A Simplified and More Efficient Solution for Stream Depletion Analysis in MODFLOW

Tuesday, September 22, 2015: 10:30 a.m.
Gengxin Ou , Nebraska Department of Natural Resources, Lincoln, NE

Current processes being used at the Nebraska Department of Natural Resources for the calculation of stream depletions requires the performance of a change analysis, both a baseline run and a scenario run of MODFLOW, to calculate the baseflow difference. The current process for obtaining a stream depletion distribution pattern is computationally inefficient because separate model runs with a pumping well in each individual grid cell are required. This study proposes a simplified and more efficient solution to estimate a stream depletion rate at a given location by using coefficients of the baseline run in the scenario runs. Groundwater internal flow coefficients are head-dependent in the unconfined layer. However, the head change between two model runs is usually minimal compared with the aquifer thickness. In the stream package, streambed conductance can be changed as the wet radius changes with stream stage. Using the assumption that the accretion flow is negligible to streamflow, the change of the stream stage can be neglected. Since flow coefficients have been calculated in the baseline model run, the solution becomes very efficient and does not require the MODFLOW input files to be repeatedly read into the scenario run. The change in the head-dependent boundary flow and storage can then be calculated using the head difference. This solution dramatically reduces computer time (in both computation and IO) and noise because the new estimate is a change in heads. The change in heads can then be canceled out in the original model because the difference is small relative to the head. This presentation will describe the simplified and more efficient solution for stream depletion analysis.

Gengxin Ou, Nebraska Department of Natural Resources, Lincoln, NE
Gengxin Ou is the Integrated Water Management Specialist in the Nebraska Department of Natural Resources.