Methodology and Interpretation

Percent Total Agriculture / Grassland within 120m of Streams
The percentage of all agricultural/grassland 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.

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. For continuity of the browser content, and consistency among maps, legend gradients are from higher values (red) to lower values (green).

Metric input GIS data:

• 244 Subwatersheds - Metadata

• 2003 Land-Cover - Metadata

• Stream Network - Metadata