lasting impact of the thornthwaite model

The Geographical Review (): –, July Copyright © by the American Geographical Society of New York

� Dr. Keim is the Louisiana state climatologist and a professor of geography at Louisiana StateUniversity, Baton Rouge, Louisiana .

THE LASTING SCIENTIFIC IMPACT OF THE THORNTHWAITEWATER–BALANCE MODEL

BARRY D. KEIM

Warren Thornthwaite’s article in the Geographical Review unveiled the ra-tionale for a new climate classification system (as opposed to the widely used cli-mate classification devised by Wladimir Köppen []). The underlying principleof Thornthwaite’s classification included analysis of the interaction between energyand moisture at the earth’s surface, rather than analyzing temperature and precipi-tation as separate variables. Although Thornthwaite’s classification system neverachieved as much popularity as Köppen’s classification, the article became a classicbecause of the methods implemented in the classification scheme. In particular,Thornthwaite’s formulation of potential evapotranspiration (pet) as a componentof the climate system was revolutionary, with applications well beyond the realm ofclimate classification.

Thornthwaite’s Inspiration

Köppen produced the most widely used climate classification system of the twenti-eth century. He introduced his initial classification in and continued to refineit for decades (Köppen , , ). Köppen took regions of common vegeta-tion and then quantified the climate of those regions in terms of average tempera-ture and precipitation. Thornthwaite and other American climatologists saw theshortcomings of the Köppen system, especially as applied to the United States. Atthe timethe sThornthwaite and Köppen interacted professionally, and thetwo occasionally corresponded about climate classification issues. Russell Matherand Marie Sanderson even presented a translated letter from Köppen to Thorn-thwaite ().

Climate Classification

Although Thornthwaite was not troubled by Köppen’s use of vegetation bound-aries, he took the approach that, “to achieve a rational quantitative classification ofclimate, definite and distinctive break points must be discovered in the climaticseries themselves” (, ). He further noted that distinctive breaks did not occurin precipitation, or in pet, where breaks in vegetation boundaries existed. There-fore, some new way of analyzing climatic data was needed to find the break pointsthat coincide with changes in vegetation.

Thornthwaite clearly believed that interpretations of evaporation and transpi-ration were the key to cracking the climate classification code. Part of his rationalefor this thinking was because the same amount of annual rainfall results in lushforest in Scandinavia but desert vegetation in Africa (Mather and Sanderson ),

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raising the notion of the effectiveness of temperature and precipitation. Thornthwaitediscussed these concepts in an even earlier article published in the GeographicalReview (). There he noted that the effectiveness of temperature is how it trans-lates to plant growth and that the effectiveness of precipitation is related to theamount of evaporation of that rainfall. As a result, a region cannot be designated aseither wet or dry using precipitation alone. Utilizing these concepts, Thornthwaitelaid out a way to determine pet based on empirical relationships of weather data(). PET represents the amount of water that would evapotranspirate if no short-age of soil moisture available in the rooting zone existed (Muller and Grymes ).In other words, it is the demand for moisture by the atmosphere, but that demandis not always met. To determine the pet values, all that is needed are temperaturedata and latitude (as a surrogate for day length) for the location or region. Fromthis concept, Thornthwaite developed a water balance that estimated the timing of

F. Average water balance for Wilmington, Delaware. Source: Adapted from Mather .(Diagram by Clifford Duplechin Jr., Louisiana State University)

lasting impact of the thornthwaite model

high and low pet, soil-moisture recharge, utilization of soil moisture, moisturedeficits, surplus, and runoff (see Figure for an example).

Using this approach, Thornthwaite devised his classification of moisture re-gions across the United States (Figure ). The “0” line delineating positive fromnegative values represents the break from wet to dry climates according to Thorn-thwaite’s moisture index. To the east of the line, values are positive, indicating a wetclimate where precipitation exceeds pet in an average year. To the west of this line,values are mostly negative, indicating a dry climate where pet exceeds precipita-tion in an average year. Straddling this boundary is Thornthwaite’s subhumid cli-mate type (C climate), which coincides with the grasslands of the American GreatPlains (Muller and Oberlander ). This region does not emerge in the Köppenclassification, demonstrating the utility of Thornthwaite’s system to find breaks inclimatological and hydrological components that coincide with breaks in vegeta-tion regions. Although interesting geographical patterns emerged that may repre-sent a more rational approach to climate classification, users found Thornthwaite’sclassification system cumbersome, and it is rarely used today.

