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  User's guide for estimating crop evapotranspiration (ETc) Using The Infrared Thermometry (IRT) Method
G.S. Jorgensen, Kenneth H. Solomon, and V. Cervinka **

CATI Publication #920701
© Copyright July 1992, all rights reserved

Crop Evapotranspiration estimates (Et c ) are used in making decisions for scheduling the quantity and frequency of irrigation water application. There are a number of well-known methods for estimating evapotranspiration (Et c ) of agricultural crops. The most popular method of Et c estimation requires utilization of empirical crop coefficients. The crop coefficient is used in conjunction with calculated reference-Et (Et o ), obtained from meteorological data to calculate Et c . Since the crop coefficient method relies on experimental data derived under various conditions, it may not accurately calculate Etc under a specific local set of field crop conditions at a given time. The Et o -crop coefficient method is used, therefore, primarily to provide information used in making decisions about irrigation scheduling.

Recent investigations using infrared thermometry (IRT) have provided results which have enabled the development of a new method for calculating Et c on a daily basis under local field crop conditions. Results from these studies were published in a scientific article titled: "A Computational Approach to Assess Transpiration from Aerodynamic and Canopy Resistance," by J. Ben-Asher, D.W. Meek, R.B. Hutmacher, and C.J. Phene. (Agron. J. 1989 pp. 776-781)

A farmer, irrigator, or consultant with an infrared thermometer and access to a personal computer and IRT-Et c software (available at a nominal charge from CIT) can now calculate how much water was used by the crop in his field the previous day. Such information, collected over a period of several days, can be utilized in making decisions about irrigation scheduling and crop water consumption.

The benefit of the IRT method to growers is that the grower can calculate daily crop water requirements under specific local environmental conditions and crop water stress can be prevented, meaning that the grower can improve water use efficiency and potentially improve yields.

This User's Guide describes the equipment, software and field techniques (including infrared measurements) necessary to use the IRT method.

Equipment and Software Requirements
To use the IRT method for calculating daily Etc, the following equipment is required:

1. A portable commercial quality infrared thermometer capable of measuring crop canopy surface temperature on either Fahrenheit or Celsius scales.

2. Access to an IBM PC or compatible microcomputer.

3. Access to CIMIS weather data. Information concerning CIMIS is available by calling (916) 653-9847. Selected CIMIS weather data is regularly available from CATI's ATI-Net computer system which can be reached by calling (559) 278-4265 for users with 1200 baud modems and (9) 278-4615 for users with 2400 baud modems. Additional information is available from the ATI-Net staff at (559) 278-4872.

4. A current copy of IRT Et software is available from the Center for Irrigation Technology by calling (559) 278-2066.

Information Required and Where to Obtain It
The grower must have several important pieces of information in order to use the IRT Et software which calculates Et c . These are called parameters and include the following:

Maximum Air Temperature: defined as the ambient dry bulb temperature, measured at noon and immediately adjacent to the field of interest. The measurement is in degrees Fahrenheit or Celsius and should be taken by the grower. Alternatively, this information may by obtained from CIMIS hourly data for the nearest weather station or any other local data.

Maximum Canopy Temperature: defined as the crop canopy surface temperature, measured at solar noon with an infrared thermometer for the crop and the field of interest. Measurement is in degrees Fahrenheit or Celsius and is taken by the grower. The infrared thermometer must be facing south at the time of the measurement within the field of view of the IRT.

Minimum Canopy Temperature: defined as the crop canopy surface temperature, measured at solar noon with an infrared thermometer immediately after irrigation of the crop in the field of interest. Measurement is in degrees Fahrenheit or Celsius and is taken by the grower. The infrared thermometer must be facing south at the time of measurement.

For practical purposes, Minimum Canopy Temperature can be approximated by measuring the surface temperature of a water saturated paper towel or a sponge large enough to cover the field of view of the IRT.

Daily Reference Evapotranspiration: obtained from CIMIS (symbolized by Et o ) for the reporting site physically nearest to the field of interest for the date on which other field measurements are taken by the grower. Note that there is a 24-hour delay in CIMIS data reporting. Therefore, the most current CIMIS information is always yesterday's weather information.

Solar Noon Net Radiation: obtained from CIMIS (symbolized as NET SOLAR RAD) for the reporting site physically nearest the field of interest and for the date on which other field measurements are taken by the grower.

Noon Relative Humidity: obtained from a local psychrometer or CIMIS data (symbolized at REL HUMIDITY) for the reporting site physically nearest the field of interest and for the date on which other field measurements are taken by the grower.

