A model for discharge in spring‐dominated streams and implications for the transmissivity and recharge of quaternary volcanics in the Oregon Cascades

A model for discharge in spring‐fed streams is described and applied to streams in the Oregon Cascades. These streams are assumed to be fed by an unconfined aquifer composed predominantly of quaternary basalts and basaltic andesites. The model is based on the unsteady Boussinesq equation and is characterized by a single parameter, a normalized length scale of the aquifer. Flow in a runoff‐dominated stream is used as a proxy for the time‐dependent groundwater recharge. Four spring‐dominated streams with 50 years of daily discharge records are studied, and modeling efficiencies are in the range of 0.76–0.89. The effective transmissivity is found to be approximately proportional to the length scale of the aquifers. Groundwater recharge rates are in the range of 66–127 cm/yr and 40–73% of the mean annual precipitation.