Evapotranspiration R Hudson VFR Research Evapotranspiration n Evaporation

Evapotranspiration R. Hudson - VFR Research

Evapotranspiration n Evaporation + Transpiration – Evaporation = change of state of water from liquid to vapour, net transfer to the atmosphere. – Transpiration = loss of water vapour from the stomata of plant leaves and replacement in the plant of water extracted from the soil. n Major losses of water from a watershed – Canadian average: 2/3 of ppt is lost to the atmosphere due to evapotranspiration R. Hudson - VFR Research

BC ET rates n In BC, mean annual evaporation varies from about 300 mm in the north to >600 mm in the southern interior and coast – ET ranges from 10 -25% of ppt on coast to as much as 90% of ppt in arid southern interior around the Okanagan. R. Hudson - VFR Research

Energy for ET n An energy balance equation for ET can be written, similar to that for snowmelt: Qn +Qh = QET + Qs + Qq – QET = energy available for ET (contains Qe term) – Qs = sensible heat stored in the atmosphere – Qq = sensible heat stored in biomass n For ET to occur, the following is needed: – a source of water R. Hudson - VFR Research

n Surface or ground water, or water contained in biomass – energy source, principally shortwave radiation, although sensible heat also contributes – vapour pressure gradient – wind n QET is the first priority n PET vs. ET – PET is potential evapotranspiration – ET = PET if not limited by water supply R. Hudson - VFR Research

– When water supply becomes limited, ET<PET n n when this occurs there is a water defecit Measurement of PET, ET – evaporation pans - standard AES method (PE) – lysimeters (PET, ET) – water balance (ET) – empirical formulae R. Hudson - VFR Research

Evaporation pan n Metal cylinder, sits on the ground and is filled with water – AES pan 4’ diameter with 10” sides filled with 8” of water – evaporation measured by monitoring the water level in the pan, usually with a float gauge, corrected for rain input. – Air and water temperature, wind speed are also measured. R. Hudson - VFR Research

– Usually pan evaporation is greater than lake evaporation or ET because of heat input to the sides and base of the pan – a pan coefficient is needed to convert the pan evaporation to ET for the desired vegetation type or to lake evaporation varies seasonally for a given type n range of 0. 3 to 0. 95, average 0. 5 to 0. 8 – there are fewer than 20 AES evaporation pans operating in BC, mostly in the southern part of the province – data from direct measurement needed to develop the pan coefficient. R. Hudson - VFR Research

Direct measurement n Lysimeter – a container is filled with soil on which vegetative cover is maintained, water balance in the container is monitored. – buried in the ground – precipitation input and drainage output are measured – water can be added to the lysimeter (PET) R. Hudson - VFR Research

PET = input - output + change in storage – input = ppt + added water – output = drainage, can be measured at drains in the bottom of the container with a tipping bucket – change in storage: container can be weighed continuously n water table and soil moisture levels can be monitored n – pro: very accurate, can measure ET or PET – con: very expensive and difficult to set up. R. Hudson - VFR Research

Direct measurement n Water balance – can be applied to small research basins – watershed must be water tight - no groundwater leakage – actual ET is back calculated from water balance equation: n ET = precipitation - streamflow – can give basin average ET forested catchments R. Hudson - VFR Research

Empirical methods n Similar to a temperature based snowmelt equation – air temperature is used as a surrogate for radiant energy available for ET – there are many such methods, (e. g. , Blainey. Criddle, Priestly-Taylor, Penman-Monteith), mostly developed for agricultural crops – most common: Thornthwaite - monthly PET R. Hudson - VFR Research

Ta = mean monthly air temperature I = annual heat index b = 0. 49+(0. 0179 I)-(0. 0000771 I 2)+(0. 000000675 I 3) R. Hudson - VFR Research