ClimateChange Assessment Guide.pdf - University of Waterloo

Guide for Assessment of Hydrologic Effects of Climate Change in Ontario40include radiation, air temperature, humidity and windspeed; radiation is the most dominant (Maidment, 1992).Crop and vegetation factors may include resistance totranspiration, plant height, leaf roughness, reflection,ground cover and rooting characteristics. Managementand environmental conditions refers to the soil nutrientrequirements for agricultural purposes. Though thework of Allen et al. (1998) focuses on evapotranspirationfrom an agricultural perspective, these factors are alsorelevant in characterizing the evapotranspiration fornon-agricultural land use designations, particularly withrespect to weather parameters and crop factors.defPotential evapotranspiration (PET) • amount ofwater transpired in a given time by a short greencrop, completely shading the ground, of uniformheight and with adequate water status in the soilprofile (Penman, 1948).Hydrologic models routinely use potentialevapotranspiration (PET) to simulate theevapotranspiration component of the water budget.This is a theoretical construct, specifying an upper limitof actual evapotranspiration; it is used within hydrologicmodels to determine the maximum amount of waterthat can be removed from the land surface, shouldthere be sufficient water available. Some hydrologicmodels include potential evapotranspiration modifiers,such as vegetative cover or growing season factors,to better represent the spatial and seasonal variationin evapotranspiration. Some hydrologic models setevapotranspiration in the winter months to zero,although there are relatively small, but significant,evaporative losses that occur during the cold seasonthrough sublimation.For example, the HELP3 model uses a potentialevaporation approach whereby evapotranspirationis estimated using the meteorological parameters ofdaily temperature and solar radiation, average annualwind speed and average quarterly relative humidity.Vegetative parameters used include leaf area index (LAI),which characterizes vegetative cover, and the length ofthe growing season defined by its start and end dates.Using the potential evapotranspiration approach tosimulate evapotranspiration demand is also employedin complex coupled groundwater-streamflow generationmodels such as GSFLOW (Markstrom et al., 2008).An alternate method of calculating actual evaporationfrom surfaces, known as the complementary approach(Szilagyi and Jozsa, 2008) uses a physically basedalgorithm that replaces the semi-empirical reductionfactors that are used in existing hydrologic models toobtain actual daily evapotranspiration from estimatesof potential evapotranspiration. The data required forthe complementary evapotranspiration algorithm is thesame as for existing model representations. Adoptionof the complementary approach in hydrological modelswill reduce the empirical nature of the calculation andimprove the confidence of users making calculations ofevapotranspiration under altered climate conditions.The response of evapotranspiration is dependant on thechanging meteorological and atmospheric conditionsthat accompany climate change. Potential changes ofthe response of evapotranspiration include:• Increasing temperatures will promote greaterlevels of evapotranspiration if vegetative and soilwater conditions are supportive. Furthermore,increasing temperatures can lengthen the growingseason thereby increasing the annual potentialevapotranspiration budget.• Under a changed climate, there may be significantincreases in evapotranspiration during winter months.Current potential evapotranspiration algorithms,built and calibrated to existing conditions, may notaccurately reflect the impact of increased temperatureon potential evapotranspiration during winter months.• The increasing atmospheric concentrations of CO 2have been observed to stimulate plant growth rates,improve water use efficiency, and result in higher plantyields. With respect to evapotranspiration, vegetationresponds to higher CO 2concentrations with increasedrates of photosynthesis and with increased stomatalresistance, the rate at which water vapour canevaporate from leaf pores. Higher stomatal resistancewill result in reduced transpiration, hence conservingwater and reducing plant water stress (Smith et al.,2005).• Uncertainty in the feedback loop arising from theinteraction of clouds and radiation also poseschallenges to understanding the response ofevapotranspiration to climate change. The capacity