Lec 8: ET III Flashcards
Define evapotranspiration
Evapotranspiration (ET) summarizes all processes that return water to the
atmosphere in vapor form.
Combined “loss” of water vapor from within the leaves of plants (“transpiration”)
and evaporation of liquid water from water surfaces, bare soil and vegetative
surfaces.
What are the 5 ways ET can be determined?
(a) Lysimeter measurements: ET = I – S
However, there are some differences between lysimeter conditions and natural conditions.
(b) Inflow-outflow measurements (water balance principles)
ET = P + R1 – R2 ± Os - ∆S
(c) Micrometeorological measurements (flux towers)
(d) Study of groundwater fluctuations
Daily rise and fall of GW table give an indication of ET losses.
(e) ET equations
They relate ET with readily available climatic data.
Which measurement types gives a direct measurement of ET?
lysimetry
What are the advantages and disadvantages of lysimeters?
- Advantages:
‒ Primary tool for evaluating weather effects on ET - Disadvantages:
‒ Difficult and expensive to construct
‒ Requires careful operation and maintenance
‒ Primarily for research
What does a lysimeter measure? (7)
- water balance
- nutrient leaching
- soil temperature and moisture
- available water
- soil co2
- electrical conductivity
- GHG emissions
What do flux towers measure?
How can evaporation be calculated from it?
Directly measure how much CO2 or H2O vapor blows in or out of a site in wind gusts
- can calculate evaporation by using measurements made at different
elevations above the land surface, along the eddy flux tower
What are the advantages and disadvantages of the eddy covariance method?
- Advantages:
‒ Direct ecosystem-level measurements
‒ Measurements are continuous and at high temporal resolution
‒ Fluxes are determined without disturbing the surface being monitored - Disadvantages:
‒ Expensive
‒ Requires the air to be turbulent for eddies to be measured
‒ Requires flat terrain and homogeneous underlying vegetation
‒ Difficult to separate ecosystem components directly
‒ When data acquisition fails, gap-filling the data is difficult
Define potential evapotranspiration (PET)
ET that would occur if you had an unlimited supply of water
determined by weather conditions and energy status
Define actual ET (AET)
Amount of ET that actually occurs based on moisture availability and radiation input
determine by weather conditions, energy status, AND water availability
In what conditions can AET = PET
At open water surfaces (e.g., lakes) and over saturated bare soils , it is possible to
have AET = PET
Define pan evaporation
Evaporation from a US Class-A evaporation pan
Define reference crop ET
ET that would take place under strictly prescribed biologic and surface moisture
conditions, e.g.:
- well-watered grass, 0.12 m high
- canopy resistance (rs) of 70 s/m
- albedo of 0.23
- actively growing, completely shading the ground
- Only climatic factors affect changes in ETr
One of the 5 ways of determining ET is ET equations. What are the 5 ET equations we saw in class, and how complex is each of them?
1) Blaney-Criddle method
- simplest
- focuses only on air temp
- computes daily PET or ET(rc)
- expressed in mm/day, but should be considered an average for periods of one month or greater.
2) Thornthwaite
- simple
- compute a monthly heat index, aggregate into a yearly one, and then compute monthly PET using that
3) Hamon
- very simple
4) Hargreaves
- medium complexity
- estimates grass reference ET (ET(rc))
- strong dependence on air temperature
- easy to use, data requirement low
5) Penman
- very complex, requires more data
- Penman Monteith: considers surface and atmospheric resistances
- mostly used in conceptual ways
Which is better for arid climates, blaney-criddle or hargreaves?
Hargreaves has better predictive accuracy in arid climates than the Blaney-Criddle due to consideration of:
– Max-min temperature difference
– Extraterrestrial radiation
What are the 3 categories of equations we saw in class? Give an example for each category.
equation categories:
1) utilizing mean daily air temperature
- Blaney-Criddle
- Thornthwaite model
2) utilizing mean daily air temperature AND humidity
- Hargreaves
- Hamon
3) complex equations
- Penman and Penman Monteith models
