ClimateChange Assessment Guide.pdf - University of Waterloo

Hydrological Impacts39frequency of hydrologic extremes are likely under bothfuture climate conditions. A shift in flooding conditionswas investigated. It was found that the 200 mm 24-hour storm, which had a 330 year return period underexisting conditions, became less likely to occur (i.e., 700year return period) under the increased warmer, drierscenario and more likely to occur (i.e., 120 year returnperiod) under the wetter scenario.The modified climate datasets were also used forcontinuous hydrologic modelling; it was found that dryweather flow and 90th percentile low flows increased by20% to 40%. Average annual maximum daily flows werereduced (~50%) due to the snowmelt period occurringearlier, lasting longer and having a melt intensity moreevenly distributed through the winter months.The study identified that current rainfall disaggregationmethods for estimating hourly data from daily data havelimited accuracy, and recommended additional researchin this field.5.1.5 The Impact of Climate Change on SpatiallyVarying Groundwater Recharge in the Grand RiverWatershed (Ontario) (Jyrkama and Sykes, 2007)Jyrkama and Sykes (2007) stress the importanceof capturing the spatial and temporal variability ofrecharge which can be considerable given variationsin vegetation, land use/land cover practices, slope,soil properties, as well as climatic conditions across awatershed. Each combination of these factors has itsown recharge signature and hence will have a uniqueresponse to changing stress conditions inducedby climate change. Preserving this level of detail,Jyrkama and Sykes (2007) modelled the impacts ofclimate change to the Grand River Watershed usingthe Hydrologic Evaluation of Landfill Performancemodel (HELP3, version 3) (Schroeder et al., 1994). Akey feature of this study is a methodology to use theHELP3 model to simulate detailed spatially distributed,temporally varying recharge conditions. This allowedfor the simulated response of climate change forcings(i.e., precipitation, temperature and solar radiation) tothe local recharge signature of unique combination ofvegetation, soil and land use/land cover properties.For the simulated climate change scenario (increasedprecipitation, temperature and solar radiation) theresults of the study showed that increasing precipitationhas the greatest influence over hydrological processes,increasing evapotranspiration, runoff and percolation,while increasing temperature had both positive andnegative effects to these model outputs.With respect to hydrologic impacts, the principalfindings include increased runoff and percolation inthe winter months with increased evapotranspirationin summer months. A key finding of the study is thenon-uniform spatial response to climate change acrossthe watershed owing to the combinations of surfacefeatures, vegetation and heterogeneous subsurfacelayering.5.2 Hydrologic ProcessesClimate change has the potential to affect all aspectsof the hydrologic cycle. The following sections describethe possible impacts to evapotranspiration, winterconditions, recharge and streamflow.5.2.1 EvapotranspirationThe parallel hydrological processes of evaporation ofwater transpired through the stomata of plant leaves andevaporation of water from watershed surfaces and nearsurface soil pores are commonly lumped into a singleprocess referred to as evapotranspiration. Sublimation offrozen water under subfreezing (winter) conditions is alsoincluded in the term. This lumping occurs because theseprocesses occur simultaneously and there is no way ofeasily distinguishing between them.For watersheds with an appreciable amount ofvegetated surface, the largest component of totalevapotranspiration is transpiration from the vegetation.Transpiration is dependent on type of vegetation,the water holding characteristics of different soilsand rooting depth as well as weather characteristics.Evaporation of intercepted water held on wettedsurfaces (vegetated and non-vegetated surfaces) isanother important component of evapotranspiration andmust be evaluated for each wetting event. Over a year,the amount contributed to annual evapotranspirationfrom intercepted water is appreciable even though thecontribution for a single wetting event is small.The rate of evapotranspiration depends on a number offactors including weather parameters, vegetation andcrop type, management practices and environmentalconditions (Allen et al., 1998). Weather parameters