Missouri Weather Stations

green line
June 6, 1995

                The Use of Evapotranspiration Estimates
                 as A Guide for Scheduling Irrigation  
                             in Missouri*

                         Wayne L. Decker
                 Professor Emeritus and Weather Consultant
                   Commercial Agriculture Program

Factors controlling the rate of water loss from crops
     The combined water loss through evaporation from the soil surface and
transpiration from the leaves of a crop is commonly called evapotranspiration
(ET).  There are several factors which determine how much water will be
lost through evaporation from a soil surface and transpiration from a
particular crop. Important among these factors are the following.
     The type of crop or vegetative cover must be considered. Grain 
sorghum does not  have exactly the same ET rate as corn, and neither of these
crops would have similar ET values to a golf green.  So the estimate of daily
ET must be crop specific.
      The stage of development of the crop is important.  The ET for a
crop immediately after it has emerged will be less than after it has grown to
the stage where it just covers the ground.  In general, for Missouri's crops
the daily ET rate increases as the plant develops, reaching a maximum late in
the season before the leaves begin to change color or fall.  Some years the
rate of development is faster than for other years.  To account for this
difference a temperature relationship has been developed to simulate the
stages of development. 
     Soil moisture content also controls ET rate.  Low soil moisture levels 
restrict the rate of water transfer to the plant and reduce ET.  But since
irrigation is used to maintain soil moisture at a proper level,  soil
moisture need not be considered in estimating the daily ET rate in an
irrigation application.
     The weather is the driving force for determining the rate of moisture
transfer to the atmosphere.  Sunny days with low humidity and high winds favor
evaporation and leads to high ET.  The maximum water that can be delivered to
the atmosphere from a plant and soil surfaces is called the potential
evapotranspiration (PET).  It is totally dependent on the weather, and methods
have been developed to estimate the PET using weather data.

*CA 155:  Working Draft May 1995

Estimation of stage of development for the irrigated crop
     One of the first steps in estimating the rate of water loss from an
irrigated crop is to determine the stage of development for the crop under
irrigation.  This is done by observing development of the crop.  For the
extension specialist or farm management advisor it may not be possible to
visit every irrigated field in a particular area.  Another indicator is
needed to estimate the stage of crop development.
     In estimating the stage of development two factors should be
considered.  The first is the planting date, because an early planted field
reaches a particular stage of development sooner than one planted at a later
date.  The second estimator is the temperature during the development period.
Here the assumption is that the crop will develop earlier when temperatures
are warmer.  To characterize temperature an index called "growing degree days"
(DD) has been defined.
     DD estimation is made by taking the difference between the average
temperature for the day and the temperature at which plant growth and
development begins.  For most Missouri field crops the minimum temperature
for growth and development is 50 degrees Fahrenheit.  In most cases the
average temperature is computed as the average of the maximum and minimum
temperatures for the day.  So, a day with a average temperature of 80 degrees
will have 30 DDs, a 55 degree average temperature 5 DD's, and so on.  For
average temperatures below 50 degrees the DD's are assigned a value of zero.
     The DD for the daily values of temperature are summed to calculate the
accumulative degree days (ADD) for a particular stage of development, and  ADD's
are used to estimate the stage of development for a particular crop.  If this
argument is followed to its final conclusion, the maximum ADD occurs for the
period from planting to maturity of a particular crop.
     The calculation of the ADD's begins at planting and continues day-by-day
through the growing season.  An irrigator could measure temperatures at the
farm and estimate the DD's for each day.  Or better yet, the irrigator could
observe the stage of crop development on the farm where the irrigation
management is being used and not bother with DD's.  For farm managers,
extension specialists, and farm advisors it's probably more practical to
obtain estimates of the ADD's and crop development from information using
observed temperatures at various locations around the state.  One source for
such information is the Commercial Agriculture's Agricultural Electronic
Bulletin Board ( AgEBB) for the automated weather stations located in areas
where irrigation is practiced.

