ClimateChange Assessment Guide.pdf - University of Waterloo

Hydrological Impacts5. Hydrologic Impacts35This section discusses climate change from a waterresources perspective. It identifies some water resourceclimatechange studies conducted in Ontario and alsoprovides a review of fundamental hydrologic processesand the effects of climate change.The hydrologic cycle describes the pathways thatwater takes as it moves between the atmosphere, landsurface, subsurface and surface water bodies. Thesepathways, representing hydrological processes, resultfrom the interaction of climatic, geologic and ecologicconditions. The radiation energy from the sun initiatesthe hydrologic cycle by evaporating water from the landand oceans. The water vapour enters the atmosphere,is transported by air masses and condenses to formclouds. The clouds produce precipitation in the form ofrain or snow. Precipitation falls on land surfaces, initiallywetting the surfaces (wetting includes interceptionby vegetated surfaces). If sufficient rain or snowmeltoccurs surface depressions are filled and overlandrunoff moves downslope to reach stream channels. Theremaining water at the land surface, after accountingfor runoff, either evaporates from the wetted surfacesor infiltrates below the ground surface where it is stored(soil water storage) or seeps down to the watertable andrecharges groundwater. Soil water storage is depletedby evapotranspiration. Groundwater storage is depletedby downslope flow to locations where groundwaterdischarges to the land surface.Climatic conditions are the driving forces of thehydrologic cycle. As such, climate change that altersthe historic pattern of variation in climate will impact allcomponents of the hydrologic cycle, both in terms oftiming and quantity. Changes in seasonality, variabilityand extremes will be more disruptive than averagechanges (Smith et al., 2005).The impacts of climate change on surface waterresources are directly experienced in all componentsof the water balance, while the impacts to groundwaterresources are experienced through water rechargereaching the aquifer (Loaiciga, 2003). Due to therelatively greater amount of water in the ground than onthe ground surface, the rates of change due to climatewill be slower and less influenced by short term events.Consequently, relative to surface water resources,aquifers are buffered from the effects of a changingclimate, and as such will become an increasinglyimportant resource (Chen et al., 2004). It is important torecognize, however, that groundwater and surface waterare essentially a single resource (Winter et al., 1998);impacts to any hydrological process will influence allother hydrological processes. For example, any increasein consumptive groundwater use has consequenceson streamflow for watercourses receiving groundwaterdischarge from the same aquifer.In general, an increase in temperature is expected toelevate potential evapotranspiration rates, diminishsnowpack accumulation and reduce the number offreezing days leading to seasonal increases in infiltration.An increase in the frequency of severe storms promotesincreased amounts of surface runoff. In other words,climate change is likely to intensify the extremes inhydrological cycle processes (Loaiciga et al., 1996).Consideration also needs to be given to societalresponses to changing climatic conditions as changes inwater demand and land use practices can also alter thelocal and regional behaviour of hydrologic systems.5.1 Studies Completed in OntarioSeveral climate change impact assessment studies havebeen completed within the Province of Ontario. Thefollowing sections describe the investigations, theirmethodology and results, and some identified gaps.5.1.1 Credit Valley Conservation (CVC) (Walker andDougherty, 2008)As part of the CVC Water Management Strategyprocess, a Hydrological Simulation Program-FORTRAN(HSP-F) based surface water flow and water qualitymodel (Bicknell et al., 2001) was setup and calibratedat the watershed level. Several scenarios representingfuture conditions were devised to test the sensitivity ofthe watershed to further urbanization and to variouswater management strategies. In addition, two climatechange scenarios were developed and used as input forsimulations. The GHG scenarios were from the familyof SRES scenarios (Table 3.1) and were based upon thesimulation results from two GCMs, the Canadian CGCM2and the UK HadCM3, for the 2050s. These GCM-GHGruns were selected to represent the largest projectedchanges in air temperature and precipitation and thus,bracketed the larger group of GCM-GHG emissionscenario combinations. The scenarios represented warmand drier conditions (CGCM2) and warmer and wetterconditions (HadCM3) in annual average terms.