Comparison of Evaporative Losses in Alberta Based on Five Evapotranspiration Models

Abstract

Water losses can occur from the earth surface to the atmosphere by two distinct mechanisms: evaporation and transpiration. As evaporation and transpiration can occur simultaneously, a collective term, evapotranspiration accounts for all processes through which water in liquid or solid form becomes atmospheric water vapour. In winter season, it is possible to obtain negative values for evapotranspiration in some locations in Alberta where the net longwave radiation from the surface is large compared to the net incoming shortwave radiation, and the vapour pressure deficit is small. Under these conditions, net condensation of water from the atmosphere is possible. We analysed 65 years (1955–2019) of climate data to estimate monthly evaporative losses in Alberta based on five models: Hamon Method, Penman Method, Penman-Monteith FAO-56 Method, Morton’s Complementary Relationship Areal Evapotranspiration Model, and Granger and Gray Method. Our analysis shows that during December-January the monthly potential evapotranspiration and actual evapotranspiration could be as low as −6 mm. The average net longwave radiation and shortwave radiation for December-January months, estimated for the same historical period, range from 54–65 W/m², to 3–33 W/m², respectively. Based on our analysis we conclude that negative evaporative losses are primarily due to the negative net radiation.