributed data. Because features are grouped by the number
within each class, the resulting map can be misleading, in that similar features can be separated into
adjacent classes, or features with widely different values can be lumped into the same class. This
distortion can be minimized by increasing the number of classes.
Metric input GIS data:
- United States Hydrologic Units (8-digit HUCs) - Metadata
- United States Coastal Change Analysis Program (CCAP) - Metadata
- Canada Hydrologic Units (Subsubdivisions) - Metadata
- Ontario Ministry of Natural Resources Data Set - Metadata