ClimateChange Assessment Guide.pdf - University of Waterloo

Hydrological Impacts43mid-winter melts would be expected, further reducingaccumulated snowpack in the future. There is morepotential for rainfall on frozen ground with futureclimates. This condition can lead to flooding andwinter soil erosion.• Reductions in the storage of snow have implicationsfor altering the dynamics and risk of floodingassociated with spring snowmelt and the timing andquantity of infiltration to aquifers. A shift in the timingand magnitude of spring melt and accompanyingaquifer recharge and a redistribution of precipitationseasonally could result in severely stressed summerlow flow conditions in streams and much higherlikelihood of winter flooding.5.2.3 Groundwater RechargeWhen discussing groundwater recharge it is importantto distinguish between infiltration and percolation.Infiltration refers to all the water that enters the soil bydownward flow from the land surface. Infiltrated waterenters the water-filled pores of the unsaturated zone andeither is stored there or continues to move down. Thestored portion is removed either by evaporation directlysupplied by soil water or by evaporation of transpiredwater after entering plant roots. Percolation refers to theportion of infiltrated water that migrates down below theevaporative zone depth (EZD) and either continues downto become recharge at the water table or moves laterallyat local barriers to enter stream channels (interflow). Thetiming of percolating water reaching the watertable isgoverned by the dynamics of unsaturated flow.Infiltration and percolation occur in the unsaturatedzone. In this zone some pores are water-filled and someare air filled. The two-phase flow of air and water createsa non-linear dynamic for the flow of water. Water flowin unsaturated porous media is a specialty subjectwithin soil physics. The unsaturated zone is of criticalimportance to groundwater-surface water interactions.The partitioning of rain/snowmelt between runoffand infiltration is controlled by the infiltrability of theupper surface of the unsaturated soil zone and thisinfiltrability is in turn controlled by the depth of the soilwater zone, its pore-size distribution and its soil watercontent (portion of pores water-filled). The proportionof infiltrated water that becomes percolation and thenrecharge is controlled by the soil water content of theunsaturated zone.The soil water storage status of the unsaturated zonealso controls evapotranspiration through the relationshipbetween soil water status and the ratio of actual topotential evapotranspiration. Soil type and rootingdepth are important in determining the relationshipgoverning the evapotranspiration to potentialevapotranspiration ratio.Factors affecting recharge include precipitation,temperature, land use, vegetation, urbanization,overland flow, slope, infiltration and flow in theunsaturated zone, and soil properties (Jyrkama andSykes, 2006). Additionally, large amounts of rechargecan occur in urban areas through leaks in the watersupply and storm drainage infrastructure (Lerner,2002). In natural environments, groundwater rechargeoccurs everywhere except at locations of groundwaterdischarge, though there may be great variability in therate at which it occurs and its spatial distribution. Theimpacts of climate change on groundwater resources areexperienced through the recharge reaching the aquifer(Loaiciga, 2003), and thus is dependent on changes to allfactors affecting recharge listed above.The degree of sophistication to which unsaturated flowis simulated in hydrologic models varies greatly frommodel to model. In physically-based models unsaturatedconditions are simulated using the Richards’ Equation(RE; (Richards, 1931)). Two-dimensional hydrologicrunoff models take a more simplified approach; theydo not typically have an extensive representation of thesubsurface soil profile. For example, GAWSER (Schroeteret al., 2000) uses the Green-Ampt model (Green andAmpt, 1911) which does not explicitly consider thestorage and nonlinear flow dynamics of water in the fulllength of the unsaturated zone. Water budget modelssuch as HELP3 also use a simplified approach thatmaintains the storage and nonlinear flow dynamics ofwater for the full length of the unsaturated zone; therebyclosely approximating the RE which gives it a strongphysical basis.Potential impacts on recharge to the water tableresulting from climate change may include the following:• Precipitation is strongly correlated to recharge;therefore increases in precipitation will lead toincreases in recharge, and conversely, decreases inprecipitation will lead to decreases in recharge.• Infiltration is dependent upon storm intensity;