Returning water to the atmosphere
EVAPOTRANSPIRATION
ANNOUNCEMENTS
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HW#3 assigned
EVAPOTRANSPIRATION (ET)
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Composed of two sub-processes: Evaporation and Transpiration
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Evaporation occurs on, 1) surfaces of open water, and 2) from vegetation,
and 3) ground surfaces.
Transpiration is the removal of water from the soil by plant roots,
transported through the plant into the leaves and evaporated from the
leaf’s stomata.
ET is typically combined in mass balance equations because the
components are difficult to partition.
Evapotranspiration
Transpiration
Evaporation
Open
Water
Soil
Vegetation Surfaces
Plants
POTENTIAL VS. ACTUAL ET
 Potential
ET (PET)- The amount of
evaporation that will occur if an unlimited
amount of water is available.
 Actual
ET (AET)- The actual amount of
evaporation that occurs when water is
limited.
EVAPORATION
Phase change of water from a liquid to a gas.
 Latent heat of vaporization - energy needed by
a molecule to leave the water surface (540
cal/g of water evaporated at 100°C.
 Rate of evaporation is driven by the vapor
pressure deficit  Function of:
1. The ability of air to hold water based on air
temperature and relative humidity.
2. The energy available to evaporate water

1.
largely based on temperature
EVAPORATION

Net evaporation ceases when the air has
reached the saturation vapor pressure.
 For evaporation to continue, some
mechanism is needed to remove the water
vapor from the evaporating surface  wind
EVAPORATION FROM OPEN WATER
Gives good estimation of PET rates.
 Effected by 4 (minor) factors:

1.
2.
3.
4.
Barometric pressure
Dissolved matter
Shape, site and situation of evaporating body.
Relative depth of evaporating body.
EVAPORATION FROM BARE SOIL
Similar to open water evaporation when soil is
saturated.
 Divided into two stages.

 Stage
1: Soil is at or near saturation
evaporation is controlled by heat energy
 Approximately 90% of maximum PET

 Stage
2: Falling stage
 Surface
starts to dry and evaporation occurs below the
soil surface.
 Controlled by soil properties rather than weather
conditions.
EVAPORATION FROM VEGETATIVE SURFACES

Interception  Water retained on plant
surfaces during and after precipitation
 Intercepted water is quickly evaporated back
to the atmosphere

10 to 25% of annual precipitation is intercepted

Plant transpiration is reduced by the amount of
intercepted water to be evaporated.
TRANSPIRATION


Transpiration-loss of water in the form of
vapor from plants
Factors that affect transpiration rates
 Type of plant
 Wind
 Plant Available Water  portion of
water in a soil that can readily be
absorbed by plant roots. Amount of
water released between field capacity
(amount of water remaining in the soil
after gravitation flow has stopped) and
wilting point (amount of water in the
soil at 15 bars of suction).
TRANSPIRATION

Field Capacity (θFC)
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Amount of soil moisture held in the soil after excess water
has drained away by gravity
Usually takes 2 – 3 days after rain and/or irrigation
Water content in the soil at -0.33 bar hydraulic head (suction
pressure)
TRANSPIRATION

Wilting Point (θWP)


Minimum soil moisture the plant requires not to wilt
Water content at – 15 bar hydraulic head (suction pressure)
TRANSPIRATION RATIO & CONSUMPTIVE USE

Transpiration ratio  ratio of the weight of
water transpired to the dry weight of the plant
 Measure
of how efficiently crops use water.
 Examples: Alfalfa (900), Wheat (500), Corn (350)

Consumptive Use = Total amount of water
needed to grow a crop
 ET
requirement + water stored in plant tissues
MEASURING EVAPORATION AND ET

Several methods
 Evaporation Pans
 PET Gages  acts as surrogates for plants
 Soil Water Depletion
 Lysimeters
 Energy Balance and Mass transfer
 measure
canopy.
average gradient of water vapor above the
PAN EVAPORATION
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Oldest / simplest method to
measure evaporation
Measure water depths in a pan
U.S. Weather Bureau has
standard Class A pan
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Cylindrical container made of
galvanized steel
10 inches deep and 48 inches in
diameter
Pan placed on a 6 inch wooden
platform
Site should be flat and free of
obstructions
Water filled to 8 inches deep