Estimating Evapotranspiration
     The first step in estimating daily ET is to calculate the daily potential
evapotranspiration (PET) from observations taken at weather stations
reporting the weather variables used to determine the PET.  These variables
include the amount of solar energy reaching the surface, air temperature,
humidity, and wind and are obtained from automated weather stations.  The
estimates are made in terms of equivalent depth of water.
     There are 14 Missouri locations--Figure 1--with measurements necessary
to estimate daily PET.  Daily estimates of PET are placed on AgEBB for use
by Missouri agricultural specialists and irrigators.  The locations have been
selected so that there is a  weather station near to most regions with
appreciable irrigation.  An irrigator should select the station nearest the
region for which irrigation strategies are used.
     The irrigator is not really interested in the PET as it does not
represent the actual water loss from an irrigated crop.  It is necessary to
adjust the PET estimate, which is obtained from weather data, to estimates of
the actual ET from the crop.  The mathematical model for making this
adjustment is simple.  It assumes that there is an adjustment factor--k--
which when multiplied by the PET for a particular day will produce an
estimate of ET for that day.  The estimation of ET is obtained by a linear
relationship shown in the following formula.
       ET = (k)(PET)
     The magnitude of k will depend on the type of crop and the stage of
development of the crop.  The stage of development is determined by the ADD's
on the day for which the estimate is being made.
     Fig. 2  shows the adjustment factor for various development stages of a
corn crop.  At bottom of the diagram the progression of the growing season is
characterized by the percent of the total ADDs at crop maturity.  At the top
of the diagram the growth and development stages of corn are presented which
correspond to the ADD's.
     Fig. 3 shows the "k" values for soybeans for the particular stage of
development and percent of the total ADD's at maturity.

Use of Estimated ET in irrigation scheduling
     ET estimates, when combined with measured rainfall, provide the necessary
data for determining the need for irrigation on a particular day.  The system
involves using a "water balance" accounting much like a balance in a bank
account is maintained.  Today's water balance is obtained from yesterday's
balance plus additions--rain and irrigation--minus losses from
      There are additional components that need to be considered in 
keeping a record of the moisture situation on a particular field. One of
these components is the water holding capacity of a particular soil.  A sandy
soil will retain much less water than a loamy soil.  An understanding of the
nature of a soil is needed so that a better use of the water held in the
soil can be made and to prevent over-irrigating beyond the soils ability to
retain water.  A sandy soil will hold an inch or less of plant available
water per foot of soil.  A loam may hold as much as 1.5 inches per foot.
     Another factor is the strategy to be used for starting an irrigation
application.  The manager must decide how dry the soil should be before
irrigation.  It is not a good conservation practice to maintain the soil at
"full" level of plant available water, and such a saturated condition may
even create a soil water condition that is not beneficial to the crop.
Irrigation in a subhumid area such as Missouri needs to take into account the
possibility of rain, and a reserve in the water balance in the soil should be
held for the rain which may occur. 
     Weather forecasts can provide a guide.  Many producers have as a goal
maintaining the soil water condition above 50% of the plant available water
in the rooting zone.
     The water balance calculation takes all of the components of water supply
into account:  the water held in the soil reservoir, evapotranspiration
during the period, and the rainfall received at the field for the period. 
The relationship is as follows.
          SMi + Rainfall + Irrigation - Evapotranspiration = SMf
Here SMi and SMf are the initial and final soil moisture estimates and the
rainfall and evapotranspiration are the values for the period between the
initial and final days.
     A final consideration should be taken by the irrigator for decisions
made late in the growing season.  The value of added water during the late
grain filling and maturation period is not as great as when the crop is going
through its flowering and early grain filling periods.  In Missouri the "best of
all worlds" would have the soil reservoir nearly empty at the end of the
growing season in order to take advantage of the cool season precipitation to
recharge the soil reservoir "for free" before the next growing season.  This
conservation practice is particularly important in the southwest and 
southeast regions of the state where cool season precipitation is likely in

Format for data of ET estimates on AgEBB for Missouri automated weather
Stations to Use
     The weather stations to use in these presentations are the automated
stations of the Commercial Agriculture network shown in Fig. 1.  These locations
include 14 stations in regions where irrigation is often practiced on
commercial farms.
Data to Use
     Daily PET estimates obtained from the basic data reduction of the
automated weather stations provide the data for the computation of the daily
estimates of ET for corn and soybeans.  It's necessary to adjust this PET
estimate, as shown in the previous equation, to obtain the value of ET for
each growing stage and crop.
     The values of the adjustment factor--k--for corn and soybeans are found in
Fig. 2 and Fig. 3.  Daily estimated ET values will then be crop and growth stage
     There must be allowance for a variance in the "start-up date" at each of
the locations of the automated weather stations.  This "start-up date" 
represents different planting dates for specific crops.  The planting dates
used at all locations are as follows.
               Corn                Soybeans
               April 1             April 15
               April 7             April 21
               April 15            April 30
               April 21            May 7
               April 30            May 15
               May 7               May 21
               May 15              May 30
               May 21              June 7
               May 30              June 15
               June 7              June 21
               June 15             July 1
 AgEBB Presentation
     For each crop and planting date ET summaries will be placed on AgEBB
each day.  There will be an estimate of yesterday's ET, a three-day total for
the previous three days, and a seven-day total--yesterday and the six
previous days.  ET estimates are presented as the equivalent depth of water
in inches or fractions of an inch.

