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Using PET for Determining Crop Water
Requirements and Irrigation Scheduling
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Contents
I. How to
Use
II.
Where to Find PET Information
III.
What is Potential Evapotranspiration (PET)?
IV. Crop
Coefficients
V.
Irrigation System Efficiencies
VI.
Understanding FAO Crop Coefficients
VII. Acknowledgments
I.
HOW TO USE PET
To calculate the water requirements of
a crop, we multiply the PET times the crop coefficient
using the following equation:
PET x Kc =
crop water
requirements
(equation 1)
where:
PET is the sum of daily PET over
the time period of interest, such as the 3-day total,
the weekly total, etc.
Kc is the crop coefficient corresponding
to the current stage of crop growth.
Example 1: the 5-day PET total
is 1.32 inches. My sorghum is in the "heading"
growth stage. What are the water requirements? (Note:
from Table 1, the "heading" crop coefficient
is 1.10)
Thus, I need to apply 1.45 inches to replace
the water used by the sorghum in the last 5 days.
Adjusting
for Irrigation System Efficiency
It may be necessary to increase the amount
of irrigation water in order to compensate for poor
irrigation system efficiency. Irrigation system efficiency
is defined in Section V of this guide. Table
4 gives the typical ranges of on-farm irrigation systems.
To adjust for irrigation system efficiency,
use the following equation:
PET x Kc ÷ Eff =
irrigation
water requirements
(equation 2)
where:
Example 2. I am irrigating with
a low-pressure center pivot. I estimate that my overall
system efficiency is 85%. What are my irrigation water
requirements for the sorghum in example 1?
Adjusting
for Rainfall
Rainfall reduces the amount of water we must supply
by irrigation to meet plant water requirements. However,
not all rainfall becomes available for use by plants
and crops. Depending on such factors as soil type, duration
and intensity of rainfall, soil moisture levels, etc.,
a portion of the rainfall will be lost to runoff and
deep percolation (water moving below the root zone).
In irrigation scheduling, the term "
effective
rainfall
" refers to that portion of rainfall
which infiltrates and is stored in the root zone. Effective
rainfall must be estimated for each field and rainfall
event. The irrigation requirement determined with equations
(1) or (2) should be reduced by the amount of effective
rainfall.
Alternatively, soil moisture monitoring devices can
be use to determine soil moisture levels and to determine
when irrigations should be re-started following rains.
II.
WHERE TO FIND PET
INFORMATION
For persons with Internet access, PET
and weather information is provided for about 10 locations
in Central and South Texas on the Texas PET Web Site.
The address is:
Persons without Internet access should
contact their water district or County Extension Agent
to see if this information is being provided locally
in another way.
Persons on the Texas High Plains should contact the
Texas A&M Research and Extension Center in Amarillo
at (806) 359-5401 about subscription to the High Plains
PET Network where PET data is sent out every three days
by FAX.
III.
WHAT IS POTENTIAL
EVAPOTRANSPIRATION (PET)?
Evapotranspiration (ET)
is a measurement of the total amount of water needed
to grow plants and crops. This term comes from the words
evaporation (i.e., evaporation of water from
the soil) and transpiration (i.e., transpiration
of water by plants). Different plants have different
water requirements, so they have different ET rates.
Since there are thousands of cultivated plants, we have
tried to simplified matters by establishing a standard
ET rate for general reference and use. The standard
is referred to as the potential evapotranspiration
(PET). This is the potential ET since
we are assuming the crop is in a deep soil and under
well watered conditions. The standard crop we are using
is a cool season grass which is 4-inches tall. The technical
term for this is the "
Potential Evapotranspiration
of a Grass Reference Crop
" or "PET"
for short.
PET depends on the climate and varies from location
to location. Special weather stations are used to collect
the climatic data for calculating PET, including temperature,
dew point temperature (relative humidity), wind speed,
and solar radiation.
The water requirements of specific crops are calculated
as a percentage of the PET. This "percentage"
is the called the crop coefficient (Kc). Crop
coefficients depend on the type of crop and its stage
of growth. Detailed information on crop coefficients
are given later.
We are using the Penman-Monteith method to calculate
PET from the weather station data. This is one of a
number of methods that can be used to determine PET
and ET. Several organizations, such as the International
Committee on Irrigation and Drainage and the Water Requirements
Committee of the American Society of Civil Engineers
have proposed establishing the Penman-Monteith method
as a world-wide standard. Such a standard would help
facilitate the sharing of PET data and development of
crop coefficients.
IV.
CROP COEFFICIENTS
Potential Evapotranspiration (PET) is
an estimate of the water requirements of a 4-inch grass
in a deep soil growing under well watered conditions.
