Evapotranspiration

1. Evapotranspiration R. Hudson - VFR Research

2. Evapotranspiration • 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. • 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

3. BC ET rates • 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

4. Energy for ET • 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 • For ET to occur, the following is needed: • a source of water R. Hudson - VFR Research

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

6. When water supply becomes limited, ET<PET • 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

7. Evaporation pan • 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

8. 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 • 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

9. Direct measurement • 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

10. 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 • water table and soil moisture levels can be monitored • pro: very accurate, can measure ET or PET • con: very expensive and difficult to set up. R. Hudson - VFR Research

11. Direct measurement • 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: • ET = precipitation - streamflow • can give basin average ET for forested catchments R. Hudson - VFR Research

12. Empirical methods • 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

13. Ta = mean monthly air temperature I = annual heat index b = 0.49+(0.0179 I)-(0.0000771 I2)+(0.000000675 I3) R. Hudson - VFR Research