ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario92Figure 6.9 Flow exceedance charts illustrate the range of flows predicted for each scenario6.6.7 Consider UncertaintyA major goal in hydrologic modelling is to identify andquantify the sources of uncertainty in the modellingprocess. Uncertainty is an unavoidable element ofclimate change hydrologic assessments and is a result ofa number of factors as follows:• Future Climate Scenarios. The most obvious sourceof uncertainty when assessing the impacts of climatechange on hydrology relates to the climate changescenarios themselves. While each climate scenariois considered to be plausible, there is a wide rangeof variability and uncertainty in predicted changes inprecipitation and temperature.• Model Parameters. There is never a unique solutionto a hydrologic modelling problem with an infinitecombination of model parameters that mightresult in a model considered to have an acceptablecalibration. While these different sets of parametersmay result in similar predictions of historicalstreamflows, they may produce different predictionsof future flows in response to climate change or landuse changes.• Model Structure and Formulation. Hydrologicmodels range in their representation of physicalprocesses. Some models are more physically basedwhile others are based on empirical formulationsderived from historical conditions. Similar to the roleof parameterization on uncertainty, the level to whicha hydrologic model represents physical processes alsoinfluences uncertainty. For example, some modelsoversimplify soil water storage and evapotranspirationand subsequently do a poor job of predicting theimpacts of climate change on those processes.• Calibration. The accuracy to which a hydrologicmodel is calibrated influences the certainty of modelpredictions; this certainty should be carried throughto the interpretation of model results. For example,if the calibrated model is shown to poorly representlow flow conditions, the uncertainty associated withpredicted impacts to low flow conditions should beconsidered when evaluating the results.