The Importance of Capturing Topographic Features for Modeling Groundwater Flow and Transport in Mountainous Watersheds

Chao Wang; Jesus D. Gomez-Velez; John L. Wilson

doi:10.1029/2018WR023863

Physics‐based distributed hydrological models that include groundwater are widely used to understand and predict physical and biogeochemical processes within watersheds. Typically, due to computational limitations, watershed modelers minimize the number of elements used in domain discretization, smoothing or even ignoring critical topographic features. We use an idealized model to investigate the implications of mesh refinement along streams and ridges for modeling three‐dimensional groundwater flow and transport in mountainous watersheds. For varying degrees of topographic complexity level (TCL), which increases with the level of mesh refinement, and geological heterogeneity, we estimate and compare steady state baseflow discharge, mean age, and concentration of subsurface weathering products. Results show that ignoring lower‐order streams or ridges diminishes flow through local flow paths and biases higher the contribution of intermediate and regional flow paths, and biases baseflow older. The magnitude of the bias increases for systems where permeability rapidly decreases with depth and is dominated by shallow flow paths. Based on a simple geochemical model, the concentration of weathering products is less sensitive to the TCL, partially due to the thermodynamic constraints on chemical reactions. Our idealized model also reproduces the observed emergent scaling relationship between the groundwater contribution to streamflow and drainage area, and finds that this scaling relationship is not sensitive to mesh TCL. The bias effects have important implications for the use of hydrological models in the interpretation of environmental tracer data and the prediction of biogeochemical evolution of stream water in mountainous watersheds. Plain Language Summary Topography controls the movement of water and solutes within watersheds and their export to streams via groundwater drainage. Mainly due to computational limitations, it is common practice to use numerical meshes that capture the high‐order (i.e., large) streams and ignore or undersample low‐order (i.e., small) streams and their ridges. However, low‐order streams are hotspots for groundwater drainage and solute export, and their ridges recharge a significant amount of water feeding aquifers and alluvial valleys downstream. By systematically comparing groundwater flow and transport characteristics from models with different degrees of topographic fidelity along streams and ridges, we find that even though ignoring low‐order streams and ridges has a modest effect in the net amount of discharge generated by the whole watershed, it causes significant changes in the complex network of subsurface flow paths, which distribute water and solutes throughout the system. These changes bias estimates of recharge and discharge, residence times, and solute fluxes to streams, and they are more evident in watersheds with tight bedrock and primarily shallow groundwater flow. This bias hampers our ability to model and predict hydrologic response and directly influences the management of water resources and water quality for human consumption and ecosystem functioning. Key Points Modeled subsurface flow and transport in mountain watersheds are strongly affected by missing low‐order streams/ridges in numerical meshes Baseflow is biased lower/higher for low‐/high‐order streams, while mean age and solute concentration are biased higher for all stream orders The magnitude of the bias is influenced by the degree of subsurface heterogeneity and reaction weathering rates for weathering products

The Importance of Capturing Topographic Features for Modeling Groundwater Flow and Transport in Mountainous Watersheds

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