10 02 Lecture
Transcript of 10 02 Lecture
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Watershed Hydrology, a Hawaiian
Prospective:Evapotranspiration
Ali Fares, PhD
Evaluation of Natural Resource
Management, NREM 600UHM-CTAHR-NREM
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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
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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).
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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.
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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
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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.
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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.
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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
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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
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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
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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)
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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.
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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.
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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.
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w~ -1.3 MPa
w~ -1.0 MPa
w~ -0.8 MPa
w
~ -0.75 MPa
w~ -0.15 MPa
s~ -0.025 MPa
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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.
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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)
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Evapo-transpiration
Transpiration
Evaporation
Rain
Runoff
Drainage
Root ZoneWater Storage
Irrigation
Below Root
Zone
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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
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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
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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
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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
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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
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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
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Effects of Vegetative Cover
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ET / Potential ET
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Available Soil Water
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ET & Available Soil Water
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