The Lasting Scientific Impact of the Thornthwaite Water-Balance Model

Warren Thornthwaite's 1948 article in the Geographical Review unveiled the rationale for a new climate classification system (as opposed to the widely used climate classification devised by Wladimir Koppen [1900]). The underlying principle of Thornthwaite's classification included analysis of the interaction between energy and moisture at the earth's surface, rather than analyzing temperature and precipitation as separate variables. Although Thornthwaite's classification system never achieved as much popularity as Koppen's classification, the article became a classic because of the methods implemented in the classification scheme. In particular, Thornthwaite's formulation of potential evapotranspiration (PET) as a component of the climate system was revolutionary, with applications well beyond the realm of climate classification. THORNTHWAITE'S INSPIRATION Koppen produced the most widely used climate classification system of the twentieth century. He introduced his initial classification in 1900 and continued to refine it for decades (Koppen 1900, 1923, 1936). Koppen took regions of common vegetation and then quantified the climate of those regions in terms of average temperature and precipitation. Thornthwaite and other American climatologists saw the shortcomings of the Koppen system, especially as applied to the United States. At the time--the 1930s--Thornthwaite and Koppen interacted professionally, and the two occasionally corresponded about climate classification issues. Russell Mather and Marie Sanderson even presented a translated letter from Koppen to Thornthwaite (1996). CLIMATE CLASSIFICATION Although Thornthwaite was not troubled by Koppen's use of vegetation boundaries, he took the approach that, "to achieve a rational quantitative classification of climate, definite and distinctive break points must be discovered in the climatic series themselves" (1948, 73). He further noted that distinctive breaks did not occur in precipitation, or in PET, where breaks in vegetation boundaries existed. Therefore, some new way of analyzing climatic data was needed to find the break points that coincide with changes in vegetation. Thornthwaite clearly believed that interpretations of evaporation and transpiration were the key to cracking the climate classification code. Part of his rationale for this thinking was because the same amount of annual rainfall results in lush forest in Scandinavia but desert vegetation in Africa (Mather and Sanderson 1996), raising the notion of the effectiveness of temperature and precipitation. Thornthwaite discussed these concepts in an even earlier article published in the Geographical Review (1931). There he noted that the effectiveness of temperature is how it translates to plant growth and that the effectiveness of precipitation is related to the amount of evaporation of that rainfall. As a result, a region cannot be designated as either wet or dry using precipitation alone. Utilizing these concepts, Thornthwaite laid out a way to determine PET based on empirical relationships of weather data (1948). PET represents the amount of water that would evapotranspirate if no shortage of soil moisture available in the rooting zone existed (Muller and Grymes 2005). In other words, it is the demand for moisture by the atmosphere, but that demand is not always met. To determine the PET values, all that is needed are temperature data and latitude (as a surrogate for day length) for the location or region. From this concept, Thornthwaite developed a water balance that estimated the timing of high and LOW PET, soil-moisture recharge, utilization of soil moisture, moisture deficits, surplus, and runoff (see Figure 1 for an example). [FIGURE 1 OMITTED] Using this approach, Thornthwaite devised his classification of moisture regions across the United States (Figure 2). The "0" line delineating positive from negative values represents the break from wet to dry climates according to Thornthwaite's moisture index. …