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An extract from Using a Programmable Calculator for Determining Potential Evapotranspiration Rates, Journal of Environmental Horticulture (HRI-ANLA) 7(3):118-121, University of Florida, Fort Lauderdale Research & Education Center, SEP 1989

  A program is presented allowing the computation of uncorrected potential evapotranspiration (ETp) rates using a low-cost (<$50) hand-held programmable calculator. This program requires the user to supply average local or on-site monthly temperatures. The first part of the program computes the annual heat index and the exponent necessary for ETp determination. The second part of the program computes monthly ETp rates by the Thornthwaite procedure as each average monthly temperature is keyed in. Comparisons of computed ETp rates with measured actual evapotranspiration (ETa) rates of various container-grown nursery crops show good correlation.

  The Thornthwaite procedure is easily used, but most growers are unfamiliar with the algebraic procedures and necessary calculations. Computer programs have been developed that make the calculations automatically, but many growers may not have access to the computer hardware necessary to run the programs. The purpose of this project was to develop a simple program to compute ETp by the Thornthwaite method that could be used on a relatively inexpensive (<$50), battery-operated, hand-held programmable calculator.

The Program
  The program involves the transcription of a computer program designed for stationary microcomputer systems. It was developed for use on a Hewlett-Packard HP-11C programmable calculator, but with minor transcriptional changes would work on other programmable calculators as well. The program is executed in 2 stages. The first stage requires the user to key in monthly average temperatures for a 12 month period … The first stage of the program computes the annual heat index and the exponent necessary for ETp calculation. The second stage of the program requires the user to again key in monthly average temperatures, but after each monthly temperature is keyed in, the monthly ETp rate appears on the display and remains on the display until the next monthly average temperature is entered or the calculator is turned off. This program is listed in 2 versions, one that uses English system terminology and the other for the metric system … The English system program has 98 lines and the metric system program has 72 lines. The annual heat index and exponent are computed in the metric system in both versions, however.

Here my versión for the HP67 calculator, with some small improvements (also calculates ETp [mm/year] and clear registers in the biginning).
In this PDF some constants are code compressed; instructions and example included.

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