Abrams D B, Haitjema H M, Feinstein D T, Hunt R J
Haitjema Consulting, Inc., 2738 Brigs Bend, Bloomington, IN 47401.
U.S. Geological Survey, Wisconsin Water Science Center, P.O. Box 11166, Milwaukee, WI 53211.
Ground Water. 2016 Jan;54(1):66-73. doi: 10.1111/gwat.12319. Epub 2015 Jan 27.
Regional finite-difference models often have cell sizes that are too large to sufficiently model well-stream interactions. Here, a steady-state hybrid model is applied whereby the upper layer or layers of a coarse MODFLOW model are replaced by the analytic element model GFLOW, which represents surface waters and wells as line and point sinks. The two models are coupled by transferring cell-by-cell leakage obtained from the original MODFLOW model to the bottom of the GFLOW model. A real-world test of the hybrid model approach is applied on a subdomain of an existing model of the Lake Michigan Basin. The original (coarse) MODFLOW model consists of six layers, the top four of which are aggregated into GFLOW as a single layer, while the bottom two layers remain part of MODFLOW in the hybrid model. The hybrid model and a refined "benchmark" MODFLOW model simulate similar baseflows. The hybrid and benchmark models also simulate similar baseflow reductions due to nearby pumping when the well is located within the layers represented by GFLOW. However, the benchmark model requires refinement of the model grid in the local area of interest, while the hybrid approach uses a gridless top layer and is thus unaffected by grid discretization errors. The hybrid approach is well suited to facilitate cost-effective retrofitting of existing coarse grid MODFLOW models commonly used for regional studies because it leverages the strengths of both finite-difference and analytic element methods for predictions in mildly heterogeneous systems that can be simulated with steady-state conditions.
区域有限差分模型的网格单元尺寸通常过大，无法充分模拟河道与含水层之间的相互作用。在此，应用了一种稳态混合模型，即将粗网格的MODFLOW模型的上层或多层替换为解析单元模型GFLOW，该模型将地表水和井表示为线状和点状汇。通过将从原始MODFLOW模型获得的逐个单元的渗漏量传递到GFLOW模型的底部，实现了两个模型的耦合。在密歇根湖盆地现有模型的一个子区域上对混合模型方法进行了实际测试。原始（粗网格）MODFLOW模型由六层组成，其中顶部四层合并为GFLOW中的单层，而底部两层在混合模型中仍为MODFLOW的一部分。混合模型和精细化的“基准”MODFLOW模型模拟出了相似的基流。当井位于GFLOW所表示的层内时，混合模型和基准模型还模拟出了因附近抽水导致的相似的基流减少情况。然而，基准模型需要对感兴趣的局部区域的模型网格进行细化，而混合方法使用无网格的顶层，因此不受网格离散化误差的影响。混合方法非常适合于对通常用于区域研究的现有粗网格MODFLOW模型进行具有成本效益的改造，因为它利用了有限差分法和解析单元法的优势，可在稳态条件下对轻度非均质系统进行预测。
