|
Using PET for Determining Crop Water
Requirements and Irrigation Scheduling
Click to download
a hard copy of the grower's guide
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
|