Evaporation,
Transpiration,
EvapoTranspiration
December 28, 2015
Losses of Precipitation
December 28, 2015
December 28, 2015
December 28, 2015
December 28, 2015
December 28, 2015
Evaporation
Evaporation happens
in several ways
Open water evaporation
Transpiration from
leaves
Evaporation from soil
and land surface
December 28, 2015
Evapotranspiration
Evaporation
Terminology
Evaporation process by which liquid
water passes directly to the vapor phase
The rate of evaporation depends upon the
water temperature and the temperature
and humidity of the air above the water.
Humidity refers to the amount of moisture
in the air; more specifically:
Absolute humidity - mass of water per unit volume of air (usually grams
water per cubic meter of air)
Saturation humidity - maximum amount of moisture the air can hold at a
given temperature
Relative humidity - the absolute humidity over the saturation humidity
(i.e., the percent ratio of the amount of moisture in the air to the total
amount it could possibly hold)
December 28, 2015
Transpiration
Transpiration - process by which
liquid water passes from liquid to
vapor through plant metabolism
Plants take up water for their own
use (i.e., for building plant tissue),
Only about 1% of what they suck up
gets used; the rest is released to the
atmosphere through leaves.
December 28, 2015
December 28, 2015
10
Evapotranspiration
When studying water in the field, one
cannot separate water lost to evaporation
from transpiration losses
It is typical to lump them together as
evapotranspiration (E-T).
Potential evapotranspiration is the water loss that would occur if there
is an unlimited supply of water available for transpiration and
evaporation.
In reality, the amount of water that transpires or evaporates is limited
by the amount of water that is available. If the amount of water
available is less than the potential, then the actual evapotranspiration
will be lower than the potential.
December 28, 2015
11
Factors Influencing
Evaporation
Energy supply for vaporization
(latent heat)
Solar radiation
Temperature
Energy input for evaporation to proceed
The higher the temperature the higher
the capacity of air to absorb more water
December 28, 2015
12
Factors influencing
evaporation
Transport of vapor away from
evaporative surface
Wind velocity over surface
Specific humidity gradient above
surface
December 28, 2015
13
Wind has a major effect on
evaporation, E
By convection wind removes vapor-laden
air
Keeping the boundary layer thin and
maintaining a high transfer rate of water
from liquid to vapor phase
Wind is also turbulent, increasing the
diffusion several orders of magnitude over
that of molecular diffusion
December 28, 2015
14
Factors influencing
evaporation
Vegetated surfaces/Crop resistance
The transpiration of cropped surface is
usually less than the evaporation of an
open water surface
Supply of moisture to the surface
Evapotranspiration (ET)
Potential Evapotranspiration (PET)
moisture supply is not limited
December 28, 2015
15
Measurement of
evaporation
(a) Evaporimeters
Class A-Evaporation pan
IS standard pan
Colorado Sunken pan
US geological survey floating pan
December 28, 2015
16
Evaporation from a
Water Surface
Simplest form of evaporation
From free liquid of permanently
saturated surface
December 28, 2015
17
Evaporation from a Pan
Sensible
Net radiation Vapor flow rate
heat to air
Rn
Hs
m v w AE
National Weather Service Class A
type
Installed on a wooden platform in a
grassy location
Filled with water to within 2.5
inches of the top
Evaporation rate is measured by
manual readings or with an analog
output evaporation gauge
December 28, 2015
CS
dh
dt
Area, A
Heat conducted
to ground
18
The most commonly use evaporimeter is the class A pan.
This is a pan with 1.21m diameter and depth of 225mm.
the pan is set 150 mm above the ground to allow air to move
freely around.
Evaporation is measured daily as the amount of water
evaporated from the pan.
At the beginning of the day, the pan is filled to the 50mm
from the top and is left to evaporate for 24 hrs.
December 28, 2015
To
determine the amount of water evaporated during the
19
Pan coefficient
Evaporation pans are not exact models
of large reservoirs and have the
following drawbacks:
They differ in heat storing capacity and
heat transfer from the sides and bottom
The height of the rim in an evaporation pan
affects the wind action over the surface
The heat transfer characteristics of the pan
are different from that of the reservoir
December 28, 2015
20
Values for Pan
Coefficient Cp
Types of Pan
Average
value
Range
Class A Land Pan
0.70
0.60-0.80
ISI pan
0.80
0.65-1.10
Colorado Sunken
Pan
0.78
0.75-0.86
USGS floating
pan
0.80
0.70-0.82
December 28, 2015
21
(b) Empirical Methods
A Large empirical equations are available to
estimate Free water evaporation using commonly
available meteorological data.
Most formulae are based on the Dalton-type
equation and can be expressed as:
EL Kf (u )(es ea )
Where;
EL = free water evaporation in mm/day
es = saturation vapour pressure at the water-surface
temperature in mm of mercury
ea = actual vapour pressure of overlying air at a specified
height
f(u) = wind speed correction
K is a coefficient
December 28, 2015
22
Meyers
equation:
E C es ea 1 0.06215V
E = evaporation in mm/month
es = saturation vapour pressure
ea = actual vapour pressure in mm
V = monthly mean wind velocity in km/hr
C = 15 for small shallow ponds
December 28, 2015
23
(c) Analytical Methods of
Estimating Evaporation
Water-budget method
Energy Balance Method
Aerodynamic method (Mass-transfer
method)
Penman Method (Combined method)
December 28, 2015
24
(i) Water-Budget Method
The water budget method is the simplest of
the three analytical methods.
