Evaporation estimates

Question 1

What is the definition of a model?

Answer 1

a model is anything that is a simplification or abstraction of reality

Question 2

Why do we rely on models?

Answer 2

When you can’t measure something directly or at the scale your interested in you have to rely on models

Question 3

What are the two categories of models?

Answer 3

theoretical or empirical

Question 4

What is a theoretical model?

Answer 4

Is a model that tries to describe phenomena from first principles, for example physics, where when you drop something velocity varies depending on the object- using newtons laws to calculate velocity of an object

Question 5

What are empirical models?

Answer 5

Empirical models attempt to describe phenomenon from observations, for example: throwing a bunch of objects out of an airplane and measure speed to see where a particular object might fall

Question 6

What are semi-empirical models?

Answer 6

Models that combine first principle and observations, for example determining the variable Cd by observations and then using that in established equations.

Question 7

What six things control evapotranspiration rate?

Answer 7

Heat
Air
Amount of water
Vapour pressure deficit
Turbulence
Gradient of water moisture in air

Question 8

How was the thornwaite model created? What type of model is this?

Answer 8

The thornwaite model was created by going out and collected data on evapotranspiration from lysimeters and comparing the values of evapotranspiration to monthly air temp and then creating an eqn.
Its a semi empirical model, was based on observations but has actual math and set coefficients

Question 9

What is I in the thornwaite model?

Answer 9

Is the heat index which is summed by calculating i- the heat index for each month and adding them up.

Question 10

How do you calculate i?

Answer 10

It’s the air temp of that month/5

Question 11

What is a in the thornwaite model? How do you calculate it?

Answer 11

a is the calibration coefficient, you calculate it by taking coefficients and multiplying it by I and then summing that up

Question 12

What’s b in the thornewaite model? How do you calculate it?

Answer 12

It’s a day length correction to account for the months that have longer days because that lead sot more evapotranspiration, you get this from a table

Question 13

What’s the entire thornwaite equation?

Answer 13

PE = 16b(10t/I)^a, this tells you the potential evapotranspiration in mm

Question 14

Why does latitude effect b- the day length correction?

Answer 14

because as you go up in latitude the days get shorter so there’s less potential evapotranspiration

Question 15

What is potential evapotranspiration?

Answer 15

This is the evapotranspiration that would occur if the water supply was unrestricted

Question 16

In practice, is evapotranspiration over land restricted?

Answer 16

Yes, sometimes not enough water in soil or plants don’t transpire

Question 17

What is open water evaporation (E0)?

Answer 17

It’s the evaporation that occurs over open water

Question 18

What is term for evaporation that actually occurs?

Answer 18

actual evaporation

Question 19

Describe the graph of potential evaporation to actual evapotranspiration?

Answer 19

When soil moisture is 100% available, actual evapotranspiration and potential evapotranspiration should be at a ratio of 100% (are the same), however depending on vegetation and available soil moisture the actual evaportranspiration will vary

Question 20

Looking at the graph (slide 17), what has more actual evaporation compared to potential evaporation at a soil moisture of 50%, lodgepole pine or native prarie grass?

Answer 20

native prarie grass at 70% compared to lodgepole pine at 50%

Question 21

What are the pros of the thornewaite model?

Answer 21

Is easy to get the required data and works well for humid regions (the conditions in which it was formed)

Question 22

What are the cons of the thornewaite model?

Answer 22

since it’s calibrated for humid regions it overestimate PE in arid regions
It can only provide estimates at monthly time scales
it Ignores many of the factors important to evaporation (such as wind turbulence), only includes vapour pressure deficit and available energy

Question 23

What is the Penman model?

Answer 23

Is a semi empirical model which includes more of the factors associated with evaporation (such as radiation, sat vap pressure, actual vap pressure, and wind velocity)

Question 24

What are the pros of the penman model?

Answer 24

Easy to solve
Gets evaporation estimates at a shorter time scale (hourly)
Includes more of the factors of evapotranspiration such as wind velocity

Question 25

What are the cons of the penman model?

Answer 25

It provides open water evaporation estimates which are more useless than potential evaporation estimates

Question 26

What is the penman-Monteith model?

Answer 26

Is a model that accounts for aerodynamic resistance, canopy resistance, vapor pressure deficit, available energy, and available water (though canopy resistance)

Question 27

What is canopy resistance?

Answer 27

Is also known as stomatal resistance, it describes how tightly plants hold on to their water, the higher the rc the more the plants hold onto water and therefore the less evapotranspiration.

Question 28

What is rc? When is it 0?

Answer 28

is the canopy resistance term, is 0 when water is freely available- which is what we assume in the thornewaite model.

Question 29

What plants often have high rc’s?

Answer 29

Plants living in arid areas

Question 30

What is aerodynamic resistance?

Answer 30

Is how efficient a surface is at mixing the atmosphere when wind blows over it, if ra is high- the mixing is less effective and the surface is low turbulence if ra is low it’s more effective at mixing and the turbulence is high.

Question 31

If aerodynamic resistance term is high what does that mean for evapotranspiration?

Answer 31

We will have low evapotranspiration as there isn’t much turbulence.

Question 32

What are the penman monteith models pros?

Answer 32

It directly calculates actual evapotranspiration
It requires few measurements (net radiation, temperature, wind speed, relative humidity, canopy, and atmospheric resistance)
It has been tested and works for a lot of diff situations

Question 33

What are the penman monteiths models cons?

Answer 33

It relies on good values of canopy resistance and aerodynamic resistance which are hard to get