L8. Evapotranspiration III Flashcards
Define evapotranspiration
summarizes all process that return water to the atmosphere in vapour form
- evaporation: direct transfer of water from open water bodies, soil surfaces, or vegetated surfaces
- transpiration: indirect transfer of water from the root-stomatal system
How can you determine evapotranspiration?
- lysimeter measurements
- inflow-outflow measurements
- micrometerorological measurements (flux towers)
- study of groundwater fluctuations
- ET equations
What is lysimetry
a scientific method and technique used to measure and study the movement of water within a specific area of land or soil, typically using specialized equipment called lysimeters
How does a lysimeter work
It is a container/chamber that surrounds a specific amount of soil. It gets weighed often and changes in weight provide data on the water balance, as any weight loss corresponds to water loss due to processes like evapotranspiration. It may also have technology to measure other things.
- Must match soil and vegetation perfectly to conditions outside of the tank.
What is a flux tower?
An instrumented structure used in environmental and ecological research to measure and monitor the exchange of gases and energy between the Earth’s surface and the atmosphere.
- measures the vertical transfer of water vapour driven by convective motion
- evaporation can be calculated bu using measurements at different elevations (along the tower)
How does the Eddy covariance principle work?
The primary measurement technique used by flux towers is the eddy covariance method. This method calculates the vertical flux of gases and energy based on fluctuations in the concentration of these gases and the vertical wind speed. It relies on the concept that turbulent eddies transport gases and energy between the surface and the atmosphere.
Pros and cons of eddy towers
Pro:
- direct ecosystem measurements
- measures are continuous and at a high temporal resolution
- no disturbances to ecosystem
Con:
- expensive
- requires are to be turbulent for eddies to be measured
- requires flat terrain and homogeneous underlying vegetation
- difficult to separate ecosystem components directly
- hard to fill gaps of data
define potential evapotranspiration
The et that would occur if there was an adequate soil-moisture supply at all time
- determined by local weather conditions (VPD, wind) and energy supply
Define actual evapotranspiration (AET)
- Actual evaporation rate from any surface under prevailing conditions of moisture availability and radiative input
- determined by weather, energy status, and water availability
Define reference crop evapotranspiration
ET that would take place under strictly prescribed biologic and surface moisture conditions (ex. well-watered grass, 0.12 m high)
What is the Blaney-Criddle method of determining ET
- focus solely on air temp
- computes daily PET
- Potential daily ET = mean proportion of daytime hours in a day (P) x mean daily air temperature (Tmean)
What is the Thornthwaite model?
A method for estimating potential evapotranspiration (PET) and understanding the water balance of a region based on temperature and latitude
- need to find monthly heat index, then yearly and use that to compute monthly potential ET
- has low data requirements, so it can be used where data are limited
- over the course of a year, air temperature tends to lag behind radiation.
What is the Penman model
- much greater data requirements than Blaney-Criddle and Thornthwaite
- equation is a combination of turbulent transfer and energy-balance approaches
What is the Hargreaves method
- tells how much ET in grass systems
- simple and easy to use
Generally what is the difference P-ET
negative in dry conditions and positive for human conditions
- relates to vegetation cover which is higher in humid climates
