Overview

REF-ET is a compiled, stand-alone computer program that calculates reference evapotranspiration. Evapotranspiration (ET) is defined as the amount of water that evaporates from vegetation (transpiration) and from the underlying soil. Reference evapotranspiration is defined as the ET that occurs from a standardized "reference" crop such as clipped grass or alfalfa. Common usage in the United States and elsewhere has been to use the two reference crops of clipped grass (cool season varieties) and full-cover alfalfa (lucerne).

The REF-ET program provides standardized calculations of reference evapotranspiration for fifteen of the more common methods and equations that are currently in use in the United States and Europe. The calculations are based on the weather data measurements that are made available by the user.

The two primary purposes of REF-ET are to:

1. Provide standardized calculations of reference ET and other intermediate micro meteorological parameters that can be compared to calculations by other programs for error checking purposes.
2. Read weather data from a wide range of data file types, data unit types and time steps.

The REF-ET program supports reference ET computation guidelines and procedures that were recommended in ASCE Manuals and Reports on Engineering Practice No. 70, "Evapotranspiration and Irrigation Water Requirements" edited by Jensen, et al., (1990) through efforts by the Committee on Evapotranspiration in Irrigation and Hydrology of the Irrigation and Drainage Council of the Environmental and Water Resources Institute of the American Society of Civil Engineers (ASCE).

REF-ET provides calculations that are compatible with United Nations Food and Agriculture Organization (FAO) Irrigation Paper No. 56 (Allen et al., 1998) and with standardized forms of the ASCE Penman-Monteith equation recommended in 2000 by the ASCE Task Committee on Standardized Evapotranspiration Calculations.

Reference ET methods calculated by REF-ET for Windows version 2 include the ASCE and FAO Penman-Monteith equations, Kimberly Penman, CIMIS Penman, FAO-24 Penman, FAO-PPP17 Penman and 1948 Penman equations, FAO-24 Radiation, Blaney-Criddle and Pan evaporation equations, and Priestley-Taylor, Turc, Makkink and Hargreaves equations. Both clipped grass and alfalfa references can be computed.

Reference evapotranspiration is defined as the ET from an extensive surface of clipped grass or alfalfa that is well-watered, and fully shades the ground. The clipped grass reference is considered to be a "cool-season" grass variety such as perennial fescue or rye grass. The FAO-24 publication by Doorenbos and Pruitt (1977) suggested that the clipped grass surface be maintained at 8 to 15 cm in height. FAO-56 has suggested that the reference definition for clipped grass be fixed at 12 cm height for computational and definition purposes. The FAO-56 Penman-Monteith equation has been recommended by FAO to represent the definition of the grass reference (albedo = 0.23, height = 0.12 m, surface resistance = 70 s/m).

The alfalfa reference is generally considered to occur from alfalfa that is greater than 0.30 m in height and that has full ground cover. The ASCE Manual 70 (Jensen et al., 1990) has suggested fixing the height of the alfalfa reference at 0.5 m for computational and definition purposes.

REF-ET SOFTWARE and INSTALLATION

REF-ET for Windows is written in Visual Basic Version 6.0, Professional Edition. REF-ET runs on computers that have Windows 95, 98, or Windows NT operating systems. The screen resolution must be set to 800 x 600 or higher. Screen resolutions of 640 x 480 will not show the entire REF-ET data screens.

The two REF-ET distribution diskettes or the REF-ETzp.exe file that is downloadable from the www.kimberly.uidaho.edu web site contain the REF-ETw.EXE program file and assorted data and example files along with required Microsoft Visual Basic system files. The distribution diskettes and the downloadable REF-ETzp.exe file contain a file named "Setup.exe" that is used to install REF-ET program onto the computer.

If the "self-extracting" REF-ETzp.exe file is utilized, it will first need to be "unzipped" in order to place the setup files onto the computer. This is done by double clicking on it in the Windows Explorer window or using some other File Manager. The REF-ETzp.exe file only needs to be "run" once to unzip it. The product will be the creation of the following six files in the folder that the user specifies.

The REF-ETzp.exe file (or two distribution diskettes) contains the following files:

SETUP.EXE
SETUP.LST
REF-ET1.CAB
REF-ET2.CAB
Examples.exe
Readme.txt

The REF-ET1.CAB and REF-ET2.CAB are Microsoft "Cabinet" files that contain the REF-ETw.exe and Visual Basic system files. The Setup.LST file contains instructions for SETUP.EXE to process when it is run during installation.