     The AgEBB table will look like the following example for corn.  This table
represents the AgEBB listing for July 15 of a particular year for the
location in Audrain County.  This information is updated each morning for all
locations with the new date indicated.  A similar table will appear on AgEBB
for soybeans.

                 July 15 Estimates of Evapotranspiration*

Location: Audrain County
Crop: Corn                           
Planting Date         YesterdayLast Three Days   Last Seven days
April 1             .25            .76              1.40
April 7             .25            .76              1.40
April 15            .25            .76              1.40
April 21            .24            .74              1.38
April 30            .24            .70              1.35
May 7               .23            .69              1.33
May 15              .22            .68              1.31
May 21              .22            .67              1.30
May 30              .22            .66              1.30
June 7              .21            .62              1.28
June 15             .20            .59              1.25
*All estimates in inches or fractions of an inch.

Figure 1 - Map of Locations With Automated Weather Stations

                  \*Corning *Albany             \
                    >*St. Joseph         *Novelty\
                    \                             \
                     \          *Brunswick          \
                      \                    *Auxvasse  \
                                      & UMC          <
                                            *Cook     \
                       *Lamar                Station    \
                      +----------------------------------/ *Portageville
                                                        /  *Steele

Figure 2 - Adjustment Factor (k) for Calculating Evapotranspiration (ET)
                from Potential Evapotranpiration for Corn 
    Planting                    Tasseling            Dough             Physio-
    |                           |                    Stage             logical
    |                           |                    |                  Mature
    |    --Vegetation           |     --Flowering &  |       --Kernel        |
    |        Stage--            |early ear formation-|    Development--      |
1.20|                                                                        |
    |                                                  .____________         |
    |                            __.______.______.___/               .       |
1.00|                          .                                      \      |
    |                         /                                        \     |
    |                       .                                           \    |
 .80|                      /                                             \   |
    |                   .                                                 \  |
    |                 /                                                    \ |
 .60|               /                                                       \|
    |              .                                                         .
    |             /                                                          |
 .40|           /                                                            |
    |          .                                                             |
    |         /                                                              |
 .20|_______.                                                                |
    |                                                                        |
    |                                                                        |
    0     10     20     30      40     50     60     70     80     90      100

               Percent of Required Growing Degree Days for Maturity

**The vertical numbers on the left (y-Axis) represent the Coefficient for
  Adjusting PE for Calculating ET (k)**

Figure 3 - Adjustment Factor (k) for Calculating Evapotranspiration (ET)
              from Potential Evapotranpiration for Soybeans

    Planting                               Flow-     Pod               Physio-
    |                                      ering     Develop-          logical
    |               Vegetative                |Floral|    Pod and Seed       |
    |                                         |Stage |    Development        |
 1.1|                                 ._____.__.______._______.              |
    |                               /                          \             |
 1.0|                            .                              \            |
    |                          /                                  \          |
  .9|                      ./                                        .      |
    |                    /                                            \      |
  .8|                   /                                              \     |
    |                  /                                                \    |
  .7|                .                                                   \   |
    |              /                                                      \  |
  .6|             /                                                        \ |
    |           /                                                           \|
  .5|          .                                                             .
    |         /                                                              |
  .4|        /                                                               |
    |       /                                                                |
  .3|     .                                                                  |
    |    /                                                                   |
  .2|_ /                                                                     |
    |                                                                        |
  .1|                                                                        |
    |                                                                        |
    0     10     20     30      40     50     60     70     80     90      100

            Percent of Required Growing Degree Days for Maturity

**The vertical numbers on the left (y-axis) represent the coefficient for
   adjusting PE for Calculating ET (k)**

Commercial Agriculture Program
S102 Animal Sciences Research Center
University Extension
University of Missouri
Columbia, Missouri 65211

[ AgEBB Home Page ] - [ Comments ]