Abstract:
Monthly water-balance models have been used as a means to examine the various components of the hydrologic cycle (for example, precipitation, evapotranspiration, and runoff). Such models have been used to estimate the global water balance (Mather, 1969; Legates and Mather, 1992; Legates and McCabe, 2005); to develop climate classifications (Thornthwaite, 1948); to estimate soil-moisture storage
... (Alley, 1984; Mintz and Serafini, 1992), runoff (Alley, 1984, 1985; Yates, 1996; Wolock and McCabe, 1999), and irrigation demand (McCabe and Wolock, 1992); and to evaluate the hydrologic effects of climate change (McCabe and Ayers, 1989; Yates, 1996; Strzepek and Yates, 1997; Wolock and McCabe, 1999).
This report provides a description of a monthly water-balance model that is driven by a graphical user interface (GUI) developed by the U.S. Geological Survey. The GUI permits the user to easily modify water-balance parameters and provide useful estimates of water-balance components for a specified location. The model, referred to as the Thornthwaite water-balance program, can be used as a research tool, an assessment tool, and as a tool for classroom instruction.
[Summary provided by the USGS.] 
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