ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment55As the climate warms and mid-winter melts becomemore prevalent, snowpack will not accumulate to thesame degree as it has historically. Rather than winterprecipitation being stored within the snowpack andreleased during intense spring melt episodes, morefrequent, but smaller snowmelt events are likely tooccur and release precipitation throughout the winter.Also, more winter precipitation will occur as rain andimmediately become runoff.Many existing hydrologic models employ relativelysimplistic representations of snowmelt processes.These representations are typically slight modificationsfrom the temperature-index method, which tracksair temperature and initiates snowmelt process oncespecified thresholds are exceeded. Meltwater thenleaves the snowpack and is provided to the soil surfacefor infiltration or runoff. These simple hydrologic modelsare less sensitive to climate change as they do notconsider more complex snowpack processes such assnow redistribution, refreeze, and sublimation.More sophisticated models consider processes/mechanisms such as liquid water holding capacity of thesnowpack, liquid water refreeze, snow accumulation,snow sublimation, and snow redistribution. Such modelsmay also consider climatic parameters supplemental toair temperature, namely solar radiation, relative humidity,and wind speed.As snow and snow accumulation are reduced in thefuture, these processes remain critical to hydrologicinvestigations, especially as it relates to climate change.Detailed snowmelt algorithms may become moresignificant in a changed climate as the frequency ofsnowmelt events is increased throughout the winter. Theredistribution of snow from fields to forest/fence edges,the development of ice layers within the snowpack,and consideration of solar energy may become muchmore significant during mid-winter melts when the airtemperature remains close to 0ºC.Guidance:Selected hydrologic models should havedetailed snowmelt and accumulation/distributionalgorithms.6.2.2.2.5 Rainfall IntensityThe proper representation of rainfall intensity is criticalfor any hydrologic investigation, but is particularlyrelevant when the study objective is high flowcharacterization or stormwater management.The intensity of a rainfall event is directly linked tothe amount of surface runoff that is created versusthe amount of infiltration that occurs. Rainfall eventsproduce significantly more runoff when rainfall isintense for a brief period, such as 10 or 30 minutes(i.e., a thunderstorm) than when it is evenly distributedthroughout the day in a slow drizzle. The ability ofhydrologic models to consider rainfall intensity is tiedto the simulation time step used. Models employinga daily time step cannot represent the impacts ofrainfall intensities or measure the impacts of changes inintensity at scales smaller than a day.Changes in rainfall intensity are expected to bea significant impact of a changed climate. As airtemperatures increase, the frequency of thunderstormsis also likely to increase. Thunderstorms are localized,quick moving systems that have durations that rangefrom a few minutes to a few hours. This shift awayfrom slow moving, regional precipitation eventsto thunderstorms may not be limited to traditionalthunderstorm months (e.g., June, July, August), but alsofor months which thunderstorms are currently rare (e.g.,spring, fall and winter months). The expected changesin rainfall intensity are currently not well understood.Most methods rely upon statistical relationships derivedfrom historical climate observations, while there is noassurance that the existing statistical relationship will bevalid in the future. Regional Climate Models (RCMs) havethe potential to represent these changes in terms ofrainfall intensity; however, the available time steps (3 and6 hours) from RCMs are not yet sufficient. Global ClimateModels (GCMs) output precipitation data at daily timesteps, and do not offer the ability to represent changesin rainfall intensity.As sub-hourly time steps are not possible with theavailable climate model output, peak flow ratesimulation using smaller time steps and urban floodlinemapping are not yet practical in the climate changeimpact assessments discussed in this document. Severalstudies are underway at municipal levels to determineclimate changed IDF (intensity-duration-frequency)