EVAPOTRANSPIRATION

Returning water to the atmosphere

ANNOUNCEMENTS

HW#3 assigned

EVAPOTRANSPIRATION (ET)

Composed of two sub-processes: Evaporation and Transpiration

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

EvaporationTranspiration

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. Barometric pressure

2. Dissolved matter

3. Shape, site and situation of evaporating body.

4. 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)

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 average gradient of water vapor above the

canopy.

PAN EVAPORATION

Oldest / simplest method to measure evaporation

Measure water depths in a pan

U.S. Weather Bureau has standard Class A pan 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

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

200 m

Turfgrass (4 in.)

Class A PanN

PAN EVAPORATION / EXAMPLE PROBLEM

Required:

Calculate the PET for July 1

Solution:

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)

t is the mean monthly air

temperature in °F

p is the mean monthly percentage

of annual daytime hours (Table 4.6)

100

tpf

MONTHLY PERCENTAGE OF DAYTIME HOURS, P

Leuven = 50o 53’ / College Station = 33o 37’ / Knoxville = 35o 57’ / Lexington = 38o 4’

BLANEY-CRIDDLE EQUATION

n

i

ifKU1

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)

SEASONAL CONSUMPTIVE USE FACTORS (K)

Mean monthly

temperatures are

available on the

web

PET ESTIMATION METHODS

Simple models require measurement of only 1

weather variable

Temperature methods

Relates PET rates to air temperature

Thornthwaite Method (good only for east-central

U.S.)

Requires average monthly air temperature

Latitude length of day

Radiation methods

Relates PET rates to solar radiation

Jensen-Haise method

PENMAN METHODS

Penman equations

Equations to account for energy required to sustain

evaporation

Solar radiation

% sunshine

Humidity

Wind

Long equations with many variables (Eqn. 4.30)

Problems

Complex equation easy to make a mistake

Need to keep units consistent

Need lots of data as inputs

PET IN TEXAS

JANUARY - DAILY PET (MM) AUGUST - DAILY PET (MM)

Penman PET Rates in Texas

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 pondInputs (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 plotInputs: precipitation, irrigationOutputs: evapotranspiration (ET), infiltration,

runoffD S = change in volume of water stored in the soil

profile

2 conditions exist for vegetated plots 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??