Conjunctive Use: Streamflow Depletion Caused by Pumping Wells on a Nearby Stream
The Hydrodynamics Group
A well pumping seasonally for irrigation that is within one quarter mile of a stream creates a cyclical streamflow rate of depletion that is in phase with the pumping and the same each year. However, as the well is further removed from the stream the aquifer tends to dampen the seasonal fluctuations. At a distance of approximately two miles from the stream, the streamflow depletion created by the well is approximately equal to pumping at a constant rate, equal to the total withdrawal averaged over the year. There is very little seasonal fluctuation. For example, if one were to pump the well two miles from the stream at 4 cfs for three months; the effect on the stream would be approximately the same as if the well were pumped continually at 1 cfs. The effect of distance is dependent upon the aquifer properties: transmissivity and storativity. In the case of the well two miles or further away from the stream, the full impact of the pumping on the stream takes a decade or more to fully develop. In the situation where wells are uniformly distributed across an aquifer, associated with a stream, the impact of the ensemble of wells on the streamflow is similar to the single well two miles or more away the stream. Again, it takes at least a decade of pumping for the full impact on the stream to occur. Were one to stop pumping an ensemble of wells, it would take at least a decade for the streamflow to return to a state where it is not impacted by the prior pumping. In other words, if one wanted to eliminate the full impact of pumping an ensemble of wells on the stream today, one would have had to stop the pumping at least a decade ago. Managing a conjunctive stream and associated aquifer system requires long-range planning because of the lag time in response created by the aquifer; it cannot be managed effectively on a short-term basis.