It involves writing the hydrological
continuity equation for the free water
storage system
The ideal way of estimating evaporation
from any lake, reservoir, or catchment
would be to measure the various
components of inflow, outflow and storage
and apply the mass balance equation.
December 28, 2015
25
The evaporation in any time interval may
be computed from
E S1 S 2 I Q
Where I and Q are the volumes of inflow
and outflow, S1 and S2 are the initial and
final storage respectively.
December 28, 2015
26
(ii) Energy Method
It is an application of the law of conservation of
energy
Continuity
CV contains liquid and vapor phase water
Hs
Liquid phase
Rn
m
v
a
dh
E
dt
d
m v w d wV dA
dt CV
CS
dh
dh
w A
E
dt
dt
0
No flow of liquid
water through CS
m v w AE
The air in contact with the ground or water surface is warmed and then m
27 H
flow of energy thru the air is termed sensible heat flux
s
December
2015
The 28,
associated
Hs
Rn
Energy Method
Energy Eq. for Water in
CV
dH
dt
Rn H s G
m
v
a
E
dh
dt
Rate of heat input to the system
Assume:
1. Constant temp of water in CV
2. Change of heat is change in internal energy of water
evaporated
dH
lv m v
dt
w AE
Recall m
:
Neglecting sensible and
1
ground heat R
fluxes
Rn H s G
E
lv w A
Er n
lv w
lv m v Rn H s G
December 28, 2015
28
H s Ts Ta
Ts Ta
es ea
lv mv es ea
Tw Ta
es ea
Rn G
EL
w L 1
Rn = net radiation, G = Heat flux into the
ground, L = latent heat of vaporization, p is
the barometric
pressure in mb
=> Sign of
changes in morning and
evening, since Hs is +ve (upward )during the
December 28, 2015
29
(iii) Aerodynamic
Method
Include transport
of vapor away from
water surface as
function of:
Humidity gradient
above surface
Wind speed across
surface
December 28, 2015
Rn
Net radiation
Air Flow
E Evaporation
30
December 28, 2015
31
Aerodynamic Method
Ea M (es ez )u z
Where Ea =evaporation by the aerodynamic
method,
M = mass-transfer coefficient, es =
saturation vapour pressure at water
temperature, ez = vapour
pressure of the air
aCE
M =0.wind
622
at level Z, u
z
Pvelocity at level Z
w
Where w = density of water, a =
density of air, P = atmospheric
pressure at level Z, CE = bulk
December 28, 2015
evaporation coefficient
32
(iv) Combined Method
Evaporation is calculated by
Aerodynamic method
Energy method
Energy supply is not limiting
Vapor transport is not limiting
Normally, both are limiting, so use a
combination method
H s lv m v
Sensible heat flux is difficult to estimate
Assume it is proportional to the vapor heat flux
= Bowen ratio
1
Rn H s G
E
Energy balance equation (G=0) lv w A
Rn lv m 1
December 28, 2015
33
Recall Vapor Pressure
17.27T
es 611 exp
237
.
3
December 28, 2015
des
4098es
dT (237.3 T ) 2
34
Combined Method
(Cont.)
Combining
Energy balance
Aerodynamic Methods
Combined Method
E
Er
Rn
lv w
Ea M (es ez )u z
Er
Ea
Well suited to small areas
with detailed data
Net Radiation
Air Temperature
Humidity
Wind Speed
December 28,2015
Air Pressure
des
4098es
dT (237.3 T ) 2
E 1.3
Er
Priestly & Taylor
35
Example
Use Combo Method to find
Elev = 2 m,
Evaporation
lv 2.501x10 6 2370T
Press = 101.3 kPa,
Wind speed = 3 m/s,
Net Radiation = 200 W/m2,
Air Temp = 25 degC,
Er
Rel. Humidity = 40%,
December 28, 2015
(2500 2.36 * 25) x103 2441 kJ/kg
Rn
200
7.10 mm/day
lv w 2441x103 * 997
36
Example (Cont.)
Use Combo Method to find
Elev = 2 m,
Evaporation
eas 3167 Pa
Press = 101.3 kPa,
Wind speed = 3 m/s,
Net Radiation = 200 W/m2,
Air Temp = 25 degC,
Rel. Humidity = 40%,
ea Rh * eas 0.4 * 3167 1267 Pa
Ea 4.54 x10 11 3167 1267 * (1000 mm / 1 m) * (86400 s / 1 day)
7.45 mm/day
December 28, 2015
37
Example (Cont.)
Use Combo Method to find
Evaporation
Elev = 2 m,
Press = 101.3 kPa,
Wind speed = 3 m/s,
Net Radiation = 200 W/m2,
Air Temp = 25 degC,
Rel. Humidity = 40%,
0.738
4098 * 3167
(237.3 25)
188.7 Pa/degC
0.262
Er
E a 0.738 * 7.10 0.262 * 7.45 7.2 mm/day
December 28, 2015
38
Example
Use Priestly-Taylor Method to find
Evaporation rate for a water body
Net Radiation = 200 W/m2,
Air Temp = 25 degC,
E r 7.10 mm/day
E 1.3
Er
Priestly & Taylor
0.738
E 1.3 * 0.738 * 7.10 6.80 mm/day
December 28, 2015
39
