The interaction of groundwater and surface water is a key watershed issue in the basin. Short-term pumping effects on streamflows are complex, and require a specialized groundwater flow model to assess this relationship. Seasonal pumping during the summer will result in a reduction in natural baseflow discharge to the Snoqualmie River that is proportional to the rate of groundwater withdrawal and the length of time that pumping occurs.
The groundwater model also indicates a time-lag between the onset of groundwater pumping and the predicted reduction in baseflow discharge. It is estimated that, after 30 days of pumping, about 30% of the yield of a well or wellfield would be be reflected as a decrease in baseflow discharge. The remaining 70% of the yield would be derived from groundwater storage. This portion of the withdrawal, when pumped directly to the river, will increase streamflow above what would occur naturally. After 180 days of pumping, the model indicates that 70% of the yield is reflected as decreased baseflow discharge. Streamflows would therefore increase by about 30% of the wellfield yield after 180 days. This relative proportion of streamflow enhancement relationship is sensitive to the hydraulic properties of the shallow and deep aquifer properties, as well as stream bed properties and the operational regime of the wellfield. Additional refinements to these relationships will be simulated with the groundwater model as additional data are collected.
|
