ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario42temperature index method (Maidment, 1992). Snowaccumulation and melt is influenced by precipitation,air temperature, solar radiation, wind speed and otherlesser factors (i.e., albedo, dewpoint, humidity).The energy balance approach applies the law ofconservation of energy to the snowpack whose lowerand upper boundaries are the ground-snow and snowairinterfaces, respectively. The energy balance approachestimates snowmelt by balancing the external energyfluxes transmitted to the snowpack with the internalenergy fluxes of the snowpack. The external fluxesinclude radiation, sensible energy (energy exchangedue to the difference in temperature between the snowsurface and overlying air), latent energy (change-of-stateenergy as ice changes to water vapour (sublimationthat causes an energy loss from snowpack) or as watervapour changes to ice (horification causes an energygain) [the direction and rate of the exchange is governedby the difference in vapour pressure between the snowsurface and the overlying air]), ground heat (energyexchanged between the snowpack volume and theground by conduction) and advection energy (energyderived from external sources, e.g., rain) (Maidment,1992). The measurement of all the parameters requiredto use this method can be excessive and limited in termsof availability and practicality of measurement. Modelssuch as HSP-F and GAWSER allow the option of usingthe temperature index method or an energy balanceapproach to model snow accumulation and melt. Inaddition to precipitation and air temperature, threevariables (i.e., solar radiation, dewpoint and wind speed)are required for the simulation with the energy balancemodel. The energy balance approach, while more dataintensive, more accurately reflects diurnal trends insnowmelt in addition to the influence of solar radiationon sunny days.As a contrast to the energy balance approach,temperature index methods can be used for snowmeltprediction whereby simple mathematical expressionsrelate temperature to observed snowmelt conditions.The advantage of this approach is that it producescomparable results to energy balance approaches overthe longer term, while the input temperature data,are relatively easy to obtain, measure, extrapolateand forecast (Maidment, 1992). As such, hydrologicbudget tools typically employ some variation of thetemperature index method. Use of such snowmeltroutines for climate change impact assessments wouldrequire an assumption of historical snowmelt conditions/temperature relationships holding under a changedclimate. This assumption is impossible to prove ordisprove at this time.More rigorous snowmelt routines, such as those in theGAWSER model, consider other processes, includingredistribution after snowfall (drifting), snowpackcompaction due to ice crystal growth, temporal changesin the porosity (liquid holding capacity) of the snowpackand sublimation (process of snow/ice changing directlyinto water vapour) and track the amount of liquid waterheld in the snowpack and refreezing of this water.Provided that the algorithms used to represent theseprocesses are physically based and not derived fromempirical representations the relationships verified withhistorical climate conditions will hold true in a changedclimate. Inclusion of these more detailed representationswill improve the confidence of users in the resultobtained from model representations of various climatescenarios.Sublimation is a particularly difficult process torepresent, largely due to the number of factorsgoverning the rate of sublimation and their spatial andtemporal variability. Factors that affect the amount ofsublimation include incident solar radiation, reflectivityand roughness of the surface, relative humidity, windspeed, temperature and slope aspect (north versussouth). In recent years specific studies of sublimation ofsnow by Pomeroy and others (Pomeroy et al., 2007) haveled to the development of physically-based algorithmsfor calculation of this process. Incorporation of thesephysically based calculations for snow sublimationinto hydrological models will improve the simulationof snowpack accumulation and ablation and will alsoconsiderably improve the confidence of the use ofhydrological models under changed climate conditions.Conceptually, changes to the accumulation and meltingof snow that may occur as a result of climate changeinclude:• The potential changes in seasonal precipitation thatare anticipated with climate change will affect thequantity of snow accumulated over the winter months.Increasing temperatures can result in a reductionof the number of freezing days, thereby reducingsnow accumulation and advancing the spring melt.In addition to the earlier spring melt, more frequent