ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario38• The adoption of a temperature-based methodologyto account for the impact of freeze/thaw cycles onhydrologic parameters (e.g., infiltration).The investigation concluded that the impact onrecharge and streamflows was largely dependent onthe GCM used. While both the HadCM2 and CGM1scenarios resulted in increases in annual streamflowvolume of +10% to +40% by 2090s, summer streamflowestimates ranged from -20% for the CGM1 scenario to+30% for the HadCM2 scenario. Due to the increase inprecipitation, water budget parameters all experiencedsignificant increases by the 2090s:• Evapotranspiration: +14 to +20%;• Runoff: +20 to +30%; and• Recharge: +13 to +30%.Estimated increases in evapotranspiration were slightlylower than increases in runoff and recharge. This is dueto large increases in winter precipitation, a season inwhich evapotranspiration will likely remain limited, evenunder a changed climate.The impact of climate change on peak flows, whichhave been theorized to increase significantly, was alsoinvestigated. Historically, peak flows within the GrandRiver Watershed have been observed during thespring freshet, when meltwater from the accumulatedsnowpack, combined with rainfall events, aggravatingflooding conditions. The climate change impactinvestigation found that large snowpacks were notgenerated because of increased winter temperaturesand increased frequency of mid-winter melts. With thereduction in winter snowpack, future peak spring flowsremained stable, or slightly reduced. Winter (Januaryand February) peak flows did show significant increases,as mid-winter snowmelts and increasingly frequentwinter rainfall events caused peak flows to occur in thisperiod that does not typically experience these types ofevents.The study identified two areas where additional work isneeded to refine climate change impact assessments.The first is to understand how evapotranspiration mayincrease during winter months. Despite temperatureincreases, a lack of sunlit hours during the winter monthswould likely limit the ability of vegetation to generatesignificant transpiration rates. Other evaporativeprocesses, including sublimation (snow to water vapour),may become more significant as winter months warm;these potential changes need to be better understoodto fully understand the impact of a warmer climate.Secondly, the study also identified the need to quantifythe increase in extreme event frequency and intensityexpected with future changed climates. If, as expected,future precipitation events become more intense, amuch higher proportion of runoff may be generatedin relation to infiltration, with higher peak flows. Thismay be significant during the winter months, where ashift from regional precipitation events to localized,convective events, may cause a reduction in infiltrationtotals. The GRCA study did not consider the projectedchanges in the intensification of precipitation events.5.1.4 Hydrologic Models for Inverse Climate ChangeImpact Modelling (Upper Thames River Basin)(Cunderlik and Simonovic, 2005, 2007)Cunderlik and Simonovic (2005; 2007) investigatedthe applicability of using inverse impact modelling todetermine the influence of climate change on waterresources. Where climate change impact modellingtypically involves developing modified climate scenariosand applying these climates to a hydrologic model, theinverse impact modelling approach involves identifyingcritical hydrologic exposures that lead to water resourcesystem failures (e.g., a flow of 100 m 3 causes flooding)and transforming these to corresponding climaticconditions. A hydrologic model (HEC-HMS) was usedin this assessment to create an inverse link betweenhydrologic and meteorological processes; in effect,creating thresholds for critical meteorological events(e.g., a 24-hour rainfall volume of 75 mm producesa flow of 100 m 3 ). GCM output, linked to a weathergenerator, was then used to generate a synthetic futureclimate dataset. Changes in the frequency of criticalmeteorological events, given the synthetic future climatedataset, can then be assessed and linked to an increasedfrequency in water resource system failures.Two GCM simulations were selected as the climatechange scenarios: one simulation with wetter conditionsand more intense precipitation events (based on theCCSRNIES GCM and the B21 GHG emission scenario)for flood assessment; and one simulation with warmer,drier conditions (based on the CSIROM2kb GCM andB11 GHG emission scenario) for drought assessment.The study found that significant changes in the