7/29/2019 10 02 Lecture

1/33

Watershed Hydrology, a Hawaiian

Prospective:Evapotranspiration

Ali Fares, PhD

Evaluation of Natural Resource

Management, NREM 600UHM-CTAHR-NREM

7/29/2019 10 02 Lecture

2/33

Objectives of this chapter

Explain and differentiateamong 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 andactual ET relationships in the

field.

Under what conditions arethey similar?

Under what conditions are

they different?

Understand and explain howchanges in vegetative cover

affect ET.

Describe methods used in

estimating potential and actual

ET

7/29/2019 10 02 Lecture

3/33

Conservation of Energy

The conservation equation as applied to energy, orconservation of energy, is known as the energybalance.

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

7/29/2019 10 02 Lecture

4/33

Water & Energy relationship There is strong link between the water and energy balance:

Partitioning of incoming radiation into the various fluxes ofenergy ( energy for ET, energy to heat the atmosphere and energyto heat the ground) depends on the water balance and how muchwater 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 availablefor ET.

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

changes. Just as we wrote water balances for a number of different control

volumes, we couldwrite energy balances for the same controlvolumes.

7/29/2019 10 02 Lecture

5/33

Evapotranspiration

S= watershed storage variation (mm): SendSbeginning

P = Precipitation (mm)

Q = Stream flow (mm)

D = Seepage outseepage in (mm)

ET = evaporation and transpiration (mm)

ET= PQS - D

7/29/2019 10 02 Lecture

6/33

Energy Budget for an ideal

surface Energy budget is:

Rn = H + LE + G

where Rn is net radiation at thesurface;

H is sensible heat exchanged with theatmosphere;

LE is latent heat exchanged with theatmosphere; and

G is heat exchanged with the ground.

7/29/2019 10 02 Lecture

7/33

Net Solar Energy Flux The net flux of solar energy entering the land surfaceis therefore given as

K = Kin - Kout = Kin (1-a)

where

K in is the incident solar energy on the surface, and itincludes direct solar radiation (i.e. that which makesit through the atmosphere unscathed) and diffuse (dueto scattering by aerosols and gases);

Kout is the reflected flux;

a is the albedo

Solar radiation is measured in specializedmeteorological stations with radiometers.

7/29/2019 10 02 Lecture

8/33

Evapotranspiration

More than 95% of 300mm inArizona

> 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 ofwater

For humid climates, vegetativecover affects the magnitude ofET and thus, Q (stream flow).

In Dry climate, effect ofvegetative cover on ET islimited.

ET affects water yield byaffecting antecedent water

status of a watershed highET result in large storage tostore part of precipitation

7/29/2019 10 02 Lecture

9/33

evapotranspiration summarizes all processes that return liquid waterback into water vapor

- evaporation (E): direct transfer of water from open waterbodies 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

7/29/2019 10 02 Lecture

10/33

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

surfacetranspiration

=> Evapotranspiration,ET

7/29/2019 10 02 Lecture

11/33

The linkage between water and

energy budgets Is direct;

the net energy available at the earths surface is

apportioned largely in response to the presence orabsence 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)

7/29/2019 10 02 Lecture

12/33

Energy Budget Net radiation:

Rn=(Ws+ws)(1- )+Ia-Ig

Rn is determined by

measuring incoming &

outgoing short- & long-wave rad. over a surface.

Rn canor +

If Rn > 0 then can be

allocated at a surface asfollows:

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

L is latent heat ofvaporization, E evaporation,

H energy flux that heats the

air or sensible heat, G is

heat of conduction toground and Ps is energy of

photosynthesis.

LE represents energy

available for evaporating

water

Rn is the primary source for

ET & snow melt.

7/29/2019 10 02 Lecture

13/33

In a watershed Rn, (LE) latentheat and sensible heat (H) are

of interest. Sensible heat can be

substantial in a watershed,Oasis effect were a well-watered plant community canreceive large amounts ofsensible heat from thesurrounding dry, hot desert.

See Table 3.2 comparison

See box 3.1 illustrates theenergy budget calculations foran oasis condition.

An island of tall forestvegetation presents more

surface area than an low-growing vegetation does(e.g. grass).

The total latent heat flux isdetermined by:

LE = Rn + H

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

Convection is the verticalcomponent of sensible-heattransfer.

7/29/2019 10 02 Lecture

14/33

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 outinto the atmosphere. The

water moves from a less

negative soil moisture

tension to a more negative

tension in the atmosphere.

7/29/2019 10 02 Lecture

15/33

w~ -1.3 MPa

w~ -1.0 MPa

w~ -0.8 MPa

w

~ -0.75 MPa

w~ -0.15 MPa

s~ -0.025 MPa

7/29/2019 10 02 Lecture

16/33

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 andplant water use, evapotranspiration.

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

There are different ways to estimate drainage.

The direct method is the use of lysimeters.

7/29/2019 10 02 Lecture

17/33

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)

7/29/2019 10 02 Lecture

18/33

Evapo-transpiration

Transpiration

Evaporation

Rain

Runoff

Drainage

Root ZoneWater Storage

Irrigation

Below Root

Zone

7/29/2019 10 02 Lecture

19/33

Calendar Days (1997)

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

DailyEvapotranspiration(mm)

1

2

3

4

5

Daily ET

ET Standard Deviation

CumulativeEvap

otranspiration(mm)

0

200

400

600

800

1000

Cumulative ET

7/29/2019 10 02 Lecture

20/33

Calendar Days

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

Std.De

v.(mm)

0

1

2

3

4

0

1

2

3

4

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

D

ailyDrainage(mm)

0

5

10

15

20

25

30

35

40

45

Cum

ulativedrainage(m

m)

0

150

300

450

600

750

900Cumulative drainage

Daily drainage

Standard Deviation

7/29/2019 10 02 Lecture

21/33

Days of the Month (April 1996)

27.0 27.5 28.0 28.5 29.0

HourlyE

T(mm)

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

7/29/2019 10 02 Lecture

22/33

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

Cu

mulativeDailyET(mm)

0

1

2

3

4

5

61.8 m2 wetting area

16.3 m2 Wetting area

7.3 m2 Wetting area

7/29/2019 10 02 Lecture

23/33

Rain/Irrig.(mm)

0

5

10

15

20

25

Drainag

e(mm)

0

1

23

4

5

6

Month Date

DailyET(m

m)

0

1

2

34

5 C

B

A

Drainage Below the Rootzone

Daily Evapotranspiration

Irrigation or Rainfall

March 30 April 9 April 19

7/29/2019 10 02 Lecture

24/33

Daily Potential Evapotranspiration (mm)

1 2 3 4 5 6

DailyEvapotran

spiration(mm)

1

2

3

4

5

6

r2 = 0.88

Y = 0.724 X

7/29/2019 10 02 Lecture

25/33

Effects of Vegetative Cover

7/29/2019 10 02 Lecture

26/33

7/29/2019 10 02 Lecture

27/33

7/29/2019 10 02 Lecture

28/33

7/29/2019 10 02 Lecture

29/33

7/29/2019 10 02 Lecture

30/33

ET / Potential ET

7/29/2019 10 02 Lecture

31/33

Available Soil Water

7/29/2019 10 02 Lecture

32/33

ET & Available Soil Water

7/29/2019 10 02 Lecture

33/33