The water requirements of other crops are determined
from PET through crop coefficients (Kc). Crop coefficients
vary for different crops. They also change depending
on the growing stage of the crop.
Unfortunately, we only have verified crop coefficients
for the Texas North High Plains for cotton, sorghum,
corn and wheat (see Tables 1-3). These coefficients
were developed by the North Plains PET Network Project
Team. Another source is the FAO (Food and Agriculture
Organization of the United Nations) who has published
a long list of generalized crop coefficients which are
used throughout the world where local values are not
available (see Tables 5&6).
Choosing
and Using Crop Coefficients
For cotton, sorghum and corn, I recommend using the
North High Plains crop coefficients. These have been
verified in research and on-farm irrigation studies,
and should vary no more than about 10% for other parts
of the state.
The North High Plains crop coefficients are listed by
stage of growth in Tables 1-3. Please note that these
dates are provided as a general guide only, as crop
growth rate is affected by many factors including variety,
current weather, soil moisture conditions, etc.
For other crops, refer to the FAO Crop Coefficients
until researchers are able to verify coefficients for
specific regions in Texas. For many crops, we would
expect these general coefficients to be within about
10%.
Soil Moisture
Monitoring
I highly recommend soil moisture monitoring using gypsum
blocks, watermark sensors, tensiometers, the "feel"
method, or other devices for measuring the current water
status in the root zone. This provides an excellent
check to ensure that irrigations are keeping up with
crop water demand.
V.
IRRIGATION SYSTEM EFFICIENCIES
No irrigation system is 100% efficient. For sprinkler
irrigation systems, we can lose anywhere from 10% to
40% of the water in the air before the water reaches
the ground depending on wind and other weather conditions.
The amount of water lost to spray drift is referred
to as the application efficiency.
For drip and surface irrigation system, our biggest
concern is how evenly distributed the water is over
the field or along laterals and rows. This is referred
to as the distribution efficiency. The term
overall
efficiency
is a combination of both the application
and distribution efficiencies.
The normal ranges in on-farm overall efficiencies are
listed in Table 4. Under some situations, we will need
to increase the amount of irrigation to compensate for
water lost due to the inefficiencies of the system.
Table
1. Sorghum Crop Coefficients.
|
GROWTH STAGE1
|
KC
|
DAYS AFTER PLANTING2
|
|
Seeding
|
0.40
|
3 - 4 |
| Emerg |
0.40
|
5 - 8 |
| 3-leaf |
0.55 |
19 - 24 |
| 4-leaf |
0.60 |
28 - 33 |
| 5-leaf |
0.70 |
32 - 37 |
| GPD |
0.80 |
35 - 40 |
| Flag |
0.95 |
52 - 58 |
| Boot |
1.10 |
57 - 61 |
| Heading |
1.10 |
60 - 65 |
| Flower |
1.00 |
68 - 75 |
| S Dough |
0.95 |
85 - 95 |
| H Dough |
0.90 |
95 - 100 |
| Blk lyr |
0.85 |
110 - 120 |
| Harvest |
0.00 |
125 - 140 |
1Sorghum will bloom at different
times depending on locating , planting date, and maturity
of the variety.
2The Days After Planting are
for a medium-early to medium late variety.
Table
2. Cotton Crop Coefficients.
|
GROWTH STAGE
|
KC
|
DAYS AFTER PLANTING
|
|
Seeding
|
.07
|
0 - 10 |
| 1st Sqr |
.22 |
32 - 40 |
| 1st Blom |
.44 |
55 - 60 |
| Max Blom |
1.10 |
70 - 90 |
| 1st open |
1.10 |
105 - 115 |
| 25% open |
.83 |
115 - 125 |
| 50% open |
.44 |
135 - 145 |
| 95% open |
.44 |
140 - 150 |
| Pick |
.10 |
140 - 150 |
Table
3. Corn Crop Coefficients.
|
GROWTH STAGE1
|
KC
|
DAYS AFTER PLANTING
|
|
Seed
|
0.25
|
0 - 5 |
| Emerg |
0.35
|
5 - 8 |
| 4-leaf |
0.45 |
24 - 28 |
| 5-leaf |
0.70 |
29 - 34 |
| 6-leaf |
0.85 |
35 - 40 |
| 8-leaf |
1.00 |
43 - 46 |
| 10 leaf |
1.15 |
51 - 58 |
| 12-leaf |
1.20 |
57 - 65 |
| 14-leaf |
1.25 |
65 - 75 |
| Tassel |
1.25 |
72 - 82 |
| Silk |
1.30 |
75 - 85 |
| Blister |
1.30 |
85 - 95 |
| Milk |
1.30 |
95 - 105 |
| Dough |
1.20 |
100 - 110 |
| Dent |
1.00 |
110 - 120 |
| ½ mat |
0.90 |
115 - 125 |
| Blk lyr |
0.70 |
125 - 135 |
| Harvest |
0.00 |
140 + |
1 Note: 50% silking occurs
in 85 days in Corpus Christi/Coastal bend area and in
75 days in the Uvalde area.
