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Irrigation—Principles & Practices

Unit 1.5 | Part 1 – 233

SUPPLEMENT 1

Evapotranspiration (ET) & Factors that Affect

ET Rates

Many factors affect ET, including weather parameters such as solar radiation, air tempera-

ture, relative humidity, and wind speed; soil factors such as soil texture, structure, density,

and chemistry; and plant factors such as plant type, root depth, foliar density, height, and

stage of growth.

Evaporation is the transformation of water from a
liquid into a gas. Water volatilizes into the air easily,
especially when it is hot and windy. Evaporation
happens only at the surface of a liquid, so the great-
er the surface area-to-volume ratio of the water, the
greater the evaporation rate. This means that you
lose more water to evaporation from water sprayed
in drops into the air than you do from water in a
drip tape line or in an irrigation canal or ditch.
The evaporation rate (i.e., the time it takes for
a certain amount of water to volatilize) for a given
day can be measured. One way is by placing a
known quantity of water in a container of a known
surface area and timing how long it takes to disap-
pear. In California, we can also look up this value
on a state-run website indexed by geographical area,
the California Irrigation Management Information
System website, http://www.cimis.water.ca.gov.
Transpiration is the transformation and use of
water by a plant. The plant uses water to transport
nutrients and air, to maintain its structure, and to
thermoregulate (maintain optimal temperature). The
transpiration rate of a plant is the amount of water
a plant uses up over a given amount of time. This
value is harder to measure, as it is difficult to assess
the minimum amount of water that a plant needs
to be healthy. The plant could be using less water
than you are giving it. You could measure this in a
very controlled environment by giving similar plants
different amounts of water and seeing the effects.
Fortunately, this can also be looked up. Transpira-
tion rates found in reference tables are generally for
mature plants; any plants that are working more
(flowering, setting fruit, at a critical stage of growth,

etc.) will transpire more. It is equivalent to breathing

for us – adults use more air than children do, you

use more when you are exerting yourself, etc.

Evapotranspiration or ET is the combined use of

water by plants and loss of water into the air (see

Appendix 1: Soil Moisture Terms, for an illustra-

tion). The evapotranspiration rate is the amount of

water that needs to be replaced over a given amount

of time to make up for the water that has been

used or volatilized. The evapotranspiration rate is

measured for mature plants in a given region on a

given day.

Precipitation and irrigation are the two primary

sources of water that plants use. Plant leaves and

soil surfaces temporarily retain some part of the wa-

ter applied to the field, but this part readily evapo-

rates. What remains percolates into the soil. Plants

extract the infiltrated water through their roots and

transport it up to their leaves for photosynthesis. In

addition to water, plants need carbon dioxide (CO 2 )

and light for photosynthesis. In order to take in CO 2

from the atmosphere, plants open their stomata,

the microscopic pores on the undersides of leaves.

It is during this process that they lose water to the

atmosphere.

Some Environmental Factors Affecting the

Rate of Transpiration

LIGHT

Plants transpire more rapidly in the light than in

the dark. This is largely because light stimulates the

opening of the stomata. Light also speeds up tran-

spiration by warming the leaf.

Supplement 1: Evapotranspiration (ET)

Evapotranspiration (ET) = Evaporation + Transpiration