ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario52While the criteria for selecting the optimum model willvary depending on the project’s objectives and goals,there are four common criteria that are applicable formost projects. They are:• Input requirements;• Hydrologic processes representation;• Required data outputs; and• Price, support and documentation.6.2.2.1 Input RequirementsSimple hydrologic models are preferred when theavailable local data are sparse; conversely, morecomplex models should be adopted when theirextensive input data requirements can be satisfied.Model sensitivity to changing climates should alsobe considered when selecting a hydrologic modelfor a climate change impact assessment. The scopeof the assessment is dependent on the model’s inputparameters. For example, hydrologic models thatrequire only temperature and precipitation as input willlimit the scope of the climate change impact assessmentto the effects of these two parameters. Models thatrequire additional parameters such as solar radiation(or sunniness/cloudiness), wind speed, humidity, etc.will expand the climate change impact assessmentto include a wider array of potential impacts. Theseadditional parameters are generally available as outputfrom Global Climate Models. The sensitivity of themodel and the scope of the assessment are related tomodel’s representation of key hydrologic processes,which are discussed in the following section.Guidance:Consider the availability of input data whenselecting a water resources model.6.2.2.2 Identify Key Hydrological Processes andMethodsThe accurate representation of key hydrologic processeswithin a water resources model is critical to assess thepossible impacts of climate change on water resources.The ability of a hydrologic model to represent both thekey hydrologic processes and the impact of a changedclimate on those processes should be the primarycriterion in selecting a hydrologic model for climatechange impact assessment.Using the above criterion to select a hydrologicmodel leads to the use of more complex models (i.e.,employing more independent climate variables) as thesemodels will generally be more sensitive to the full natureof future climates.When comparing existing and future conditions, theimplicit assumption is that the relationships containedwithin the model are valid for both time periods. Whenselecting a hydrologic model, this assumption shouldbe evaluated; if relationships are not expected to bevalid under future conditions, a different model may berequired.Although studies may have different hydrologic processthat are important to the specific focus of the study (e.g.,water budget, reservoir yield analysis, low flow habitat),there are certain hydrologic processes that are importantto every hydrological investigation. These hydrologicprocesses are discussed in the following subsectionsand include evapotranspiration, soil water accounting,infiltration, snow accumulation and melt, and rainfallintensity.Guidance:The method used by each candidate hydrologicmodel to simulate key hydrologic processesshould be understood. The applicability of thatmethod under a changed climate should beevaluated.6.2.2.2.1 EvapotranspirationEvapotranspiration is the collective process ofevaporation from water surfaces or water stored inthe soil column and transpiration from vegetation.In Ontario, evapotranspiration (ET) is the dominantprocess in the water cycle; it is responsible for up to60% of the water lost from watersheds on an annualbasis. Spatially averaged evapotranspiration can bereasonably estimated for basins where net groundwater