ClimateChange Assessment Guide.pdf - University of Waterloo

Climate Change Assessment53inflow/outflow is negligible (generally basins greaterthan a few hundred square kilometres) by subtractingmean annual streamflow from annual precipitation.Estimating evapotranspiration at the local scale (specificland use/geology combination), however, is difficultto estimate or monitor; it is estimated through theapplication of calibrated hydrologic models. Due to thesignificance of evapotranspiration to the overall waterbalance, misrepresentation of this process will introducesignificant errors into other water budget components(e.g., runoff, groundwater recharge).Under a changed future climate, evapotranspirationis expected to be significantly modified from existingconditions. In particular, this change is expected to belargest in the late fall, winter, and early spring seasons(see Section 5.2). Currently, evapotranspiration duringthese seasons in Ontario is close to zero, as vegetationbecomes dormant and air temperatures drop belowfreezing. Evaporative losses during this time periodrelate mainly to sublimation of the snowpack. Astemperatures increase, days with evapotranspirationshift later into the fall and earlier into the spring. Winterevapotranspiration rates also increase due to increasedsublimation and the increased prevalence of mid-wintermelts. While summer evapotranspiration rates also havethe potential to increase, the availability (or lack thereof)of soil water content within the upper soil zone limits theamount of actual evapotranspiration.Evapotranspiration algorithms in the hydrologic modelshould rely on physically based representations ofevapotranspiration and should not rely upon historicalobservations or empirical relationships developed fromhistorical observations. Physically based representationsof evapotranspiration may include consideration ofair temperature, solar radiation, wind speed, relativehumidity and dewpoint temperature, and attemptto calculate evapotranspiration from first principles.Methods for estimating potential evapotranspirationfrom climate data are discussed in Section 6.5.6.Hydrologic models which use historical pan evaporationobservations or have set monthly evapotranspirationrates based on existing conditions would not beappropriate for use in climate change impactassessments, as there is no clear rationale for adjustingfuture evapotranspiration rates.Guidance:The selected hydrologic model should havean evapotranspiration (ET) algorithm that isphysically-based in order to represent futureclimate changed rates and fluxes.6.2.2.2.2 Soil Water AccountingWater stored within a soil matrix (pores and spacesbetween soil grains) is termed “soil water content”.This water is removed by evapotranspiration, whichis evaporation or plant uptake (transpiration). Theevapotranspiration rate moves to zero when the soilwater content reaches the wilting point of the soil;evapotranspiration remains at zero until an infiltrationevent replenishes the soil water content. Soils withlarge amounts of soil water content can typically sustainevapotranspiration rates longer into dry periods thansoils with smaller soil water content. The amount ofwater held within the soil column also affects infiltrationrates.More complex hydrologic models consider the impactof soil water availability on evapotranspiration rates. Bytracking and accounting for inputs and losses from thesoil water reservoir, these models can replicate the effectof soil water availability on evapotranspiration rates,reducing or ceasing evapotranspiration when soils reachtheir wilting point.The hydrologic model’s ability to consider the impact ofsoil water availability is critical when assessing climatechange impacts. Under a changed climate, summermonths are expected to have significant temperatureincreases, with corresponding increases in potentialevapotranspiration rates. However, increases in summerpotential evapotranspiration rates may not result incorresponding increases in actual evapotranspirationdue to limitations on the availability of soil water.Under the existing climate, typical years see soil wateravailability reach zero in the mid-late summer, with thisdeficit extending into the early fall.Models which do not consider the ability of soil watercontent to limit evapotranspiration would over-estimatefuture evapotranspiration rates. This overestimation