ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario80Scenarios can be sorted according to annual simulatedchanges in air temperature and total precipitation. Thescenarios associated with the percentiles of interestin the Percentile Method (Section 6.4.3.2) can bedetermined from these sorted groupings.The meteorological inputs to the hydrology model canbe adjusted to reflect the simulated change fields on amonth by month basis for each scenario selected. Thesemonthly change fields represent average changes in theclimate parameter and are applied to each value in theinput data time series. This process changes the meanvalues for the time series but will not affect the variabilityor frequency of occurrence of extreme conditions. It is,therefore, not suitable for assessments that focus onpeak flow and flooding issues.While most hydrologic models and climate changeimpact assessments focus on the effects of airtemperature and precipitation, other meteorologicalparameters may be required. Models such as HSP-F canuse solar radiation and wind speed to simulate snowpackbehaviour and to modify evapotranspiration. Hydrologicmodels are not as sensitive to wind and solar radiationas they are to temperature or precipitation; therefore,this aspect of future climates is of lesser importance. Forthis reason, the GCM change field method is generallyadequate for adjusting these parameters for use inclimate change impact assessments. This is also true inassessments involving downscaled climates.Assessment of climate impacts is discussed in Section6.6. Examples of studies conducted in Ontario using thechange field method and typical change field values arediscussed in Section 5.1.6.5.3 Statistical Downscaling MethodVarious statistical downscaling models are currentlyavailable for climate downscaling. The StatisticalDownscaling Model (SDSM) is one tool that is availableon the Environment Canada website (www.cccsn.ca). Thismodel was developed in the UK by Drs. R. Wilby and C.Dawson. It is a hybrid of a stochastic weather generatorand a regression-based downscaling method, packagedwith a user friendly Windows interface. The modelallows users to generate single site scenarios of dailyweather variables (i.e., air temperature and precipitation)for current and future climates. SDSM helps the useridentify large scale predictors (i.e., mean sea levelpressure, surface vorticity or specific humidity) thatexplain much of the climate variability at a site. Statisticalmodels are built using these predictors to estimate thepredictand values. Predictands are the local climatevariables of interest (i.e., temperature and precipitation).As predictors tend to change in the future, so toowill the simulated predictands, through the statisticalrelationships determined by the model.Guidance:To examine climate variability, at least onestatistical downscaling model scenario should beapplied to develop future climates. The GCMemissionscenario used in the climate simulationshould match one of the scenarios chosen for thesuite of GCM-emission scenarios, if possible.The SDSM application process requires predictor timeseries for current and future conditions. For currentconditions, predictor time series are created from theGCM runs, available at GCM grid points using the NCEP(National Centre for Environmental Prediction) model.NCEP is a reanalysis of recent weather that creates aretrospective accounting of the atmospheric conditionsbased on observed data. In effect, the NCEP runs fillin values for a wide variety of predictors by modellingexisting and recent conditions. A large number of NCEPvariables are available for all of North America for theperiod 1961 to 1990.GCM-based runs that extend well into the future(typically to 2100) provide the predictor time series forfuture emission scenarios, for various time horizons(i.e., 2050s, 2070s etc.). The local climate predictandsare generated using the same statistical relationshipsbetween the predictors and predictands developed forcurrent conditions.To acquire a downloaded copy of SDSM with the user’smanual as well as a brief summary of the modellingprocess, visit the Environment Canada website, www.CCCSN.ca. Click Downscaling Tools on the left handside of the screen and select SDSM from the drop downmenu. Review the summary version of model applicationand refer to the full version of the manual for detaileddescriptions and a well documented example for