AZMET Evapotranspriation


                             Paul W. Brown
                      Extension Biometeorologist


Proper irrigation management is essential to the successful production
of turfgrass in Arizona.  The Arizona Meteorological Network (AZMET) presently
provides weather information to assist irrigation management throughout 
southern and central Arizona including the Phoenix and Tucson metropolitan
areas.  AZMET information can be particularly useful when determining how much
water has been used by a turfgrass surface since the previous irrigation.
This document discusses the proper procedures for using AZMET information for
estimating turf water use.


      Evapotranspiration (ET) is the loss of water from a vegetative surface
through the combined processes of plant transpiration and soil evaporation
(ET is equivalent to and frequently referred to as consumptive use). 
Both environmental and biological factors affect ET. Important environmental
factors include solar radiation, temperature, atmospheric dryness (vapor 
pressure deficit), wind and soil moisture.  Biological factors affecting ET
include type of vegetation, foliage geometry and foliage density.

      Several methods have been developed to estimate crop ET.  Most methods
use weather data to provide an estimate of reference (or potential)
evapotranspiration (ETo), often convert the ETo to "actual" ET using a
multiplicative factor known as a crop coefficient (Kc):

                                    ET = Kc x ETo

The Arizona Meteorological Network provides daily estimates of ETo for
all locations served by AZMET's remote weather station network.  AZMET ETo 
values are determined using a weather-based model known as the Penman Equation.
Weather parameters utilized in the Penman calculation include solar radiation,
temperature, humidity and wind speed.


      Reference crop evapotranspiration (ETo) is an estimate of the water used
by a well-watered, full-cover grass surface, 8-15 cm in height (the reference
crop). As previously mentioned, a correction factor or crop coefficient (Kc)
is required to convert ETo to ET for a specific crop.  Crop coefficients for
turfgrass depend on the type of grass (warm or cool season), cutting height
and desired turf quality.  The following Kc recommendations are based on
results of research conducted at the University of Arizona:

    Type of Grass       Cutting Height           Quality             Kc

     Warm Season          4 -   5 cm       Acceptable (Park)         0.65
     Warm Season          2 - 2.5 cm       High (Golf Course)        0.76

     Cool Season          4 -   5 cm       Acceptable (Park)         0.65
     Cool Season          2 - 2.5 cm       High (Golf Course)        0.72


      The following example shows the procedure for using AZMET ETo to
estimate daily water use of turf.  Remember Turf ET = Kc x ETo.

      Step 1.     Obtain the AZMET ETo estimate from an AZMET Daily
                  Weather Summary.

                  From Figure 1, the ETo estimate is 0.33"

      Step 2.     Select the Kc value appropriate late for your turf.
                  Assume a high quality warm season turf:  Kc = 0.76

      Step 3.     Determine turf ET value.
                  ET = Kc x ETo

                      = 0.76 x 0.33

                      = 0.25"


      Use AZMET ET estimates with appropriate caution.  We suggest using the
Kc's provided with this document, then adjusting the coefficients up or down
for your particular situation.  Adjustments should be made on the basis of
turf quality. Increase the value of the coefficients if unacceptable turf
quality results from their use.  By the same token, you may wish to reduce
the coefficients if resulting turf quality is higher than deemed necessary.

      AZMET ET estimates provide a convenient means of determining turf water
use, and thereby, provide a method of calculating how much water to apply with
each irrigation. Remember, irrigation system efficiency and water quality must
be taken into consideration when determining the final amount of water to
apply.  Use of inefficient systems and/or irrigation water with a high salt
content will require applications of water over and above that lost by ET.

     NOTE ! :  
     ETo is a Reference Evapotranspiration.

     This is NOT equal to a Pan Evaporation value.

     ETo values can be considered equal to evaporation from a large body 
     of water, such as a pond or lake.  However, for smaller, shallower 
     bodies of water this relationship does not apply.
     Many factors affect lake/pond evaporation including surface area,
     depth, water temperature, and turbidity.
     To date, not enough research has been done to determine the
     minimum body of water size at which ETo is still equal.

     To get an approximate Pan Evaporation value from the ETo value
     use this 'rule-of-thumb' conversion: divide the ETo value by a 
     conversion constant. In winter or cooler times of the year, divide 
     by 0.7. During summer or warmer periods, divide by 0.6.

       ETo           Winter             ||       ETo           Summer    
     -------  =  Approx. Pan Evap.      ||     -------  =  Approx. Pan Evap.  
       0.7                              ||       0.6                      

      ETo units can be in 'English' (inches) or Metric (millimeters).

      Many factors can affect the rate of evaporation from an open body
      of water; depth of water, area of the water, temperature of water,
      topography and vegetation surrounding the body of water, etc.
      The rate of evaporation from a irrigated field (drip,flood, sprinkler)
      can be affected by the same factors as an open body of water (above).
      The soil type, texture, color and porosity will also influence evaporation.

                                     SEP 1 1987

                           MAX.    MIN.    MEAN     TOTAL      UNITS
   TEMPERATURE             103.2   81.5    91.4                 DegF
   RELATIVE HUMIDITY        35.6   11.1    20.9                 %
   VAPOR PRESS DEF.                         4.1                 KPas
   SOLAR RADIATION                                   592.9      Langleys
   PRECIPITATION                                      0.00      Inches
   SOIL TEMP. 2 IN         103.9   81.4    90.5                 DegF
   SOIL TEMP. 4 IN         102.9   85.2    92.8                 DegF
   WIND SPEED               16.7            5.3                 MPH
   WIND VECTOR MAG.                         3.1                 MPH
   WIND VECTOR DIR.                         110                 Degrees
   REF. EVAPOTRANSPIRATION                          |  0.33  |  Inches   ETo
       HEAT UNITS       86/55F              86/50F               86/45F
                     DAY     CUM         DAY      CUM         DAY      CUM
   INTEGRATED        30.4    1000        35.4                 40.4
   SINE CURVE        30.2     982        35.2     1157        40.2     1332
                 DAY : Daily Heat Unit Total
                 CUM : Heat Units Accumulated Since Installation

Figure 1.   AZMET Daily Weather Summary for Phoenix-Greenway.
            The boxed area encloses the daily ETo value.


 1.   Penman, H.L.  1948.  Natural evaporation from open water, bare soil and
      grass.  Proc.  Roy. Soc. London, A193:120-146.

 2.   Pruitt, W.O., and J. Doorenbos.  1977.  Empirical calibration a
      requisite for evapotranspiration formulae based on daily or longer mean
      climatic data.  Proceedings of the International Round Table Conference
      on "Evapotranspiration", Budapest, Hungary.  20 pages.

 3.   Slatyer, R.O., and I.C. McIlroy.  1961.  Practical microclimatology,
      CSIRO, Melbourne (UNESCO).

 4.   Tanner, C.B., and W.L. Pelton.  1960.  Potential evapotranspiration
      estimates by the approximate energy balance method of Penman.
      J. Geophys. Res., 65:3391-3413.

 5.   van Bavel, C.H.M.  1966.  Potential evapotranspiration: The combination
      concept and its experimental verification.  Water Resources Res.,