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