Methodology and Interpretation

Percent total agriculture within 120 meters of streams
The affects of runoff from areas adjacent on streams is strongly influenced by land cover type within approximately 120 meters of the streambank. Thus, the percentage of all agricultural land cover within 120 meters of streams is calculated by summing the total number of pasture and cropland land cover cells underneath stream segments in the reporting unit and within a four cell buffer (120 meters) and dividing by the stream corridor's total land area (i.e., all cells within 120 meters of streams minus those classified as water). Cells inside the cell buffer zone but outside of the reporting unit boundary are ignored. 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. Other distances may be more appropriate, depending on the runoff chemical constituent(s), flow dynamics, soil conditions, stream channel morphology, and other landscape characteristics.

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

• United States National Elevation Dataset (NED) - Metadata