ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment854. STMND - minimum daily air temperature indegrees K.5. STMXD - maximum daily air temperature indegrees K.6. SWMX - mean amplitude of sustained wind at 10 min m/s, averaged over 6 hours.7. Other parameters of interest – 58 variables areavailable.• Select Next, Choose Text format. Check the orderand Click Submit to finish the order.Within a few days the data will arrive as a compressedfile containing hundreds of individual files. Individual filescontain one month of data for each weather variable.Each file contains at least 4 grid point sites, with up to 36sites depending on the resolution value set on the orderform.RCM data requires some post processing to convertunits and disaggregate to smaller time steps asdiscussed below. Once converted to the desired timestep this data is directly applicable as future climateinput data for modelling the 2041 to 2070 conditions.Guidance:If the high resolution RCM data is found tocontain many days with small amounts ofprecipitation and very few dry days, then thedistribution of precipitation may be unrealisticand unsuitable for application to assessments. Inthis case this method should not be included inthe study.The largest potential benefit of the RCM downscalingapproach is the ability to better characterize changesin rainfall intensity and variability at the local scale.However, as RCM data is only available at 6 hour timesteps, this type of characterization is not yet possible. Asthe science advances, it is expected this limitation will beresolved.6.5.6 Disaggregate Daily Data and PotentialEvapotranspiration GenerationMany hydrologic models require sub-daily time stepsto simulate important diurnally variable processessuch as evapotranspiration, snowmelt and snowpackaccumulation. Sub-daily time steps are particularlyimportant in smaller watersheds as the onset of ameteorological event can widely and rapidly affectconditions and runoff. Streamflow at the mouth of asmall watershed will change rapidly as weather changes.Appendix D contains a description of the methodsadopted for the case study to convert RCM data tothe units and the one hour time steps necessary forhydrologic modelling. Since the RCM data is provided in6 hourly and daily time steps it represents a special casecompared to the GCM output used for the change fieldand statistical downscaling methods.When managing GCM and statistically downscaleddata, disaggregation can be used to convert dailydata to hourly (or shorter) time steps (Appendix D).The most common disaggregation operations involveconverting daily precipitation to hourly, converting dailyminimum and maximum temperatures to hourly values,and possibly converting daily total solar radiation andwind speed to hourly values. Disaggregation methodsare typically straightforward approaches, for examplegenerating hourly precipitation data by simply dividingdaily precipitation totals by 24.Disaggregation tools are provided with WDMUtil, awatershed time series data management tool, providedby the U.S. EPA (Hummel et al., 2001). WDMUtil isavailable for download at no cost at Hwww.epa.gov/waterscience/modelsH/ as part of the BASINS modellingpackage. WDMUtil can be used to store, manageand view time series data. Utilities are available fordisaggregation of solar radiation, air temperature,evapotranspiration, wind and precipitation.Many hydrologic models use potentialevapotranspiration as input. Typically, actualevapotranspiration is calculated by the model byreducing the potential evapotranspiration according totemperature, wind, solar radiation, soil water content,leaf area index, etc. Since potential evapotranspiration isa function of air temperature and solar radiation, both ofwhich are expected to change in the future (Section 6.2),