SIDEBAR 4.1 Potential Evapotranspiration
Until the 1940s, practical algorithms were unavailable for reliably evaluating the relative wetness or dryness of climates, relationships between precipitation and stream runoff, the amount of irrigation necessary to maximize crop yield, and a number of other hydroclimatic problems. What was missing was an easy-to-use, reliable, and yet physically realistic way of estimating the time-integrated atmospheric demand for land surface moisture. Surrogates for atmospheric moisture demand, such as pan evaporation and air temperature, were used out of necessity, but they were conceptually flawed and often produced highly biased estimates.
Working with colleagues at the Laboratory of Climatology in New Jersey during the 1930s and 1940s, C.W. Thornthwaite devised a relatively straightforward characterization of atmospheric moisture demand (termed "potential evapotranspiration," or E0) and a practical means of estimating it (Wilm et al., 1944). Thornthwaite's contribution to climatological understanding has endured not only because his E0 concept is physically grounded, but because relatively reliable estimates of E0 can easily be made from measurements (or estimates) of monthly air temperature (T) and day length (h).
Thornthwaite's E0 as well as his climatic water-budget algorithms (Thornthwaite and Mather, 1955) have found many varied uses, ranging from evaluating local hydroclimatic problems to assessing the geographic variability of evapotranspiration on regional, continental, and even global scales. His separation of E0, E, and β, and representation of E as E = E0 β—where E is the actual evapotranspiration and β is a dimensionless measure of land surface moisture conductance—in particular, suggests which environmental characteristics should be observed and estimated. Thornthwaite's conceptualization, in other words, has significantly advanced our understanding of how to sample the environment to assess the hydroclimate of a place or region.