ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario44increases in storm intensity promote relatively greateramounts of overland flow and runoff relative toinfiltration. Thus, if an increase in annual precipitationis solely attributed to increases in storm intensity thenrecharge to groundwater can be expected to remainthe same or perhaps be reduced.• Due to increasing temperatures, the timing of majorrecharge events associated with the spring snowmeltmay shift to earlier in the year. Additionally, theduration of major recharge events may be extended,and thus recharge volume increased, during thesnowmelt season.• Possible increases in potential evapotranspiration withclimate change, may cause the unsaturated soil zoneto dry faster (spring) or take longer to return to wetconditions (fall), thereby reducing recharge in thoseseasons.5.2.4 StreamflowWhile there are various definitions available, for thepurposes of this discussion streamflow contributionsinclude groundwater discharge from seeps andsprings, interflow and surface overland runoff as wellas anthropogenic sources such as municipal andindustrial effluents and diversions. Precipitation fallingdirectly on the stream also contributes to streamflow;however, its role is typically insignificant relative to othercontributions.Baseflow is the portion of streamflow that persists duringdry periods with no rain or snowmelt. Baseflow maycome from depletion of surface storage but in SouthernOntario most baseflow comes from groundwaterdischarging to surface ditches, drains, streams andrivers. Baseflow from groundwater occurs throughout theyear at varying rates according to groundwater storageconditions that, in turn, are dependent on previousrecharge amounts. Discharge of groundwater to streamsis dependent on the elevation of the local groundwatertable being higher than that of the streambed.Interflow, also referred to as subsurface storm flow, isthe lateral movement of water that enter fissures andmacropores in the upper soil profile from localizedpockets of saturation either in the vadose zone, orjust below the water table surface, and discharge tothe receiving stream channel during the storm event.Flow discharging to the stream channel from buriedpipe agricultural drains (tile drainage) is, strictlyspeaking discharge of groundwater but has the sametiming characteristics as interflow and is commonlyincluded with interflow in hydrologic models. Theprime requirement of interflow is a sloping soil horizonof high infiltrability and lateral permeability with animpeding layer at shallow depth. This is quite commonin agricultural areas, and is almost always present onforested valley side slopes. Interflow is most common inthe spring following snowmelt when soil water storageamounts are typically high.Surface runoff is the portion of overland flow that directlyflows into the stream over the surface of the land. Itis recognized that overland flow in humid, vegetatedcatchments is a rare occurrence both spatially andtemporally (Freeze and Cherry, 1979). Surface runoffduring storms and snowmelt periods is responsible forflooding events. Surface runoff is particularly importantin areas with low permeability soils (i.e., clays and clayloams and frozen ground conditions) and in areas withthe water table at the soil surface. For these surfaceslittle or no infiltration occurs and most, or all, waterreaching the surface as rain, snowmelt or surface flowfrom upslope, leaves the area as surface runoff, or isponded above the surface as depression storage.The anthropogenic sources of streamflow (irrigationreturn flow, wastewater effluent, urban stormwater) arestrongly influenced by future management decisions andland use changes and this complicates the assessmentof climate-change effects. Effluent discharge rates underfuture conditions are likely to be modified from currentrates by changes in population, urbanization trends,water consumption practices, technologies employed,altered climate and external forces (i.e., policy,legislation). Likewise, anthropogenic water sources suchas stormwater and other non-contaminated dischargeswill be influenced by human activity and trends in thefuture. Any assessment of streamflow with climatechange must account for these other flow sources andtheir likelihood of change.Hydrologic models sum baseflow, interflow and runoffcomponents to create an outflow hydrograph from aspecified spatial area or catchment. This catchmenthydrograph is added to the stream network, and usingchannel characteristics such as slope, length, roughnessand cross section geometry, is routed downstream.Hydrographs are also routed through online ponds, lakesand reservoirs. Storage within the channel/ponds acts to