Well-Induced Stream Depletion and Groundwater Return Flow: Estimating Impact Schedules with a Finite-Difference Spreadsheet

Abstract

Allocation of water from stream-aquifer systems must account for stream depletion from well withdrawal and for accretion from groundwater return flow. Both are subject to attenuation and delay. Transient impact assessment tools include: simplified analytical solutions; special finite-difference groundwater modeling software, such as MODFLOW; and the Delayed Impact Calculator (Calculator), a spreadsheet programmed by the author for streamlined finite-difference estimation of dynamic schedules. Well-induced stream depletion results from the Calculator are compared to results from established tools for a previously documented scenario. Similar comparisons are described for modified cases. Calculator results match MODFLOW (version 1.12.00) results (y=1.0000x, r2=1.0000, 3 sets, 90 pairs each). The finite-difference tools estimate somewhat quicker stream response to groundwater impulse than do analytical solutions derived for ideal semi-infinite aquifers. A previously documented schedule estimate for a simple groundwater return flow scenario is confirmed by the Calculator. An example of Calculator utility is given, treating return flow to a reach above an active surface diversion.