Simulating groundwater flow and runoff for the Oro Moraine aquifer system. Part I. Model formulation and conceptual analysis

A steady-state groundwater model that incorporates the vadose zone in an approximate fashion is developed to account for situations where recharge may be significantly affected by heterogeneity above the water table. The model is intended for basin-scale applications where the exact representation of the unsaturated zone is no longer feasible. The pseudo-unsaturated model is compared to a variably saturated model making use of an idealized cross-section approximating the complex Oro Moraine aquifer system. Differences in predicted flow regimes between the two models arise from the coarser mesh used in the pseudo-unsaturated simulations, but are minimal below the water table. The pseudo-unsaturated model yields an accurate overall system water mass balance although a loss in spatial resolution of predicted fluxes may occur. Such accuracy losses are offset by drastically lower computer memory requirements. The flow simulations further show that near-surface heterogeneity has a profound impact on the sustainable capacity of a groundwater system and the location of sensitive recharge areas. The pseudo-unsaturated model is then coupled to a high-conductivity layer atop the model domain to simulate rainfall runoff. An important advantage of this model formulation is that precipitation and evapotranspiration measurements can be directly used as model input, while groundwater infiltration is part of the flow solution. Runoff and baseflow contributions to long-term streamflow generation are also calculated and can be compared to data from gauging stations during model calibration.