Running the IRT Et Computer Program
The IRT Et computer program is designed to run on IBM PC or IBM-compatible computers. The program requires the user to start the program by typing IRT at the DOS prompt and then enter the data elements described in the preceding section (Maximum Air Temperature, Maximum Canopy Temperature, Minimum Canopy Temperature, Daily Reference Et (Et o ), Noon Net Radiation, and Noon Relative Humidity). The IRT-Et c program can be operated optionally using either metric or English scale data, except that Net Radiation must be entered in watts/square meter as reported by CIMIS using the metric option.

After entering these data, the IRT-Et c program asks if you wish to have a printed copy of the output of the program. If you do, turn your system printer on and type "Y" followed by a carriage return. The program quickly performs its calculations and displays the results on the computer screen and optionally on the system printer.

Interpreting the Results Calculated by the IRT Et Program
Table 1 is a sample of the output from the IRT-Et c program. Information in each of the columns is described in terms of its content and use to the grower.

Explanation of the Printed Output
The first six printed lines of output are listings of the input data elements which the user of the IRT-Et c program typed in while running the program. These data are given back to the user for checking the accuracy of the data entries.

Table 1
Sample IRT-ET c Program Output
(Using metric data equivalents)

Air Temperature  30°C Max.
Canopy Temp. 32°C Min.
Canopy Temp. 28°C
Reference Et 9mm
Net Radiation  600 W/Sq.m

Relative Humidity 33%

(1) Canopy Temp °C : 32
(2) Crop Coefficient : 0.52450
(3) mm/Day : 4.7162
(4) Theoretical CWSI : 0.4760
Row 1: Canopy Temperature is the Maximum Canopy Temperature (except where corresponding Et c values would be less than 0.0)

Row 2: Crop Coefficient is the ratio between Et c and Reference Et (Et o ). A value of 1.00 means that plants in the field of interest transpired the maximum amount of water possible under the prevailing environmental conditions at the time measurements were taken. A value of less than 1.00 means that plants in the field of interest transpired less than what was possible given the environmental conditions at the time of measurement. A value of less than 1.00 therefore indicates a soil water deficit.

Row 3: Et c is the amount of water used by the plants in the field of interest on the day measurements were taken, measured in either inches or millimeters (depending upon the option selected when running the IRT-Et c program). Since the IRT-Et c method is used on a daily basis for each field, this Et c value should be recorded in a "field log" in which not only daily Et c entries can be made, but where cumulative water consumption can be calculated by the grower. The cumulative Et c determined by the grower on the basis of daily measurements is essential for determining the amount of water to be applied at the next irrigation. Knowing the cumulative Et c will permit the grower to avoid problems associated with under- and over-irrigation.

Row 4: CWSI (Crop Water Stress Index) is an indicator of when to irrigate the field of interest. When the CWSI value on a given day is between 0.2 and 0.7 (depending on the crop and the experience of the grower), the grower should irrigate in an amount appropriate given the cumulative Et c calculated by the grower on the basis of daily measurements. Crops which are sensitive to soil water stress should be irrigated when CWSI is between 0.2 and 0.5 while more drought-tolerant crops can be irrigated when CWSI is between 0.5 and 0.7.

Limitations of the IRT Et Method
The IRT-Et c method described in this guide is an experimental method which has been validated using selected field crop data in Central California. Although the IRT program is "self-calibrating" (that is, it doesn't require any information other than that specified above), it has not yet been tested under actual commercial field conditions nor has it been fully tested on a complete range of crops.

The IRT-Et c program is applicable only to mature plants with a well- developed canopy. It is not applicable to calculations of bare soil evaporation.

It should be noted that sharp climatic changes may cause low canopy temperatures even when soil water is limited. Alternatively, under such conditions, high canopy temperatures may be observed when soil water is not limiting.

Any comments concerning the operation and usefulness of the IRT Et program would be welcome and may be submitted to:

IRT Et Program
c/o Center for Irrigation Technology
California State University, Fresno
5370 North Chestnut Avenue
Fresno, California 93740-0018

About the Authors
The work described in this guide was carried out during 1985-86 at CATI in cooperation with the USDA Water Management Research Laboratory by Dr. Jiftah Ben-Asher, visiting scientist from the Jacob Blaustein Institute for Desert Research of Ben Gurion University and Dr. Claude J. Phene, soil scientist, USDA-ARS Water Management Research Laboratory. Further information about the IRT-Et c method can be obtained by writing to:

Dr. Ben-Asher
Ben Gurion University of the Negev
Sede Boqer Campus, 84990, Israel

Dr. Ben-Asher may also be reached via BITNET. His BITNET ID is DCAR 100 at BGUNUE. AC. IL
FAX # 972-57 555058