Water–Balance Modeling

Despite the unpopularity of the Thornthwaite climate classification, the underlyingmodel used to derive the climate regions is very much in use today, more than sixty

F. The Thornthwaite climate classification for the United States. Source: Adapted from Mullerand Oberlander . (Cartography by Clifford Duplechin Jr., Louisiana State University)

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years after its introduction. After his seminal article in , Thornthwaite himselfpublished extensively on the virtues of the water balance model and its applications(for example, Thornthwaite a, b, , a, b; Thornthwaite and Mather, , ). Among the many researchers who have utilized the Thornthwaitemodel are:

• Robert Muller, who investigated impacts of land use on water surplus andrunoff in the northeastern United States (, );

• Marie Sanderson, who used the model to interpret the hydrology of the LakeErie Basin ();

• Russell Mather, who wrote on further applications of the water balance model(, );

• Marlyn Shelton, who addressed the impacts of land use on runoff in Oregon’sDeschutes Basin ();

• Peter Gleick, who strongly advocated using the water balance model to assesspotential impacts of global climate change (a, b);

• Stewart Cohen, who used the model to estimate impacts of global climatechange on water supply in the Great Lakes Basin ();

• Robert Muller, myself, and Judy Hoff, who assessed flooding regimes in Loui-siana ();

• Myself, Gregory Faiers, Robert Muller, John Grymes, and Robert Rohli, whoinvestigated trends in rainfall and runoff in Louisiana ().

• Johannes Feddema, who examined the impacts of global warming and de-sertification on Africa’s water resources (); and

• Andrew Grundstein, who analyzed changing climate across the United Statesover the past century using components of the water balance model ().

Furthermore, even though a plethora of new models of pet have been developedsince Thornthwaite’s work in the s and s, Thornthwaite’s pet estimates arestill accepted in the scientific literature and often rival more sophisticated methodsin accuracy (for example, Khandelwal and Pandey ).

Impact

A recent search for citations of Thornthwaite’s article using the Google Scholarsearch engine resulted in more than , hits, many of them since . Few pub-lished articles illicit such an impact, and for such a protracted period of timewithno end in sight for Thornthwaite. His water balance model was also utilized in an-other famous publication, Meteorological Drought (), in which Wayne Palmerintroduced his methods to define drought using Thornthwaite’s water balance com-ponents. The Palmer Drought Severity Index (pdsi) and Palmer HydrologicalDrought Index (phdi) are still routinely calculated and utilized today by the Na-tional Oceanographic and Atmospheric Administration’s National Drought Miti-gation Center and Climate Prediction Center to assess drought across the UnitedStates and beyond. Numerous research articles utilize the pdsi and the phdi for

lasting impact of the thornthwaite model

research purposes; see, for example, Aiguo Dai, Kevin Trenberth, and Taotao Qian(), who address drought, agricultural issues, and climate change. Dai, Trenberth,and Qian attribute development of the model to Thornthwaite, but many research-ers using these indices are probably unaware of the underlying Thornthwaite waterbalance model that produces the index. As a result, the number of publicationsutilizing Thornthwaite’s methods would be difficult to quantify, but the direct num-ber is already large, and it would be even larger for those that were indirectly influ-enced by his work. For example, Palmer’s research paper was cited more than, times, according Google Scholar.

Thornthwaite’s article was revolutionary in many ways. It inadvertentlyassisted in advancing the field of geographical climatology from merely being de-scriptive and regional. Ironically, that is the essence of climate classification. How-ever, it was the underlying water balance model needed to produce the rationalclimate classification that led to the advancements. Thornthwaite complained thatcontemporary climatology of the daypresumably the s, s, and swasnothing more than statistical analysis of meteorological observations and that, as aresult, some scientists regarded climatology as “statistical meteorology,” mostlydevoid of scientific merit (, ). Thornthwaite’s work helped change the direc-tion of climatological research to applied, with merit. His work also changed per-ceptions of climatology from outside the field, for his publications touched numerousdisciplines, among them agriculture, hydrology, and climatic change.

References

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–: Relationship with Soil Moisture and Effects of Surface Warming. Journal of Hydrom-eteorology (): –.

Feddema, J. J. . Future African Water Resources: Interaction between Soil Degradation and Glo-bal Warming. Climatic Change (): –.

Gleick, P. H. a. Methods for Evaluating the Regional Hydrologic Impacts of Global ClimateChanges. Journal of Hydrology (–): –.

———. b. Regional Hydrologic Consequences of Increases in Atmospheric CO and Other Trace

Gases. Climatic Change (): –.Grundstein, A. . Evaluation of Climate Change over the Continental United States using a Mois-

ture Index. Climatic Change (–): –.Keim, B. D., G. E. Faiers, R. A. Muller, J. M. Grymes III, and R. V. Rohli. . Long-Term Trends of

Precipitation and Runoff in Louisiana, U.S.A. International Journal of Climatology (): –.Khandelwal, M. K., and V. Pandey. . Comparison of PET Computed by Various Methods in

Different Agroclimatic Stations of Gujarat State. Journal of Agrometeorology : –.Köppen, W. . Versuch einer Klassifikation der Klimate, vorzugsweise nach ihren Beziehungen

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———. . Water Balance Evaluation of the Effects of Subdivisions on Water Yield in MiddlesexCounty, New Jersey. Association of American Geographers Proceedings : A–.

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———. a. Climate and Soil Moisture in the Tropics. Geographical Review (): –.———. b. Recent Climate Studies in Australia. Geographical Review (): –.———. . The Water Balance in Tropical Climates. Bulletin of the American Meteorological Society

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thwaite Associates.Thornthwaite, C. W., and J. R. Mather. . Climate in Relation to Crops. Meteorological Monographs

(): –.———. . The Water Balance. Publications in Climatology (). Centerton, N.J.: Laboratory of

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