Table
4. Typical overall On-Farm Efficiencies
| System |
Overall
Effeciency
|
| Surface |
0.50 - 0.80 |
| a. average |
0.50 |
|
b. land leveling and delivery pipeline
meeting
design standards
|
0.70 |
| c. tailwater recovery with (b) |
0.80 |
| d. surge |
0.60 - 0.901 |
| Sprinkler |
0.55 - 0.753 |
| Center Pivot |
0.55 - 0.903 |
| LEPA |
0.90 - 0.95 |
| Drip |
0.80 - 0.902 |
1. Surge has been found to increase efficiencies
8 to 28 percent over non-surge furrow systems.
2. Trickle systems are typically designed
at 90 percent efficiency; short laterals (<100ft)
or systems with pressure compensationg emitters may
have higher efficiencies.
3. Under low wind conditions.
VI.
FAO CROP COEFFICIENTS
With the FAO method, crop coefficients (Table 5) are
represented by straight lines connecting four general
growth stages, as indicated in the following figure.
Table 6 gives a more detailed explanation of the crop
coefficient values and their respective growth stages.
Table
5. FAO Mean Crop Coefficients, Kc, for Arid
Climates1
| CROP |
Kc12
|
Kc2 |
Kc3 |
| Alfalfa Hay3 |
0.30 |
1.25 |
1.10 |
| Artichokes |
0.95 |
1.00 |
0.95 |
| Asparagus |
0.30 |
0.95 |
0.30 |
|
Banana, 1st
year
|
0.50 |
1.15 |
1.10 |
|
Banana, 2nd
year
|
0.70 |
1.20 |
1.10 |
| Barley, Wheat, Oats |
0.30 |
1.15 |
0.20 |
| Beans, Green |
0.40 |
1.00 |
0.90 |
| Beans, Dry and Pulses |
0.40 |
1.15 |
0.35 |
| Beets, Table |
0.40 |
1.05 |
0.95 |
| Berries |
0.30 |
1.05 |
0.40 |
| Carrots |
0.50 |
1.10 |
0.80 |
| Castor Beans |
0.40 |
1.15 |
0.50 |
| Celery |
0.35 |
1.10 |
1.00 |
| Citrus |
0.65 |
0.80 |
0.65 |
|
Citrus w/cover or
weeds
|
0.70 |
1.05 |
1.05 |
| Clover Hay3 |
0.60 |
1.20 |
1.05 |
| Coffee |
0.90 |
0.90 |
0.90 |
| Conifer Trees |
1.20 |
1.20 |
1.20 |
| Corn, Field |
0.40 |
1.15 |
0.60, 0.354 |
| Corn, Sweet |
0.40 |
1.15 |
1.05 |
| Cotton |
0.40 |
1.20 |
0.65 |
|
Cucumber, Fresh
Market
|
0.35 |
0.95 |
0.75 |
|
Cucumber, Machine
Harvest
|
0.35 |
0.95 |
0.95 |
| Crucifers5 |
0.40 |
1.05 |
0.90 |
| Dates |
0.95 |
0.95 |
0.95 |
| Deciduous Orchard |
0.50 |
1.00 |
0.656 |
|
Deciduous Orchard
w/cover or weeds
|
0.80 |
1.25 |
0.856 |
| Eggplant |
0.40 |
1.05 |
0.85 |
| Flax |
0.30 |
1.10 |
0.20 |
| Grapes |
0.30 |
0.80 |
0.40 |
| Grass Pasture |
0.80 |
0.80 |
0.80 |
| Groundnuts |
0.40 |
1.05 |
0.60 |
| Hops |
0.30 |
1.00 |
0.60 |
| Kiwi |
0.30 |
1.05 |
1.05 |
| Lentil |
0.30 |
1.15 |
0.25 |
| Lettuce |
0.30 |
1.00 |
0.95 |
| Melons |
0.40 |
1.00 |
0.75 |
| Millet |
0.30 |
1.10 |
0.25 |
| Mint |
0.60 |
1.10 |
1.10 |
| Olives |
0.40 |
0.70 |
0.70 |
| Onion, Dry |
0.50 |
1.05 |
0.80 |
| Onion, Green |
0.50 |
1.00 |
1.00 |
| Open Water |
1.15 |
1.15 |
1.15 |
| Palm Trees |
0.95 |
0.95 |
0.95 |
| Peas, Fresh |
0.40 |
1.10 |
1.05 |
| Peas, Dry/Seed |
0.40 |
1.10 |
0.30 |
| Peppers, Fresh |
0.35 |
1.05 |
0.85 |
| Pistachios |
0.20 |
1.10 |
0.40 |
| Potato |
0.40 |
1.10 |
0.758 |
| Pumpkin |
0.40 |
1.00 |
0.75 |
| Radishes |
0.30 |
0.85 |
0.80 |
| Rice |
1.10 |
1.25 |
1.00 |
| Safflower |
0.35 |
1.15 |
0.20 |
| Sorghum, Grain |
0.30 |
1.05 |
0.50 |
| Sorghum, Sweet |
0.30 |
1.20 |
0.50 |
| Soybeans |
0.35 |
1.10 |
0.45 |
| Spinach |
0.30 |
1.00 |
0.95 |
| Squash |
0.30 |
0.95 |
0.75 |
| Strawberries |
0.40 |
0.90 |
0.70 |
| Sugar Beet |
0.30 |
