L6. Evapotranspiration I

Question 1

What is the definition of evaporation?

Answer 1

direct transfer of water from open water bodies, soil, and vegetation surfaces to the atmosphere
- is a function of the ability of air to hold water, based on air temperature and relative humidity (RH)

Question 2

How does evaporation occur?

Answer 2

Water molecules are always moving, some break away from water surface into the atmosphere, while some cross back to water surface. When there is more leaving than joining then net evaporation is occuring.

Question 3

What are the two basic controls on evaporation?

Answer 3

1. Evaporation requires energy ( energy comes mostly solar radiation, can also come from sensible or ground heat flux)
2. Evaporation is driven by the vapour pressure deficit (VPD: the difference between the current amount of moisture in the air and the maximum amount of moisture air can hold when it is saturated)

Question 4

What is saturation vapour pressure?

Answer 4

The maximum amount of moisture that air can hold at a given temperature

Question 5

Actual vapour pressure

Answer 5

Actual amount of moisture that air holds at a given air temperature. To derive air temp and relative humidity OR dew point temp is needed

Question 6

What does the magnitude of evaporation depend on?

Answer 6

• solar radiation
• VPD
• Temperature
• Wind
• Quality of water

Question 7

What is the water budget method for estimating evaporation?

Answer 7

Water Input (P) = Water Output (ET + R + I) ± Change in Water Storage
- advantages: simple
- disadvantages: difficult to estimate surface seepage losses, and subsurface runoff and unreliable

Question 8

What is the energy budget method for estimating evaporation?

Answer 8

The energy budget gives information about the energy exchanges that drive this process. Evaporation is the conversion of liquid water into water vapour and is influenced by the availability of energy and water.

• most accurate method
• difficult all terms and energy balance equation must be simplified

Question 9

How do evaporation pans help estimate evaporation?

Answer 9

They measure the rate of evaporation in a specific location.

• easy method, inexpensive
• can be poor indicator of land surface evaporation or lake evaporation