ClimateChange Assessment Guide.pdf - University of Waterloo

Hydrological Impacts375.1.2 Toronto Region Conservation Authority (TRCA)(Walker, 2006)In the development of the Rouge River Watershed Plan(TRCA, 2008), hydrological analyses of the effects offuture land use and climate change on both surfaceand groundwater were conducted. A conventionaldeterministic hydrologic model was used to assesschanges to streamflow and water quality; a groundwaterflow model coupled with a surface water budget modelwas used to investigate the groundwater regime.Four climate change scenarios were used in these twocalibrated models including the same two GCM-GHGruns used in the CVC study described above (CGCM2and HadCM3) combined with two urban developmentschemes. In one scheme, conventional SWM practiceswere modelled and in the other, sustainable communitydesign features (i.e., low impact development, LID) wereassessed.The change field method was used to develop futureclimate time series in the same manner as used in theCVC study. There was no attempt to downscale theclimate. Two separate hydrologic impact analyses wereconducted, one using the surface water hydrologymodel.The results of climate change simulations using thesurface water hydrology model indicated that streamflowwould likely decrease throughout the Rouge Riversystem, especially in summer months due to reducedinfiltration and recharge, and much higher anticipatedevapotranspiration. Total annual streamflow waspredicted to decrease by as much as 31%. Peak flowrates (i.e., top 10 percentile) also decreased by up to33% compared to simulations using the current climatetime series. In the two climate change scenarios, thewater quality was predicted to deteriorate as summerstreamflow rates decreased and winter erosionincreased.Results of the groundwater and coupled water balancesimulations differed from the surface water simulations.Where the surface water simulations indicated adecrease in infiltration and groundwater recharge as aresult of increased evapotranspiration, the groundwatermodel simulations indicated an increase in groundwaterrecharge as increased precipitation offset the increase inevapotranspiration.In general, the Rouge River Watershed was shown to besensitive to possible future climates and concomitantstresses, especially in terms of reduced summerstreamflow and elevated contaminant concentrations.The groundwater system was also judged to besensitive to climate change but further investigation wasrecommended given the uncertainty in the direction ofeffects. Similar modelling assessments of surface andgroundwater response by the TRCA for the HumberWatershed Plan (2008) resulted in virtually identicalconclusions.As in the CVC study, evapotranspiration was estimatedwith considerable uncertainty and the frequency ofextreme events was not changed in the input data.Results differed significantly between scenarios,highlighting the uncertainty that results from the use ofonly two climate change scenarios. Further, the disparityin the results of the two different modelling techniquesindicated that model type and structure, as well ascalibration, may significantly affect the outcomes ofhydrologic impact assessments.5.1.3 Grand River Conservation Authority (GRCA)(Bellamy et al., 2002)As part of the Grand River Water Budget project in 2000,the GRCA investigated the impacts of a future climateon the hydrology of the watershed. Output from twoGCMs (HadCM2 and CGM1 using the IS92a emissionscenario) was used to adjust a 1961-1999 climate datasetto 2030s, 2060s and 2090s conditions. Changes inmonthly precipitation and temperature from the GCMswere applied directly to observed climate data usinga change field approach. The climate data were notdownscaled.The adjusted climate datasets were used as primaryinput into a continuous, physically-based, hydrologicmodel, GAWSER (Guelph All-weather SequentialEvents Runoff Model, (Schroeter & Associates, 2004))constructed for the watershed. Several enhancementswere made to the GAWSER algorithm to better estimatethe impacts of a future climate, including:• The use of the Linacre evapotranspirationmethod (Linacre, 1977) for estimating potentialevapotranspiration (temperature based); and