ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario906.6.5 Compare Changes in Climate DataGuidance:A quantitative evaluation and summary of theselected climate data scenarios should becompleted before completing the hydrologicsimulations.The evaluation of the selected climate data scenariosshould summarize changes in monthly and annualtemperature and precipitation as comparedto the reference regime. Estimated potentialevapotranspiration (PET) may be included in thiscomparison where it is included as input climate data;otherwise, potential evapotranspiration may be assessedas a hydrologic change (below).This assessment of monthly climate data isrecommended to develop a qualitative understandingof the potential hydrologic changes. This understandingshould supplement the modelling process and verifythat the types of changes in hydrologic responseare consistent with expectations. Climate summaryinformation is discussed in more detail in Section 6.5.7and Table 6.7.6.6.6 Compare Hydrologic Changes by ScenarioThis task must answer the fundamental question: Whatis the hydrologic impact of climate change? Each ofthe climate change impact scenarios will producetime series of streamflow at one or more locations andwater balance information at a variety of scales. Thefollowing methodology is recommended to compare thehydrologic impacts projected by each scenario:• Compile the output time series into an electronicformat that is suitable for data manipulation, analysisand comparison between other scenarios. Relationaldatabases are preferable over spreadsheets due totheir ability to aggregate and compare large volumesof data.• Calculate simple metrics for each scenario (i.e., meanannual flow, mean monthly flow) and tabulate similarto Table 6.10. Compare the metrics with the climatedata summarized in Section 6.6.5 to ensure that theresults make sense. Aggregate the metrics usingan approach that is consistent with the number ofclimate change scenarios evaluated. Where thereare fewer future climates, the aggregation may onlyinclude mean/median, minimum and maximumstatistics. Where there are a larger number of futureclimates, the hydrologic changes may be summarizedusing parametric statistics (e.g., upper quartile, upperdecile) that better represent the range in variability ofpredicted impacts.• Calculate simple low flow and high flow metrics (i.e.,2-year flow, 7Q 10) for each of the scenarios. Aggregatethe results from each of the scenarios.• Calculate IHA or EFC parameters if necessary toprovide more insight into potential ecological impactsof climate change.When compiling and comparing scenario results it isimportant to separate the GCM change field resultsfrom the downscaled climate results. This is necessarybecause the downscaled climates are likely based uponsome of the same GCM change field climate scenarios.Lumping the downscaled climate results with the GCMchange field results would bias the results towards theover represented scenarios.Guidance:A quantitative evaluation and summary of theselected climate data scenarios should becompleted before completing the hydrologicsimulations.Graphical tools effectively summarize hydrologic impactsof the climate change scenarios, particularly with manyclimate scenarios. Figure 6.8 illustrates a histogramshowing an example range of variability associated with57 different climate change scenarios. When evaluatingspecific metrics, predicted impacts may range fromincreases to decreases in predicted variables and thehistogram helps to visualize the statistical distribution ofresults and the most likely impacts. Refer to Appendix Efor a case study with more examples of histograms andother graphical tools mentioned in this section.Flow exceedance charts are often used by hydrologiststo illustrate the variability of a streamflow regime. The