Watershed Hydrology, a Hawaiian
Prospective:
Evapotranspiration
Ali Fares, PhD
Evaluation of Natural Resource
Management, NREM 600
UHM-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
ET = P – Q – ΔS - ΔD
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)
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
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 rootstomatal 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 changes
That happens to any of these 3.

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- & longwave 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 wellwatered 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 lowgrowing vegetation does
(e.g. grass).
The total latent heat flux is
determined by:
– LE = Rn + H


Advection is movement of
warm air to cooler plantsoil-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
• There are different ways to estimate drainage.
• The direct method is the use of lysimeters.
 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.
Water Mass balance Equation
S =(I + R + U) - (D + RO + ET)

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ET = Evapotranspiration
R, I = Rain & Irrigation
D = Drainage Below Rootzone
RO = Runoff
S = Soil Water Storage variation
U = upward capillary flow
Rain
Transpiration
Evapo-transpiration
Irrigation
Evaporation
Runoff
Root Zone
Below Root
Zone
Water Storage
Drainage
1000
5
Daily ET
Daily Evapotranspiration (mm)
800
4
600
3
400
2
200
1
ET Standard Deviation
0
0
30
60
90
120
150
180
210
240
Calendar Days (1997)
270
300
330
360
Cumulative Evapotranspiration (mm)
Cumulative ET
45
750
35
30
600
25
450
20
15
Daily drainage
300
10
150
5
Std. Dev. (mm)
0
0
4
3
2
1
0
4
3
2
1
0
Standard Deviation
0
30
Jan
60
Feb
90
Mar
120
Apr
150
May
180
Jun
210
Jul
240
Aug
Calendar Days
270
Sep
300
Oct
330
Nov
360
Dec
Cumulative drainage (mm)
40
Daily Drainage (mm)
900
Cumulative drainage
1.8 m2 wetting area
16.3 m2 wetting area
0.6
7.3 m2 wetting area
Hourly ET (mm)
0.5
0.4
0.3
0.2
0.1
0.0
27.0
27.5
28.0
28.5
Days of the Month (April 1996)
29.0
Cumulative Daily ET (mm)
6
5
1.8 m2 wetting area
16.3 m2 Wetting area
4
7.3 m2 Wetting area
3
2
1
0
27.0 27.2 27.4 27.6 27.8 28.0 28.2 28.4 28.6 28.8 29.0
Days of the Month (April 1996)
20
A
15
10
Drainage (mm)
5
0
6
5
4
3
2
1
0
Daily ET (mm)
Rain/Irrig. (mm)
Irrigation or Rainfall
25
5
4
3
2
1
0
B
Drainage Below the Rootzone
Daily Evapotranspiration
C
March 30
April 9
Month Date
April 19
6
Y = 0.724 X
r2 = 0.88
Daily Evapotranspiration (mm)
5
4
3
2
1
1
2
3
4
Daily Potential Evapotranspiration (mm)
5
6
Effects of Vegetative Cover
ET / Potential ET
Available Soil Water
ET & Available Soil Water