Finite Element Implementation of SAFE for Asymmetric Stream-Aquifer Flow Exchange

Climate change and human activities increasingly affect interconnected stream-aquifer systems, motivating continued evaluation of how stream-aquifer interactions are represented in integrated groundwater-surface water models. Most groundwater models quantify stream-aquifer exchange using an empirical riverbed leakance parameter that is predefined or calibrated from observed head differences between the stream and aquifer. Here, we describe the numerical implementation of the analytical stream aquifer flow exchange (SAFE) formulation (Morel-Seytoux et al., 2018) within the finite-element-based Integrated Water Flow Model (IWFM). SAFE represents stream-aquifer exchange using physically based parameters and provides an alternative to commonly used first order exchange (FOE) formulations, such as the MODFLOW River Package (RIV). A key extension is the ability to quantify asymmetric stream-aquifer exchange arising from different groundwater heads on opposing sides of a stream, a capability not available in standard stream-aquifer interaction packages. Numerical experiments using hypothetical test cases and a large-scale regional application demonstrate that differences in conductance and connectivity parameterizations produce distinct spatial and temporal patterns of simulated exchange. Compared with FOE, the SAFE formulation produces smoother temporal transitions and reduced short-term variability in simulated exchange. These results demonstrate the importance of conceptual model formulation when interpreting simulated stream-aquifer exchange and provide a physically based alternative for large-scale integrated groundwater-surface water modeling.