Limitations to Upscaling of Groundwater Flow Models dominated by Surface Water Interaction

Different upscaling methods for groundwater flow models are investigated. A suit of different upscaling methods is applied to several synthetic cases with structured and unstructured porous media. Although each of the methods applies best to one of the synthetic cases, no performance differences is formed if the methods were applied to a real 3D-case. Furthermore, we focus on boundary conditions such as Dirichlet, Neumann and Cauchy conditions, that characterize the interaction of groundwater with e.g. surface water, recharge. It follows that the inaccuracy of the flux exchange between driving forces on a fine-scale and the hydraulic head on a coarse scale causes additional errors that are far more significant than the errors due to an incorrect upscaling of the heterogeneity itself. Whenever those errors are reduced, the upscaled model was improved by 70%. It thus follows that in practise, whenever we focus on predicting groundwater heads, it is more important to correctly upscale the boundary conditions than hydraulic conductivity.