Methodology and Interpretation

Land-Cover Percent Agriculture / Grassland Change
The percentage of all agricultural land-cover and grassland is calculated (in each of the following time periods: early 1970s, 1980s, 1990s, and 2003) by dividing the number of all agricultural land-cover (i.e., pasture and crop) and grassland (e.g., non-agricultural grassy or herbaceous plant areas) 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). Row crop 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. Maintained areas of grassy or herbaceous plants (e.g., golf courses or lawns) may involve the application of fertilizers, which can be transported to streams or lakes in runoff. Thus, change in this metric may indicate an increase, decrease, or no change in vulnerability to these ecological services over time, among reporting units.

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

• 1970s Land-Cover - Metadata

• 2003 Land-Cover - Metadata