Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of
Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.
The University of Arizona College of Agriculture and Life Sciences is an equal opportunity employer authorized to provide research,
educational information, and other services only to individuals and institutions that function without regard to sex, religion, color, national
origin, age, Vietnam era Veteran’s status, or disability.
Converting Reference Evapotranspiration
into Turf Water Use
Turf Irrigation Management Series: II
Turf/Pasture Grass Specialist
This information has been reviewed by university faculty.
Turf Irrigation Management Series
Accurate estimates of turf water use are required to
effectively manage a turf irrigation system. In Volume I
of this series entitled “Basics of Evaporation and Evapo-
transpiration (ET),” we indicated that actual turf wa-
ter use (ET
) is rarely measured in the real world.
stead, we use meteorological data and mathematical
models known as the Penman or Penman-Monteith
Equation to estimate reference evapotranspiration (ET
— the ET from a tall, cool-season grass that is supplied
with adequate water.
In the lower elevations of Ari-
zona the ET
value would seem of limited value since
we rarely grow turf that is equivalent to the reference
However, we get around this problem by ad-
justing the ET
value to account for differences in turf
type, turf quality and stage of development.
ment describes the procedures used to adjust ET
use on managed turf surfaces in Arizona.
ESTIMATING TURF ET FROM ET
An adjustment is necessary to convert ET
estimates of turf ET (ET
; Fig. 1).
The adjustment pro-
cess is actually quite easy and consists of multiplying
by an adjustment factor known as a crop coeffi-
The procedure can be completed in seconds with a hand
calculator provided you have access to an ET
and an appropriate K
value for your turfgrass.
Reference ET values can be obtained from a private,
on-site weather station or public weather networks
such as the Arizona Meteorological Network (AZMET)
which provides daily ET
values for 23 southern Ari-
zona locations via the Internet at
Given that information on ET
readily available, the problem of estimating
down to one of selecting an appropriate K