Refill when water drops to 7 inches
deep
Water level measurements made
using a hook gage

Measurements to 0.01 inch
DETERMINING PAN FACTORS


EPET = kp Epan
Lake evaporation

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Typically taken as 70% of
pan evaporation
PET

Pan evaporation times a
coefficient ranging from
0.6 to > 1.0.
PAN EVAPORATION / EXAMPLE PROBLEM

Given:
 Set up below with a class A pan
 Average wind speed = 4.3 km/hr
 Average relative humidity = 67%
 Measured water change in pan on July 1 = 7.5 mm
200 m
Class A Pan
200 m
Turfgrass (4 in.)
N
PAN EVAPORATION / EXAMPLE PROBLEM

Required:
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Calculate the PET for July 1
Solution:
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Fetch =
Wind speed =
Set up =
Kp =
PET = Kp x depth change =
PET =
ANNUAL PAN EVAPORATION ESTIMATES FOR
TEXAS
LYSIMETERS

Allow an area to be isolated
from the rest of the field while
carefully measuring the
individual components of the
water balance.
 Weighing
 Non-weighing-measure
drainage from the bottom
ESTIMATING ET
SCS Blaney-Criddle Method
Estimates seasonal AET.
Can be used for monthly estimates
if monthly crop coefficients are
locally available (Table 4.8)
Assumes mean monthly air
temperature and annual day time
hours can be used as an substitute
for solar radiation to estimate the
energy received by the crop.
Monthly consumptive factor (f)
tp
f 
100
t is the mean monthly air
temperature in °F
p is the mean monthly percentage
of annual daytime hours (Table 4.6)
MONTHLY PERCENTAGE OF DAYTIME HOURS, P
Leuven = 50o 53’ / College Station = 33o 37’ / Knoxville = 35o 57’ / Lexington = 38o 4’
BLANEY-CRIDDLE EQUATION
U is the seasonal
consumptive use in
in./season
 K is the seasonal
consumptive use
coefficient for a crop with a
normal growing season
(Table 4.7)

n
U  K  fi
i 1
SEASONAL CONSUMPTIVE USE FACTORS (K)

Mean monthly
temperatures are
available on the
web
PET ESTIMATION METHODS
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Simple models require measurement of only 1
weather variable
Temperature methods
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Relates PET rates to air temperature
Thornthwaite Method (good only for east-central
U.S.)
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Requires average monthly air temperature
Latitude  length of day
Radiation methods
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Relates PET rates to solar radiation
Jensen-Haise method
PENMAN METHODS
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Penman equations
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Equations to account for energy required to sustain
evaporation
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Solar radiation
% sunshine
Humidity
Wind
Long equations with many variables (Eqn. 4.30)
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Problems
 Complex equation  easy to make a mistake
 Need to keep units consistent
 Need lots of data as inputs
Penman PET Rates in Texas
JANUARY - DAILY PET (MM)
PET IN TEXAS
AUGUST - DAILY PET (MM)
LONG TERM WATER BALANCES
Basic equation for a control volume:
 I - O = DS
 Inputs – Outputs = Change in Storage
Control volumes in hydrology
 Pond, cultivated field, subdivision, watershed,
river basin, etc.
Example1: Control volume is a pond
Inputs (I)
precipitation, runoff, water pumped in
Outputs (O)
Discharges, seepage losses, evaporation
Change in Storage (DS)
Change in volume of water stored in pond
LONG TERM WATER BALANCES
Example 2: Control volume is a vegetated plot
Inputs:
precipitation, irrigation
Outputs: evapotranspiration (ET), infiltration,
runoff
D S = change in volume of water stored in the soil
profile
 2 conditions exist for vegetated plots
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If the soil profile is kept very wet ET is
maximized.
If the soil profile dries naturally ET is limited by
available water in the soil profile
QUESTIONS??