ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario36The change field method employed average monthlyGCM-based change fields for four variables (i.e.,precipitation, air temperature, solar radiation andwind speed). Changes were applied to a time seriesof historical meteorological data, adjusting individualobservations by the monthly change field values.Downscaling was not conducted and this method doesnot account for changes in extreme events. Figure 5.1shows the precipitation and temperature change fieldsby month applied to a station used in the CVC WaterQuality Strategy to characterize the two future climatesfor the 2050s.Simulation results indicated that the warmer futureclimates would result in much less snowpackaccumulation and greatly reduced spring freshets. Inboth scenarios, the annual hydrograph with climatechange displayed much higher fall and winterstreamflow. Summer results differed in that the drierCGCM2 case resulted in lower summer flow rates andthe wetter HadCM3 case resulted in a small increasein summer flows. Potential evapotranspiration wassignificantly higher in both climate change cases dueto warmer air. Actual evapotranspiration was elevatedin the warmer wetter case and similar in the warmerdrier case. Water quality was greatly impacted withboth future climates as bare winter soils in these casesresulted in elevated erosion.One of the most significant sources of error in thisanalysis is likely due to the method of estimatingpotential evaporation and the method ofobtaining actual evapotranspiration from potentialevapotranspiration. The method used is sensitive tochanging air temperature and solar radiation, butdoes not account for potential changes in the soilwater/vegetation relationship and may not accuratelyaccount for winter conditions. The assessment is alsonot appropriate for storm event analysis as the changefield method does not alter the frequency of extremeevents in the climate time series. Results differedsignificantly between the scenarios tested, highlightingthe uncertainty involved in using only two GCM-basedscenarios in the estimation of future climates and theinterpretation of output.Figure 5.1 Monthly change fields for precipitation and air temperature for 2050s in the Credit River Watershed asprojected by two GCMs (CGCM2 and HadCM3) (Walker and Dougherty, 2008)