ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario56curves that will be the basis for stormwater infrastructuredesign and risk assessment.Guidance:Hourly (or shorter) time steps should be used forwatershed scale studies to properly represent thedynamic between rainfall intensity and runoff/infiltration.6.2.2.2.6 BaseflowAs discussed in preceding sections, a major impact ofclimate change on water resources may be the timingof when groundwater recharge occurs, not necessarilythe amount of groundwater recharge. As the typicalgroundwater recharge season is pushed earlier into thespring and later into the fall, the number of months withgroundwater recharge occurring will likely decreaseand the amount of water recharging during the wintermonths will likely increase (see Section 5.2). Also, a largerportion of total precipitation may fall during significantstorms with longer inter-event periods of drought,further disrupting groundwater recharge patterns.This potential increase in rainfall intensity and decreasein precipitation event frequency will represent afundamental shift in how groundwater systems arerecharged. Although the total annual amount ofrecharge may remain the same or even increase, theshift in the timing may cause undesirable impacts tothe groundwater system. These impacts may includereaching summer low flow conditions earlier in the yearor reduced summertime streamflow.As discussed in the Section 6.2.1, streamflow generationmodels can be linked to groundwater flow modelsthrough estimated groundwater recharge; thus providinga three-dimensional representation of the groundwatersystem. Steady state groundwater models, usingaverage annual input conditions, have been the standardof practice for investigating groundwater flow systems.These models assess impacts at an annual scale, butcannot consider the seasonality of inputs includinggroundwater recharge.Since steady state groundwater flow models cannotrepresent the possible seasonal shift in groundwaterrecharge, it is recommended that transient groundwaterflow models be used for climate change impactassessment. Transient groundwater flow models aresimilar to continuous streamflow generation models, inwhich they sequentially step through a specified timeperiod. This allows for the representation of monthlyshifts in groundwater recharge and the determinationof the resulting impacts. Both FEFLOW and MODFLOWgroundwater flow models can be run in transient mode.Guidance:For those investigations which seek to determinethe impact of climate change on groundwaterdischarge or groundwater levels, a threedimensionalgroundwater flow model such asFEFLOW or MODFLOW should be used, inaddition to a streamflow generation model.To understand the seasonal or monthly impactsto the groundwater system, the groundwater flowmodel should be applied in transient mode.6.2.2.3 Required OutputThe objectives and needs of the project will determinethe required output parameters. For climate changeimpact assessments, the selected model(s) should atleast be able to output the following:• Daily and hourly streamflow at a variety of locationswithin the watershed;• Water balance information, including monthlyevapotranspiration, direct overland runoff andgroundwater recharge; and• Groundwater conditions including aquifer water levelsand discharge to streams.A discussion of model assessment parameters andmetrics is provided in Section 6.6.2.