Watershed Hydrology, a Hawaiian Prospective:

Evapotranspiration

Ali Fares, PhDEvaluation of Natural Resource

Management, NREM 600UHM-CTAHR-NREM

Objectives of this chapter Explain and differentiate

among the processes of evaporation from a water body, evaporation from soil, and transpiration from a plant

Understand and be able to solve for evapotranspiration (ET) using a water budget & energy budget method

Explain potential ET and actual ET relationships in the field.

Under what conditions are they similar?

Under what conditions are they different?

Understand and explain how changes in vegetative cover affect ET.

Describe methods used in estimating potential and actual ET

Conservation of Energy

The conservation equation as applied to energy, or conservation of energy, is known as the energy balance.

How precipitation is partitioned into infiltration, runoff, evapo-transpiration, etc., similarly, we can look at how incoming radiation from the sun and from the atmosphere is partitioned into different energy fluxes (where the term flux denotes a rate of transfer (e.g. of mass, energy or momentum) per unit area).

Water & Energy relationship There is strong link between the water and energy balance: Partitioning of incoming radiation into the various fluxes of

energy ( energy for ET, energy to heat the atmosphere and energy to heat the ground) depends on the water balance and how much water is present in soils and available for evapotranspiration.

the partitioning of precipitation into the various water fluxes (e.g. runoff and infiltration) depends on how much energy is available for ET.

Just as changes in water balance were reflected in changes in storage in water amounts (soil moisture in a root zone; level of a lake) changes in energy balance are reflected in temperature changes.

Just as we wrote water balances for a number of different control volumes, we could write energy balances for the same control volumes.

Evapotranspiration

ΔS= watershed storage variation (mm): Send–Sbeginning

P = Precipitation (mm)

Q = Stream flow (mm)

ΔD = Seepage out – seepage in (mm)

ET = evaporation and transpiration (mm)

ET = P – Q – ΔS - ΔD

Energy Budget for an ideal surface

Energy budget is: Rn = H + LE + G where Rn is net radiation at the

surface; H is sensible heat exchanged with the

atmosphere; LE is latent heat exchanged with the

atmosphere; and G is heat exchanged with the ground.

Net Solar Energy Flux The net flux of solar energy entering the land surface

is therefore given as K = Kin - Kout = Kin (1-a) where K in is the incident solar energy on the surface, and it

includes direct solar radiation (i.e. that which makes it through the atmosphere unscathed) and diffuse (due to scattering by aerosols and gases);

Kout is the reflected flux; a is the albedo Solar radiation is measured in specialized

meteorological stations with radiometers.

Evapotranspiration More than 95% of 300mm in

Arizona > 70% annual precipitation in

the US In General: ET/P is

– ~ 1 for dry conditions– ET/P < 1 for humid climates &

ET is governed by available energy rather than availability of water

For humid climates, vegetative cover affects the magnitude of ET and thus, Q (stream flow).

In Dry climate, effect of vegetative cover on ET is limited.

ET affects water yield by affecting antecedent water status of a watershed high ET result in large storage to store part of precipitation

evapotranspiration summarizes all processes that return liquid water back into water vapor

- evaporation (E): direct transfer of water from open water bodies or soil surfaces

- transpiration (T): indirect transfer of water from root-stomatal system• of the water taken up by plants, ~95% is returned to the

atmosphere through their stomata (only 5% is turned into biomass!)

• Before E and T can occur there must be:• A flow of energy to the evaporating or transpiring

surfaces• A flow of liquid water to these surfaces, and• A flow of vapor away from these surfaces.

• Total ET is change as a result of any changesThat happens to any of these 3.

Evapotranspiration

Three main factors affect E or T from evaporating & transpiring surfaces:– Supply of energy to

provide the latent heat of evaporation

– Ability to transport the vapor away from the evaporative surface

– Supply of water at the evaporative surface

Source of energy? Is solar radiation

What take vapors away from evaporating surface? Wind and humidity gradient

Evaporation includes:– Soil -- vegetation

surface – transpiration– => Evapotranspiration,

ET

The linkage between water and energy budgets

Is direct; the net energy available at the earth’s surface is

apportioned largely in response to the presence or absence of water.

Reasons for studying it are:– To develop a better understanding of Hydrological

cycle– Be able to quantify or estimate E and ET (soil, water or

snowmelt)

Energy Budget Net radiation:

Rn=(Ws+ws)(1- α)+Ia-Ig

Rn is determined by measuring incoming & outgoing short- & long-wave rad. over a surface.