1.15 |
1.009 |
| Sugar Cane |
0.40 |
1.25 |
0.70 |
| Sunflower |
0.30 |
1.15 |
0.35 |
| Tea |
1.00 |
1.00 |
1.00 |
| Tomato |
0.40 |
1.20 |
0.65 |
| Walnut Orchard |
0.50 |
1.00 |
0.65 |
| Winter Wheat |
0.30 |
1.15 |
0.20 |
Footnotes to Table 5.
1Values for Kc2
and Kc3 represent those for an arid climate
(RHmin ~ 20%) with moderate wind speed (0-5
m s-1, averaging 2 m s-1) and
are used for general irrigation water requirements.
For humid or windier conditions, Kc2 and Kc3
can be modified as follows:
Kc3=Kc3 table
- 0.0015 RHmin + 0.01 U2 when
Kc3 > or = 0.4
Kc3 = Kc3 table
+ 0.001 (RHmin-20) when Kc3
< 0.4
2General values for Kc1
under infrequent soil wetting (~each 10 days). Kc1
is a function of wetting interval and potential evaporation
rate during the initial and development periods.
3 The coefficients for hay crops represent
immediately following cutting; full cover; and immediately
before cutting, respectively. An overall mean coefficient
for alfalfa hay which takes into account cutting effects
is 1.05 and for clover is 1.00. Kc3 for alfalfa
seed, which is never cut for forage or hay is about
0.5.
4 The first Kc3 value is for harvest
at high grain moisture. The second Kc3 value
is for harvest after complete field drying of grain
(to about 18%).
5 Crucifers include cabbage, cauliflower,
broccoli, and Brussel sprouts.
6 The Kc3 value represents the
Kc prior to leaf drop.
7 Cool season grass varieties include dense
strands of bluegrass, ryegrass, and fescue. Warm season
varieties include bermuda grass and St. Augustine grass.
The 0.90 values for cool season grass represent a 0.06
to 0.08m height under general turf conditions.
8 The Kc3 value for potatoes is
about 0.40 for long season potatoes with vine kill.
9The Kc3 value for sugar beets
is 0.60 if no irrigation occurs during the last month.
Table
6 . FAO Generalized Crop Growth Stages
|
Kc Values
|
Growth Stage
|
Description
|
|
Kc1
|
Initial
|
The average KC value from planting
to about 10% ground cover.
|
|
Kc1-Kc2
|
Rapid Growth
|
From 10% ground cover to 75% cover or to peak
water use, which ever comes first.
|
|
Kc2
|
Mid season
|
The average value from the end of the rapid
growth stage until water use begins to decline
due to crop aging.
|
|
Kc2-Kc3
|
Late season
|
From when KC begins to decline until
harvest or when water use ceases or becomes minimal.
|
|
Kc3
|
Harvest
|
The average value at harvest or the end of the
water use season.
|
Note: For the rapid growth and
the late season stage, it is assumed that KC values
increase or decrease linearly with time.
VII. ACKNOWLEDGEMENTS
Information on crop growth periods, time periods after
planting, and other advice provided by Charles, Stickler,
Associate Professor and Extension Agronomist, Texas
A&M Center-Uvalde.
For More Information
For more information on the Penman-Monteith equation
and other methods for determining PET, see the book:
Evapotranspiration and Irrigation Water Requirements,
edited by M.E. Jensen, R.D. Burman, and R.G. Allen.
Published by the American Society of Civil Engineers,
New York, NY. 1990. 332pp.
Written by Guy Fipps, Associate Professor and Extension
Agricultural Engineer, Texas A&M University, College
Station..
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