This dataset provides the dominant and secondary hydrologic class within each 8-digit Hydrologic Unit Code across the United States. Hydrologic classes were created using statistics calculated from discharge information from 2,618 (unregulated) USGS streamflow gages with reference, semi-disturbance, or pre-dam regulation data (USGS National Water Information Serve: http://waterdata.usgs.gov/usa/nwis/si). Hydrologic classes are groups of streams that share similar hydrologic properties, i.e. simliarity in streamflow characteristics, and may respond similarly to impoundment. Hydrologic classes provide a template to describe ecological patterns, generalize hydrologic responses to disturbance, and stratify research and management needs. Hydrologic classes were assigned to the 2,618 gages using clustering analysis. For the remaining gages (non-reference), predictive models using landscape and climate information were used to predict class membership.
The 2,618 unregulated USGS stream gages were mapped in ARC GIS 9.3 by latitude/longitude coordinates. Theisen polygons [Analysis (Proximity) toolset] were created for each USGS, and each dominant and secondary hydrologic class was assigned to 8-digit Hydrologic units. In addition, the number of hydrologic classes represented within each 8-digit hydrologic unit was summarized.
Class Names and Descriptions are provided below:
- Intermittent Flashy 1 = High intermittency, long high-flow duration; Mixed US distribution, mostly western (centralized to ND, SD).
- Late Timing Runoff = somewhat stable, later annual maximum (different reasons for late timing in various geographical areas - e.g. hurricanes (FL), rainfall (west). Coastal areas but very mixed (inland).
- Perennial Runoff 1 = higher variability, different timing; Southeast Piedmont and Uplands.
- Perennial Runoff 2 = higher baseflow, different timing than PR1; Northeast Piedmont and Uplands
- Super Stable Groundwater = very high baseflow, stable, probably occuring around spring-fed areas; Mixed US distribution.
- Stable High Baseflow = relatively high BF, stable, relatively high runoff; SE blue ridge mountains
- Intermittent Flashy SW = High intermittency, long high-flow duration, short low-flow duration; SW California, AZ.
- Snowmelt 2 = slightly higher BF, different timing max flows and flow durations; Western Rockies, great lakes.
- Unpredictable Intermittent = Low predictability, somewhat flashy, later annual max than class PR2; Eastern US
- Intermittent Flashy 2 = High intermittency, late maximum; Mixed US distribution.
- Western Coastal Runoff = distinct wet/dry seasons, early maximum (altered timing); Western Coast (CA)
- Stable High Runoff = higher runoff, stable; Upper Western Coast (WA, OR)
- Harsh Intermittent = long periods of intermittency, and periods of episodic flows; SW US.
- Snowmelt 1 = slightly lower BF, different timing max flows and flow duration; Eastern Rockies
- Glacial High Runoff = predictable, minimum occurs in winter, long high-flow duration; high elevation areas (MT, WY) and arctic (AK)"