ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment51This approach uses a streamflow generation modelrunning continuously to generate spatially-differentiated,temporally-averaged recharge rates for a specifictime period. The groundwater flow model is then runin steady state mode using the temporally-averagedrecharge rate estimated by the streamflow generationmodel. More recent advances in computing capacityhave allowed transient runs with the groundwaterflow model. The commonly applied groundwater flowmodels, FEFLOW and MODFLOW, can be linked to thestreamflow generation models listed above.The streamflow generation model may be refined basedupon the groundwater flow model simulations, thus,adding a feedback loop to the modelling approach.In the Credit Valley Conservation (CVC) Subwatershed19 Study, the HSP-F groundwater component wasconfigured to account for major subsurface fluxes ofgroundwater to and from adjacent watersheds andsubwatersheds (Appendix E). The groundwater flowmodel, MODFLOW provided the streamflow generationmodel with steady state fluxes across catchmentboundaries and subwatershed boundaries. Running themodels in this sequence allows regional groundwaterflow systems and their contributions to the surface waternetwork to be more accurately represented than usinga streamflow generation model alone. This study alsoemployed transient groundwater model simulationsusing monthly simulated groundwater recharge rates topredict seasonal changes in the flow system.6.2.1.3 Coupled or Conjunctive ModelsA relatively new modelling approach is the applicationof fully coupled or conjunctive models to simulate thehydrologic response of a watershed. This approachrelies on a single model that includes completerepresentations of the surface water system and threedimensionalgroundwater flow system. For example,GSFLOW is a conjunctive model that uses MODFLOWand PRMS (Precipitation-Runoff Modeling System) forthe subsurface and surface regimes, respectively. Theadvantage of this approach is a degree of efficiency andlinkage that more effectively uses the best model foreach regime, surface and subsurface. However, due tothe complexity of conjunctive models and the benefitsof separate surface water and groundwater models,conjunctive models should be selected with carefulconsideration.6.2.1.4 Modelling Approach SelectionThe study objectives together with the availableresources will determine the appropriate modellingapproach. For screening level studies or studies thatare focussed on determining changes to total annualstreamflow volume, a streamflow generation modelalone may suffice. In runoff-driven catchments (e.g., clayplains and urban areas), surface runoff models alone maybe adequate to investigate the hydrologic impacts ofclimate change.For studies investigating the impacts of climate changeon either the groundwater system or baseflow (wherebaseflow is predominantly comprised of groundwaterdischarge), a comprehensive modelling approach usinglinked surface water and groundwater flow modelsis most appropriate. While a variety of streamflowgeneration models have been successfully applied,FEFLOW and MODFLOW-based models are the mostcommon groundwater simulation tools applied acrossOntario.It may be appropriate to consider using a conjunctivemodelling approach in watersheds where the surfacewater and groundwater systems are highly integratedand require very detailed information on the volume andtiming of groundwater discharge. The costs associatedwith proceeding with this approach should be weighedagainst the expected benefits.Guidance:Select a realistic modelling approach thatmaximizes model sophistication whileconsidering available resources and modellingcapacity.6.2.2 Select Water Resources ModelFollowing the selection of the modelling approach,the specific water resources model can be selected.Much like the modelling approach, the objectives,issues, needs and available resources play a large role inselecting the optimum water resources model.