ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment89When completing a climate change impact assessment,the selection of hydrologic change metrics dependson several factors; these include the scale and scope ofthe investigation as well as the specific ecological andhydrologic functions in the stream or watershed beingevaluated. If the assessment focuses on ecologicalimpacts, then many of the IHA and EFC-type ofparameters will be useful to predict and to identifyspecific functions that might be affected by climatechange.Guidance:A set of comparative hydrologic metrics shouldbe selected that represents all functions of thehydrologic system.After selecting the hydrologic metrics, the study teammust decide how to compare the climate changeimpacts to the reference regime. The following methodscan be used to compare the estimated impacts witheach metric:• Absolute Change. Estimate the absolute change inthe hydrologic metric (e.g., 7Q 10decreases from 10L/s to 7 L/s).• Relative Change. Estimate the percent change in thehydrologic metric (e.g., 7Q 10decreases 30%).• Frequency Change. Estimate the change in thefrequency of exceedance for a metric (e.g., frequencyof overbank flow increases from 2.5 times per year to2.8 times per year).The method of comparing impacts depends on thetype of hydrologic metric and the accuracy of theestimated parameter. Sometimes the goal of thestudy is to develop a general understanding of theexpected impacts of climate change; in these casesrelative percent changes in metrics may be sufficient. Instudies such as determining changes to low flows for awastewater assimilation study, the assessment may needto estimate specific impacts to low flows and requireabsolute metrics to be evaluated.6.6.3 Setup Future Conditions Model(s)Comparing the hydrologic impacts due to climatechange alone is often of interest in hydrologic studies;thus, eliminating the changes due to other futureconditions such as land use or water managementpractices from the analysis. If this is warranted, then thesame model would be run under baseline and futureconditions with only climate data altered; however,once the potential hydrologic impacts due to climatechange alone are quantified, the study team shouldbuild in changes due to future land uses, populationgrowth, wastewater effluent, future water demands,technological innovation (e.g., low impact developmentor LID), and climate change adaptation measures intothe model (Section 6.4.5) as these would be morerealistic of the future scenarios. The cumulative impactsof a changing climate and future conditions can beassessed by simulating all of these future conditionsunder both the baseline and climate change scenariosusing the hydrologic metrics described in the previoussection.When considering both future land development andclimate change scenarios, the practitioner shouldconsider the following:• The procedure used to quantify and presenthydrologic metrics must be designed to ensure theestimated hydrologic changes from land developmentare clearly separated from climate change impacts.• The results should be analyzed and presentedto demonstrate the cumulative impacts of landdevelopment and climate change.6.6.4 Model Climate Change Impacts by ScenarioMany model simulations will exist when the study teamis dealing with multiple climate change scenarios andmultiple future conditions. Management of modelsimulations and good file management practices areessential to ensure that model simulations are run andprocessed as intended. As such, a relational databaseis recommended to manage, manipulate, and report ondata input and simulation results.