ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment93With climate change scenarios, the impact of uncertaintyshould be considered the same as in any traditionalhydrologic assessment. Future climate scenarios simplyadd another level of uncertainty to those alreadyassociated with model parameters, model structure,and model calibration. While it is important to reduceuncertainty wherever possible, properly addressinghydrologic uncertainty involves estimating the impact ofuncertainty on model predictions.Guidance:Estimate the impact of uncertainty from futureclimate scenarios, model parameterization,model structure and calibration upon modeloutput.The simulation results should be used to answer thefollowing questions rather than focussing on onlyabsolute results:• What are the potential impacts of climate change?The results of the climate change scenarios representplausible hydrologic impacts due to climate change.• What are the likely impacts of climate change? Thelikely impacts of climate change can be estimatedby aggregating all scenarios. For example, thescenarios may lead to statements such as “Eight outof ten climate change scenarios project that the 7Q 10streamflow will be reduced by 40%.”Figure 6.10 contains the histogram of 7Q 20streamflowestimates (i.e., the lowest average 7-day flow witha 20-year return period) based on 57 future climatescenarios; it illustrates the importance of uncertaintywith projecting climate change impacts. The simulationswere based on a calibrated streamflow generationmodel of a subwatershed in Southwestern Ontario.The impact to 7Q 20streamflow is estimated to rangefrom -50% to +25%. While this appears to be a largerange of uncertainty, statistical methods help to betterdescribe this change. From this example it is possibleto make statements such as “70% of the climate changescenarios project that 7Q 20will decrease, and 50%project that 7Q 20will decrease by up to 25% of thecurrent value.”Figure 6.10 Histogram of estimated 7Q 20streamflow using 57 climate change scenarios