Rn can – or +

If Rn > 0 then can be allocated at a surface as follows:

Rn = (L)(E) + H + G + Ps

L is latent heat of vaporization, E evaporation, H energy flux that heats the air or sensible heat, G is heat of conduction to ground and Ps is energy of photosynthesis.

LE represents energy available for evaporating water

Rn is the primary source for ET & snow melt.

In a watershed Rn, (LE) latent heat and sensible heat (H) are of interest.

Sensible heat can be substantial in a watershed, Oasis effect were a well-watered plant community can receive large amounts of sensible heat from the surrounding dry, hot desert.

See Table 3.2 comparison See box 3.1 illustrates the

energy budget calculations for an oasis condition.

An island of tall forest vegetation presents more surface area than an low-growing vegetation does (e.g. grass).

The total latent heat flux is determined by:– LE = Rn + H

Advection is movement of warm air to cooler plant-soil-water surfaces.

Convection is the vertical component of sensible-heat transfer.

Water movement in plants Illustration of the energy

differentials which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere. The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere.

Yw~ -1.3 MPa

Yw~ -1.0 MPa

Yw~ -0.8 MPa

Yw~ -0.75 MPa

Yw~ -0.15 MPa

Ys~ -0.025 MPa

Soil Water Mass Balance

Lysimeters have a weighing device and a drainage system, which permit continuous measurement of excess water and draining below the root zone and plant water use, evapotranspiration.

Lysimeters have high cost and may not provide a reliable measurement of the field water balance.

• There are different ways to estimate drainage.• The direct method is the use of lysimeters.

Water Mass balance Equation

ET = Evapotranspiration R, I = Rain & Irrigation D = Drainage Below Rootzone RO = Runoff S = Soil Water Storage variation U = upward capillary flow

S =(I + R + U) - (D + RO + ET)

Evapo-transpiration

Transpiration

Evaporation

Rain

Runoff

Drainage

Root ZoneWater Storage

Irrigation

Below RootZone

Calendar Days (1997)

0 30 60 90 120 150 180 210 240 270 300 330 360

Dai

ly E

vap

otr

ansp

irat

ion

(m

m)

1

2

3

4

5

Col 3 vs Col 4 Col 1 vs Col 1

Daily ET

Col 1 vs Col 1 Col 3 vs Col 7

ET Standard Deviation

Cu

mu

lati

ve E

vap

otr

ansp

irat

ion

(m

m)

0

200

400

600

800

1000

Col 1 vs Col 1 Col 3 vs Col 8

Cumulative ET

Calendar Days

0 30 60 90 120 150 180 210 240 270 300 330 360

Std

. Dev

. (m

m)

0

1

2

3

4

0

1

2

3

4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Dai

ly D

rain

age

(mm

)

0

5

10

15

20

25

30

35

40

45

Cu

mu

lati

ve d

rain

age

(mm

)

0

150

300

450

600

750

900Cumulative drainage

Daily drainage

Standard Deviation

Days of the Month (April 1996)

27.0 27.5 28.0 28.5 29.0

Hou

rly

ET

(m

m)

0.0

0.1

0.2

0.3

0.4

0.5

0.61.8 m2 wetting area

16.3 m2 wetting area

7.3 m2 wetting area

Days of the Month (April 1996)

27.0 27.2 27.4 27.6 27.8 28.0 28.2 28.4 28.6 28.8 29.0

Cum

ulat

ive

Dai

ly E

T (

mm

)

0

1

2

3

4

5

61.8 m2 wetting area

16.3 m2 Wetting area

7.3 m2 Wetting area

Fig. 6

Rain

/Irrig. (mm

)

0

5

10

15

20

25

Drain

age (mm

)

0123456

Month Date

Daily E

T (m

m)

0

1

2

3

4

5 C

B

A

Drainage Below the Rootzone

Daily Evapotranspiration

Irrigation or Rainfall

March 30 April 9 April 19

Daily Potential Evapotranspiration (mm)

1 2 3 4 5 6

Da

ily E

va

po

tra

ns

pir

ati

on

(m

m)

1

2

3

4

5

6

PEo vs ET Col 7 vs Col 8

r2 = 0.88

Y = 0.724 X

Effects of Vegetative Cover

ET / Potential ET

Available Soil Water

ET & Available Soil Water