az1195 - Cooperative Extension Turf Irrigation Management...

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Cooperative Extension 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 12/2000 AZ1195 Turf Irrigation Management Series: II T HE U NIVERSITY OF A RIZONA C OLLEGE OF A GRICULTURE T UCSON , A RIZONA 85721 Paul Brown Biometeorology Specialist Dave Kopec Turf/Pasture Grass Specialist This information has been reviewed by university faculty. Turf Irrigation Management Series INTRODUCTION 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 T ) is rarely measured in the real world. In- stead, we use meteorological data and mathematical models known as the Penman or Penman-Monteith Equation to estimate reference evapotranspiration (ET o ) — the ET from a tall, cool-season grass that is supplied with adequate water. In the lower elevations of Ari- zona the ET o value would seem of limited value since we rarely grow turf that is equivalent to the reference surface. However, we get around this problem by ad- justing the ET o value to account for differences in turf type, turf quality and stage of development. This docu- ment describes the procedures used to adjust ET o for use on managed turf surfaces in Arizona. ESTIMATING TURF ET FROM ET O An adjustment is necessary to convert ET o values to estimates of turf ET (ET T ; Fig. 1). The adjustment pro- cess is actually quite easy and consists of multiplying ET o by an adjustment factor known as a crop coeffi- cient (K c ): ET T = K c x ET o The procedure can be completed in seconds with a hand calculator provided you have access to an ET o value and an appropriate K c 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 o values for 23 southern Ari- zona locations via the Internet at http://ag. . Given that information on ET o is readily available, the problem of estimating ET T boils down to one of selecting an appropriate K c value for your turf.
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az1195 - Cooperative Extension Turf Irrigation Management...

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