ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario50• A description of temporal considerations applied inthe study; and• A listing of all data and resource needs for thestudy along with other relevant information in eachinformation type category.6.2 Select Hydrologic Modelling MethodsThis section provides guidance on the selection, setupand testing of hydrologic models used to assess theimpacts of climate change. Key hydrological processesare discussed, with a focus on their dependence uponclimate and their likely sensitivity to future climates.To evaluate the possible impacts of climate change onwater resources, a water resource metric (e.g., 7Q 20orannual groundwater recharge) from the changed climateis compared to that of the existing climate conditions.These metrics are calculated through the use of numericmodels which simulate hydrologic processes of awatershed.Traditional water resource modelling approachesrange from relatively simple to very complex. The levelof modelling complexity is typically related to theobjective of the study. Screening level investigationsemploy simplistic approaches as these are sufficient toanswer basic questions. Objectives of these screeninglevel investigations are often limited to broad waterresource assessments such as, what is a subwatershed’sthe mean annual streamflow? Complex approaches arerequired where detailed future conditions are comparedand evaluated, or where a high level of confidence isrequired in the modelling results.Guidance:A modelling approach should be clearly definedas a first step to modelling method selection.When selecting the assessment approach and modellingtools, consideration should be given to the studyobjectives, the specific issues the study is meant toaddress, the needs of the study, and the availableresources (see Section 6.1).6.2.1 Select Water Resources Modelling ApproachWater managers use three assessment approachesto investigate and facilitate understanding of thehydrology and the hydrogeology of a watershed. Theseapproaches are described in the following subsections.6.2.1.1 Streamflow Generation ModelA numeric streamflow generation model partitionsprecipitation into the water balance componentsof evapotranspiration, direct overland runoff, andgroundwater recharge. The groundwater system isconceptualized as a simplistic representation, typicallya linear reservoir, which receives groundwater rechargeand returns this water through groundwater discharge towatercourses. Total streamflow is generated by summingdirect overland runoff and groundwater discharge.Streamflow is routed through channels to downstreamlocations.Streamflow generation models are typically runcontinuously over long time periods (i.e., decades)to generate average annual water balance fluxes andstatistics. Examples of streamflow generation modelsapplied in Ontario include HSP-F, GAWSER, PRMS andSWAT. Event based models are not recommendedfor water balance related assessments as they do notprovide interevent flows or a consistent estimate of keywater balance fluxes and storage.6.2.1.2 Linked Streamflow Generation andGroundwater ModelsLinking two separate models - a streamflow generationmodel and a groundwater flow model, is becomingmainstream in Ontario. Spatially distributed estimatesof groundwater recharge generated by the streamflowgeneration model provide the recharge for thegroundwater model. The groundwater flow modelprovides a detailed three-dimensional representationof the groundwater system and includes representationof the effects of pumping wells and the identificationof locations of groundwater discharge to streams.Calibration and verification of the groundwater modelis based both on comparing observed water levels inwells and piezometers with model results and comparingstream baseflow measurements with simulatedgroundwater discharge rates.