Keim, Barry D. 2010. The Lasting Scientific Impact of the Thornthwaite Water-Balance Model. The Geographical Review 100 (3/July): 295-300.


Barry D. Keim in his article “The Lasting Scientific Impact of the Thornthwaite Water-Balance Model” admires the new climate classification system made by Warren Thornthwaite in 1948 and writes climate is “an analysis of the interaction between energy and moisture at the earth’s surface, rather than analyzing temperature and precipitation as separate variables” (p. 295). Further, he admires Thornthwaite’s formulation of potential evapotranspiration (PET) as a component of the climate system to be revolutionary and justifies why Thornthwaite’s classification was more scientific than Köppen’s classical classification.

Keim claims that Thornthwaite’s water-balance model was to “achieve a rational quantitative classification of climate, definite and distinctive breakpoints” (p. 295) and clarifies that the precipitation alone is insufficient to classify a region as wet or dry. On that note, Thornthwaite purposed PET based on empirical relationships of weather data that represents the amount of water that would evapo-transpirate, and, hence, he developed the water-balance model. Though the model was unpopular, it was widely used by the geographers after sixty years of its introduction for various purposes such as land use on water surplus and runoff, hydrology, impacts of global climate change, drought, agricultural issues, etc. (p. 298). His model is still accepted in scientific literature because of its accuracy.

Keim admits that Thornthwaite’s water-based model was revolutionary, and was accepted by almost all the researchers. His article was cited more than 2,700 times (p. 298).  Thornthwaite’s work not only changed perceptions of climatology but also helped change the direction of climatological research so that it could be applied with merit. For Keim, though Thornthwaite’s revolutionary work remained uncovered for decades, it added new dimensions in the field of agriculture, hydrology, and climatic change.

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