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The Science of
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Thornthwaite and Mather
Water Balance
Affordable Software for classroom demonstration and lab exercises, and for practicing professionals

Easy-to-Use Analysis Software

Two fully interactive versions of the water budget that estimate monthly potential and actual evapotranspiration, directly from C. W. Thornthwaite and J. R. Mather, 1957. The Water Balance, Publications in Climatology, Vol. X, No. 3, Drexel Institute of Technology, Centerton, NJ.

This is easy-to-use software that calculates potential and predicted evapotranspiration (and runoff) based on mean monthly temperature, precipitation, and latitude works in any DOS environment (DOS-Prompt in WINDOWS 98/2000). The program's algorithms are created from and conform to Thornthwaite and Mather's original tables as revised in 1957.

Full reference is in the accompanying brochure, which also contains theory, operations, limitations, demonstration data, and sample output. The program is supplied on a 3-1/2” floppy disk or, if specified (by email) on a CD.

Each version:

  • Requires mean or actual monthly temperature, precipitation and, if desired, runoff;
  • Provides screen or print options;
  • Produces mean annual tabular water budget organized by months, climatogram, and comparison of predicted and actual runoff, if entered;
  • Functions in a DOS or WINDOWS 9x environment, and;
  • Comes mailed postage paid on a 3-1/2" disk (or CD) with a four page explanatory brochure.
  • "Revisiting the Thornthwaite and Mather Water Balance." The Thornthwaite and Mather method of calculating the monthly pattern of annual the water balance has been criticized, but is also misunderstood. Download a free copy of a recent paper published in the Journal of the American Water Resources Association to set the record straight. Click here

    LATE NOTES: Both editions now operate in WINDOWS XP Professional (as well as WINDOWS 97 or 98). You can also re-start your computer in DOS MODE, entering the subdirectory where the software is located and entering the word SOLVE.

  • It is easiest to right click on the SOLVE.EXE file and send an icon to the desktop. Then, whenever you wish to start the software, click on the icon. Remember to exit the SOLVE software with the F4 key and a "Y" (as instructed).

  • The software also operates in any DOS environment (on 286, 386, or 486 PCs) or, in WINDOWS 9x, where you can use PRINT SCREEN or screen capture software to store the printed output as a file that can then be pasted into a WORD document, for example, which also enables making use of the software's color graphics).

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School Version: $30
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This version was designed for ninth grade earth science classes. It features monthly tabular and graphed outputs of the annual water budget, and graphic comparison of measured and predicted runoff. Output may be viewed on the screen and/or printed. This version will be especially useful for ninth-grade earth science, general or natural science, biology, geography, or for meteorology or climatology teachers.


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Professional Version: $100
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This version is the same as the school version, but includes the opportunity to perform sensitivity analyses by making multiple runs once the input data are supplied. Successive “what-if?” runs may be made with (1) different soil storage capacity, (2) latitude (effecting a "translocation" of a sloping watershed at some aspect to an equivalent latitude), (3) elevation (reflecting varying snowmelt runoff models), and (4) significant hydrological watershed characteristics, land use considerations, and unusual hydrological factors. This will be of particular value to those working on water balance research projects as well as consultants in wetland creation and management, and sanitary landfill calculations.

© Peter E. Black Updated May 16, 2008