Evaluation of Validity of Reponse Functions in a Thick Unconfined Aquifer

For many decades ground water hydrologists have taken advantage of the linearity of the confined ground water flow equation to simplify hydrogeologic analyses by application of superposition principals.   Surface water capture response functions have been one of those applications that have provided a broader understanding and visualization of impacts of aquifer stresses on the exchange of ground water and surface water.  In the unconfined Spokane Valley – Rathdrum Prairie aquifer in northern Idaho, response functions were developed from a numerical ground water flow model with the belief that system response was near linear and therefore errors in calculated response functions were negligible.  To test this assumption, surface water responses to an arbitrary aquifer stress were compared for three versions of the model: 1) unconfined with piece-wise linear boundaries, 2) a time-averaged constant thickness representation with piece-wise linear boundaries, and 3) a fully linear representation.  Departure from linearity created greater than expected impacts for stresses in several locations in the aquifer.  Some of the non-linear effects are due to non-linear boundary conditions, but an unexpectedly large effect results from seemly negligible changes in aquifer thickness in a portion of the aquifer that is gradually thinning between two rivers.  The results suggest that subtle changes in aquifer thickness have the potential to “steer” background recharge and discharge to a degree that invalidates use of response functions in some locations under unconfined conditions.