Lecture4 2 interception and ET dung

Evapotranspiration Returning water to the atmosphere... Evapotranspiration ET • Composed of two subprocesses – Evaporation occurs on surfaces of open water or from vegetation and ground

Evapotranspiration

Returning water to the atmosphere

Evapotranspiration (ET)

• Composed of two subprocesses

– Evaporation occurs on surfaces of open water or from vegetation and ground surfaces

– Transpiration is the removal of water from the soil by plant roots,

transported through the plant into the leaves and evaporated from the leaf’s stomata

• Typically combined in mass balance equations because the components are difficult to

The Processes

• Free-water evaporation

– Open water surfaces

• Lakes, rivers;

– Other sources of water

• Vegetation surfaces, soil surface

• Transpiration

• Roots  Stem  Leaves  Stomata  Atmosphere

Control on Evaporation

Transpiration

 Transpiration is the loss of water

in the form of vapor from plants

 Factors that affect transpiration

Transpiration coefficient (TC): Water amount of

transpiration/dry mass production

TC: 200 đến 1.000

Actual Evapotranspiration (AET)

The quantity of water that is actually removed from a surface due to the processes of evaporation and

transpiration

Potential Evapotranspiration (PET)

A measure of the ability of the atmosphere to remove water from the surface through the processes of

evaporation and transpiration assuming no control

on water supply

Potential evapotranspiration (PET)

Definition by Penman (1948)

The amount of water transpired in unit time

by a short green crop completely shading the ground, of uniform height and never short of water

Therefore, PET based on atmospheric

conditions and a specific vegetation

type

More general definition:

Potential evapotranspiration is a water loss from the soil surface completely covered by

vegetation

ET for watershed scale

Actual Evapotranspiration (AET)

The quantity of water that is actually removed from a surface due to the processes of evaporation and

transpiration

Actual evapotranspiration from watersheds cannot be measured directly by any practical method!!

However,

Evaporation

Measurements

Free water evaporation

- Pans and tanks

- Evaporimeters

Example of Pan Setup

• A hook gauge is used to measure the water level inside

the pan and A cup anemometer is

placed beside the pan to measure the surface wind

movement over it

Pan evaporation method

Epan : Evaporation of pan (mm)

Kp: Pan coefficient

+ Kp = 0.35-0.85; Mean: 0.7

Pan evaporation methods

Pan evaporation

Epan = W – [V2-V1]

where

W = precipitation during D t

V1 = the storage at the beginning of D t

V2 = the storage at the end of D t

For American Class-A pan, Kohler et al (1955) developed an empirical equation to account for energy exchange through sides of a pan, and adjust daily pan evaporation, Epan, to free water evaporation, Efw [mm day -1 ] (Equations 7-41 and 7-42).

Pan evaporation methods

ET measurement

ET measurement

• Water balance (water budget):

Q = P - ET

– Can measure Q (runoff);

– Can measure P (precipitation);

– Difficult to measure ET (evapotranspiration);

• Usually determine ET by:

ET = P - Q

Example: Elk River Catchment

Atmospheric Energy Balance

Solar energy as driving force of hydrological cycle

Shortwave: 0.1 and 0.7 micrometers (µm)

Shortwave radiation: direct solar radiation (Ws)

and diffuse radiation (ws)

Albedo ( α) : Short wave reflectivity, total

shortwave radiation that was reflected from

surface

Net shortwave radiation

Net shortwave radiation:

(Ws+ws)- α(Ws+ws) or (Ws+ws)(1- α)

Net longwave radiation

Longwave: 0.7 and 100 micrometers (µm)

Longwave radiation: emitted from

atmosphere and all terrestrial objects

Soil and plan surface reflect only small portion of total downward longwave

Net longwave radiation: la - lg

• Internal energy

– Sensible heat – heat content that can be

measured and is proportional to

temperature

– Latent heat – “hidden” heat content that

is related to phase changes

Heat energy

LE: represents energy available for evaporating water

Net radiation

LE = R n - H - G - Ps

R n = (L)(E) + H + G + Ps

Rate of energy use for evaporating water

If we assume Ps is very small relative

to the other energy

LE = R n - H - G

v w

eb

L p

Pw: density of water (1000 kg/m 3 )

Lv: latent heat of vaporization (2.47x10 6 J/kg)

Net radiometer

Measurement of flux of short and longwave

 Lysimeters account for change in water storage

 i.e Measure actual evapotranspiration

input (Rainfall R and Additional water A) and output (Percolated water P)

collected in the receiver, then PE can be estimated from the equation:

PE = R + A – P

Lysimeter for measuring potential evapotranspiration

R A

P

– how much water the crop used

• At midnight (or some standard time)

– water tank below the lysimeter is filled with water that can be used for

irrigation during the day – ie no weight change as a result of irrigation during the day

Three tank method

Tanks A and B: no bottom side

How measure ET and Percolation?

Example for Lysimeter

Good method, but

expensive for

installation!!

Mass balance measurement using lysimeter

A t

m

m eFLUX

n p

L

G R

s

s E

Figure 1 Evapotranspiration over grass at Vancouver Airport

Actual ET based

on lysimeter Estimated ET

/ ) (

a st

a a

s p n

r r

r e

e C

R PET



 D

Specific heat of air

Air density Vapor pressure of surface and air

L

u f e e

R

) (

) ( )

(





 D

Bordeaux (Le Bray site): Eddy covariance installations at canopy level in a French pine forest

Evapotranspiration at various locations in

forested areas