ClimateChange Assessment Guide.pdf - University of Waterloo

Hydrological Impacts41for clouds to absorb and reflect solar radiation as wellas absorb and emit long wave radiation means theirinfluence can either cool or warm the Earth’s surface(McCarthy et al., 2001). Thus, uncertainty from thecloud feedback loop can have a potently significantimpact on evapotranspiration as it influences twokey parameters that control its rate, temperatureand radiation, thereby having the potential to bothincrease and decrease the evapotranspiration budget.5.2.2 Winter ConditionsIn Ontario it is necessary to consider the effects ofwinter conditions on hydrological processes which aremanifested through the freezing and thawing of porewater in the soil, and the accumulation and meltingof snow on the ground surface. These processesare dependent on solar radiation, temperature,precipitation, wind patterns (for snow redistribution),albedo (affects amount of reflected solar radiation) andsoil water content.5.2.2.1 Frozen SoilSoil moisture has a controlling influence overthe partitioning of infiltration, overland flow andevapotranspiration processes. Soil water in the vadose(unsaturated) zone can greatly alter surface hydrologiccharacteristics when it freezes. Capturing this dynamicis important to understanding transient relationshipsbetween overland flow and infiltration during the onsetof the winter and spring seasons when the freezing andthawing of soil water typically occurs. When the water insoil pores freezes the void space becomes increasinglyrestricted. This increases resistance to water movementthrough a decrease in hydraulic conductivity, hencereducing infiltration. As the soil water freezes it expandsto become ice which deforms the soil matrix. The impactof freezing soil has implications when the soil thawsduring the spring, increasing the hydraulic conductivityand allowing for the rapid infiltration of snowmelt(Jyrkama, 1999). Due to the decrease in hydraulicconductivity of the soil, seasonally frozen groundessentially inhibits infiltration which has a significanteffect on promoting the occurrence and distribution ofrunoff.Despite its significant influence on cold-climatehydrology, the consideration of changes in infiltrability ofsoil in winter (increased viscosity of cold water, possibilityof frozen soil water) is not common among hydrologicmodels. For those models which do have the ability toconsider frozen ground conditions (i.e., HSP-F, (Bicknellet al., 2001); GAWSER (Schroeter & Associates, 2004)), itis typically considered by one of two methods: the useof average seasonal adjustment factors which reduceinfiltration capacity in winter, and increase it in summer;or alternately through the use of a temperature indexmethod, which reduces infiltration when accumulateddegree-days drop below a specified threshold.The average seasonal adjustments would not beappropriate to represent frozen ground conditionsunder a changed climate because they have beengenerated based on existing climate data. A procedureto analyse the adjusted temperature dataset to arrive atfuture freeze/thaw cycles and revised average seasonaladjustment factors would be needed to use this method.Models that use a temperature index method wouldautomatically adjust for the warmer condition andsimulate a shorter annual frozen ground period.Under the influence of increasing temperatures that areprojected due to climate change, the impacts on frozensoils may include:• A later onset of freezing soil conditions allowingfor relatively greater amounts of infiltration tooccur at the beginning of the winter season whenevapotranspiration demands are diminishing.• A reduction in the number of freezing days candecrease the depth to which freezing conditionspenetrate the soil horizon. This implies a more rapidthaw in spring, allowing recharge associated with thespring melt to occur earlier in the year.• An end of frozen soil conditions can allow increasedrainfall, and more frequent snowmelt events toinfiltrate into the soil and generate additionalrecharge than is currently experienced.5.2.2.2 SnowThe accumulation, redistribution and melt of snow arecomplex processes that are dependent on many factors.A snowpack develops when precipitation occurs as snowand is allowed to accumulate on the ground surface.The combined effect of sublimation and ablation(melting) is typically modelled either by using a processoriented,energy balance approach, or an operational,