Calculation of recent streamflow condition in hydrologic units

Calculation of historical runoff in hydrologic units

A. Calculation of recent streamflow condition in hydrologic units

For each streamgage:

1. The k-day average streamflow value is compared to all historical k-day average streamflow values for the day of the year. For example, the k-day average streamflow value for the k days prior to November 9, 2003 is compared to all other k-day average streamflow values prior to November 9 for all years in the historical database. The k-day average streamflow value and all the historical k-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 k-day average streamflow  percentiles (from step 2) 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 k-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.
   

B. Calculation of historical runoff in hydrologic units

USGS daily streamflow data for water-years 1900-2002 were used to estimate runoff (streamflow per unit area) for the hydrologic cataloging units in the conterminous United States following the approach of Krug et al. (1987). This was accomplished by first identifying streamgages with the following characteristics: (1) complete data for each water-year, (2) drainage basins contained entirely within a cataloging unit and (3) drainage areas that did not overlap with the drainage basins of other streamgages in the same cataloging unit. The average daily flow for the water-year then was divided by the drainage basin area for each streamgage to calculate runoff, and a drainage-basin-area weighted average runoff value was computed for each cataloging unit. An adequate number of streamgages were identified using the above criteria for only for some cataloging units during some years. For the remainder of the cataloging units, the runoff estimates were based on (1) streamgages with small drainage basins with an unknown degree of overlap, (2) runoff values for neighboring cataloging units, and (3) average runoff values for larger accounting units.

Reference

Krug, W.R., Gebert, W.A., and D.J. Graczyk, 1987, Preparation of average annual runoff map of the United States, 1951-80. U.S. Geological Survey Open-File Report 87-535, 414 pp.