Reservoir-Aquifer Seepage Interaction

This GoldSim model simulates the dynamic seepage interaction between a surface water reservoir and the surrounding groundwater aquifer. Its primary purpose is to calculate the bidirectional flow across the lakebed based on the relative hydraulic heads of the two systems. The calculated seepage is then used directly in the reservoir's water balance calculations within the "Pool1" element. The model operates under the assumption that the aquifer volume is significantly larger than the reservoir, meaning the groundwater elevation (GWEL) is treated as independent of reservoir seepage fluxes and is not dynamically affected by them within the simulation. Groundwater levels must be specified externally (e.g., fixed value, time series, function, or stochastic method), and groundwater volume balances are not tracked in this model.

In this implementation, GWELis dynamically simulated using a History Generator element configured with a Random Walk method, generating a stochastic time series representing fluctuations driven by external factors. The surface reservoir itself is modeled using a Pool element. This element dynamically calculates the reservoir's water volume, water surface elevation (WSEL), and the corresponding water surface area based on the reservoir's defined geometric properties.

Seepage flow (Q) between the reservoir and aquifer is governed by Darcy's Law, calculated as the product of the lakebed's hydraulic conductance (C) and the head difference (dh = WSEL - GWEL) between the reservoir and the aquifer, acting across the specified lakebed thickness (Tb). The conductance term is defined as C = (Klb * A_seep) / Tb, incorporating the lakebed material's hydraulic conductivity (Klb, specified via a Data element) and the effective seepage area (A_seep). A positive calculated Q signifies outflow from the reservoir to the aquifer, while a negative Q represents inflow from the aquifer into the reservoir; equilibrium occurs when WSEL = GWEL and Q = 0.

Conditional logic is applied to determine the effective seepage area (A_seep). When potential for inflow exists (GWEL > WSEL), A_seep is set to the maximum potential lakebed area (A_max), reflecting that groundwater can flow upwards across the entire connected lakebed interface. Conversely, during potential outflow conditions (GWEL <= WSEL), A_seep is dynamically set to the current water surface area (A_wetted), as calculated by the Pool element. The 2D water surface area is a simplification and using the 3D wetted surface area of the lakebed might be a refinement if available.

The model's primary outputs are the calculated seepage flow rates, which directly influence the water balance of the reservoir simulated in the "Surface_Reservoir" container and can also be linked to other external GoldSim models if needed.

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