Publication Summary

Automated hydrograph separation techniques were used to evaluate the groundwater contribution to total streamflow (baseflow) at active and inactive stream gaging stations throughout Washington State. Discharge records for 582 gaging 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.

Automated hydrograph separation techniques were used to evaluate the groundwater contribution to total streamflow (baseflow) at active and inactive stream gaging stations throughout Washington State. Discharge records for 582 gaging 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.

Annual unit-area baseflow for the 294 stations free of significant regulation or snowmelt effects ranged from <1 to 11 ft3/sec/mi2 with a median value of 2.9 ft3/sec/mi2. Unit-area baseflow for stations located west of the Cascade Mountain crest averaged approximately 3.2 ft3/sec/mi2. Stations located east of the crest averaged approximately 0.4 ft3/sec/mi2. On average, groundwater discharge represented approximately 68% of total annual streamflow for the stations modeled. Estimated groundwater contributions to streamflow for the typical low flow months of July, August, September, and October averaged 86%, 86%, 77%, and 69% respectively. This suggests that reductions in groundwater discharge to streams during this period, due to increased groundwater withdrawals, may significantly impact the instream flows needed to sustain fish and maintain water quality. This highlights the importance of managing surface water and groundwater as a single interconnected resource.

The baseflow estimates provided in this report are best used as basin-scale averages. Any attempt to apply these values as absolute representations of groundwater inflow on either a basin scale or stream segment scale is inappropriate.

Study results were compiled in an ArcView GIS project for spatial analysis, and a master database was assembled. A compact disc (pub# 99-327data) containing the tabular data and ArcView project files generated during the study is included with this report.