The Examples.exe file is a "self-extracting" zipped file that contains various example data sets and associated definition files that are described later. This file can be unzipped into the REF-ET folder or other folder after it has been created during installation of REF-ET. Examples.exe is unzipped by double clicking on it in the Windows Explorer or other File Manager Software.

REF-ET can be installed directly from the distribution diskettes or the files can be copied into a temporary folder (directory) on the hard drive. If the self-extracting REF-ETzp.exe file is used, then this file can be unzipped into a temporary folder as well (by running it once). More in depth installation instructions are contained in the readme file.

Disclaimer

NOTE: Neither the American Society of Civil Engineers, the United Nations Food and Agriculture Organization, the University of Idaho, nor the author of REF-ET (R.G. Allen) accept any liability for errors or incorrect results from the REF-ET program nor any problems associated with installation of the software onto the user's computer system.

The REF-ET program includes commonly used algorithms and assumptions required in estimating reference ET. However, application of the REF-ET program to various combinations or types of data may occasionally violate assumptions or may require extrapolation beyond endpoints of algorithms, resulting in incorrect ET and parameter estimates. Therefore the user of this software assumes all responsibility and reliability for accuracy and correctness of estimates.

Distribution and Support Limitations

All source code and executable code of the REF-ET program is copyrighted by the author, Dr. Richard G. Allen and the University of Idaho, 1999, 2000. Users are given permission by the author to distribute the executable code contained on the distribution disks or in the downloadable REF-ET.zip file provided that the University of Idaho logo and author titles are retained, and with the understanding that users of distributed copies will subsequently register their copies of REF-ET with the University of Idaho upon use of their copy. An invoice for payment of the registration can be obtained from http://www.kimberly.uidaho.edu/ref-et/ or by making a request to REFETinv@kimberly.uidaho.edu. Registration fee is $49.00 plus a $5.00 shipping and handling fee ($15.00 shipping and handling addresses outside of the North American continent).

When REF-ET is distributed to students of Universities, a $10.00 license fee must be sent to the University of Idaho for each copy of REF-ET distributed. Students are expected to register the software with the University of Idaho upon graduation from their university and if subsequent use of the software is for commercial or professional purposes. No permission is granted to distribute any modifications of either the executable nor source code of the REF-ET program without prior, written consent by the author. In addition, No permission is granted to distribute the REF-ET program commercially.

The University of Idaho will provide limited telephone and written support to registered users of REF-ET who have purchased copies of the program directly from the University of Idaho. No telephone nor written support is available to nonregistered users or to users who have obtained or purchased copies from sources other than the University of Idaho.

Benefits of Registration

The registered user of REF-ET receives:

• The REF-ET.exe program on CD or diskettes
• A printed copy of the Users Manual
• Support via a special support electronic mail system
• Subscription to the REF-ET discussion list (you can unsubscribe if you wish)
• Automatic upgrade for the next software version
• Automatic alert of any errors or bugs discovered in the software
• Automatic notification of upgrades and user support

The special Email support will attempt to provide a response to the registered user within 48 hours of receipt of the Email inquiry.

In addition to the above benefits, registration provides financial resources to the University of Idaho and software developers to enable them to continue making improvements and updates to the software.

All registration information will be treated confidentially and will not be distributed to others.

If you feel that you are, or have been, getting $49 worth of benefit from the REF-ET program, then please, please register. The $49 benefit can be calculated by multiplying the value of your time by the number of hours of time that REF-ET may have saved you (for example, savings in time programming a spreadsheet to make similar calculations; or time saved in chasing down an error in outside calculations that REF-ET was able to point out fairly quickly).

The cost for the registered copy is 49 US dollars. We don't bill for the registration; however, the following two links provide a registration which the user can download, fill out, and mail with a check in the amount of 49 US dollars plus $5.00 shipping and handling ($15.00 shipping and handling addresses outside of the North American continent) to the address on the form.

The University of Idaho will provide limited telephone and Email or other written support to registered users of REF-ET who have purchased copies of the program directly from the University of Idaho. No telephone nor written support is available to non registered users or to users who have obtained or purchased copies from sources other than the University of Idaho.