Applications of integrated surface water/groundwater modelling - Techniques and perspectives

P.J. Thompson, M.A. Marchildon, E.J. Wexler, J.D.C. Kassenaar

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater-Surface Water Interactions III

ABSTRACT: Integrated modelling is emerging as a powerful tool to assess and understand the interactions between surface water and groundwater processes. This trend is being driven by engineering and water management challenges such as water budgeting, well permitting, and cumulative impact assessment that must now consider complex issues such as ecological flow needs under both drought and high-flow conditions. The new USGS GSFLOW integrated surface water and groundwater model is an open source code (Markstrom et al., 2008) that is based on the proven PRMS surface water model (Leavesley et al., 1983) and the robust, modern MODFLOW-NWT groundwater model (Niswonger et al., 2011). Hydraulic processes such as reservoir and stream routing are fully simulated, and lake, stream, and wetland features can interact with multiple components of the groundwater system. Overland flow and interflow is routed to surface water bodies with a distributed cascade approach based on surface topography and soil-zone interactions are modelled using a soil moisture accounting scheme coupled to the groundwater system. Importantly, GSFLOW allows the surface and groundwater sub-models to be built with differing spatial scales. A detailed surface water model, based on a high resolution DEM and detailed soil and land cover information, can be coupled to a locally refined groundwater model so that engineering scale problems can be solved using fast and computationally efficient models. Traditional un-coupled, steady state modelling approaches often do not provide the necessary temporal scale needed to offer insight into many surface water/groundwater interactions. For example, vernal pools that fluctuate seasonally both in volume and extent depending on the depth to water table can significantly affect runoff generation. Hydrologic interactions vary not only seasonally or monthly, but between individual storm events. Accordingly, integrated transient models typically constructed with hourly precipitation inputs can be calibrated to daily streamflow or groundwater level targets. Detailed topography and land-use data, distributed NEXRAD radar precipitation data and a regionally calibrated snowpack model allow processes to be simulated at a high spatial resolution. Transient, high resolution modelling has required the development of new software tools to process, visualize, and communicate model results. Several case studies employing transient GSFLOW models are presented to demonstrate this approach: 1) An impact assessment of a proposed large-scale land development on a permeable southwest Florida aquifer system. Lakes, wetlands, stormwater ponds, ditches, dykes, canals and rivers were all simulated over karst geology. Pre- and post-development scenarios were compared to assess the impacts of urbanization on groundwater recharge, wetland hydroperiod and surface runoff volumes. Uniquely, Pre-Columbian land use and vegetation conditions in the study area were approximated to estimate a finaturalfl condition against which to compare the predicted impacts of urbanization. 2) A catchment-scale water budget simulation of municipal wells located at the base of a re-entrant valley cut into the Niagara Escarpment (Ontario, Canada). Complex lake and wetland interactions were represented with 475km of mapped stream tributaries, 143 shallow wetlands, quarry lakes, and ponds as well as many diversions, quarry discharges, and surface water takings. Several large reservoirs dominate the surface water system and day-to-day operations of individual gates and spillways at the reservoirs were simulated. The model was calibrated to reservoir stage, total streamflow and static groundwater water levels at 2684 water wells and 100 transient monitors. The model clearly demonstrated that filling the adjacent reservoir in the spring provided significant recharge to the wellfield aquifer system, a connection confirmed by temperature monitoring in the municipal wells. 3) A comparative analysis of low-impact development (LID) urban design scenarios for the purpose of groundwater recharge protection, runoff mitigation and preservation of groundwater discharge to wetlands and streams. Green roofs, rain barrels, bioswales, infiltration galleries and permeable pavements were all considered at a fine resolution (10m hydrologic model cells). The 30.4km²

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