Calculation of 7-day average streamflow condition in hydrologic units 

For each streamgage:

1. The 7-day average streamflow value is compared to all historical 7-day average streamflow values for the day of the year. For example, the 7-day average streamflow value for the 7 days prior to November 9, 2003 is compared to all other 7-day average streamflow values prior to November 9 for all years in the historical database. The 7-day average streamflow value and all the historical 7-day average streamflow values for the day of the year are sorted from lowest to highest. The percentile of the current daily streamflow value is its "position" relative to the other values in the sorted dataset and rescaled to range between 0 and 100.

For each hydrologic unit:

2. A median of the 7-day average streamflow  percentiles (from step 1) is computed for each regional hydrologic unit (2-digit hydrologic unit code, also know as a "huc2"), subregional hydrologic unit (huc4), hydrologic accounting unit (huc6), and hydrologic cataloging unit (huc8). (The cataloging units are subdivisions of the accounting units; the accounting units are subdivisions of the subregional units; and the subregional units are subdivisions of the regional units.) For each individual hydrologic unit, the median is based on all the streamgages located within the hydrologic unit. A median is computed for a hydrologic unit only if there are at least four streamgages with a 7-day average streamflow percentile located within the hydrologic unit.
3. When creating the hydrologic unit map, the cataloging unit (huc8) medians are drawn on top of the accounting unit (huc6) medians; the accounting unit (huc6) medians are drawn on top of the subregional unit (huc4) medians, and the subregional unit (huc4) medians are drawn on top of the regional (huc2) medians. This drawing procedure gives precedence to the hydrologic units with finer spatial detail.