ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario54could result in under-estimation of future runoff,recharge and streamflow. It is recommended that anyhydrologic model used for climate change impactassessment should have the ability to track soil watercontent and limit evapotranspiration rates based on soilwater content.Guidance:Hydrologic models used should have theability to track soil water content and limitevapotranspiration rates based on soil watercontent.6.2.2.2.3 InfiltrationInfiltration is the amount of precipitation that entersthe top of the soil column. When precipitation exceedsthe infiltration rate of the soil, overland runoff occurs.Infiltration is affected by soil structure, land cover andwater content of the soil.Climate-related variations in infiltration rates are relevantto climate change impact analysis. Seasonal shifts ininfiltration, due to the freezing and thawing of surficialsoils, can dramatically alter the soil infiltration rate.Under existing climate conditions, frozen soil conditionsprevalent throughout the winter months in Ontariogreatly reduce infiltration rates and promote overlandrunoff. As soils thaw during the spring, infiltrationrates increase, resulting in less overland runoff andmore recharge. A major impact of climate changeon water resources is expected to be a reduction inthe prevalence of frozen ground conditions, with acorresponding increase in winter infiltration capacities.The ability of a hydrologic model to represent thisshift will be important in understanding the impacts ofclimate change.Water resource models account for the freezing andthawing of soils in one of two ways. The first methoduses seasonal adjustment factors which vary theinfiltration capacity of soils on a monthly or seasonalbasis. These adjustment factors are usually calibrated torepresent average freeze/thaw conditions and cannotrepresent freeze/thaw cycles that deviate from thataverage. The second method relies on the model tocontinually modify infiltration rates according to airtemperatures and possibly snowpack conditions. Thistemperature-based method typically tracks accumulatedfreezing degree-days and reduces infiltration parametersas specified thresholds are exceeded. When snowpackis considered, it is generally to increase or decreaseinfiltration depending upon the presence or absence ofthe pack.Models that rely on temperature-based and snowpackmethods to simulate monthly/seasonal infiltrationcapacity modifications are preferred to methods utilizinguser specified seasonal infiltration adjustments. Inaddition to being able to represent the thawing of soilsduring an event that deviates from the average (e.g.,January melt), the temperature-based method canautomatically account for the increased occurrence ofunfrozen soils that would occur under a changed climatein Ontario.Methodologies relying on user specified seasonaladjustments can also be used for climate change impactassessments; however, the monthly/seasonal adjustmentfactors would have to be manually modified to reflectthe future climate for each possible climate scenario.For studies investigating a large number of climatescenarios, this may prove cumbersome and somewhatarbitrary.Guidance:Infiltration algorithms used in hydrologicmodelling should be sensitive to the onset andthawing of frozen ground, especially as theseconditions are altered by climate change.6.2.2.2.4 Snow Accumulation and MeltingSnowmelt is a key process for Ontario watersheds. Thespring snowmelt period usually produces the majorityof overland runoff; it is one of the major groundwaterrecharge periods. Any hydrologic model assessing theimpact of climate change within Ontario must havethe ability to simulate snow accumulation and meltingprocesses.