Automated hydrograph separation techniques were used to evaluate the
groundwater contribution to total streamflow (baseflow) at active and
inactive stream guaging stations throughout Washington State. Discharge
records for 582 guaging stations, with at least three complete water years
of daily mean streamflow data, were downloaded from the U.S. Geological
Survey (USGS) National Water Information System. Station characteristics
were compiled for each gage, including period of streamflow record, type
and degree of regulation affecting the gage, watershed drainage area, USGS
station number, station name, and gage location.
Summary statistics were calculated for annual mean streamflow and
annual 7-day low flow for all 582 stations. Monthly, and in some cases
annual, statistics for baseflow were then estimated using a USGS
hydrograph separation software program called HYSEP (Sloto and Crouse,
1996) for those stations judged to be free of significant snowmelt or
regulation effects.