Editing Potential evapotranspiration (section)

=== Thornthwaite equation (1948) ===
<math>PET = 16 \left(\frac{L}{12}\right) \left(\frac{N}{30}\right) \left(\frac{10T_d}{I}\right)^{\alpha} </math>

Where

<math>PET</math> is the estimated potential evapotranspiration (mm/month)

<math>T_d</math> is the average daily temperature (degrees Celsius; if this is negative, use <math>0</math>) of the month being calculated

<math>N</math> is the number of days in the month being calculated

<math>L</math> is the average day length (hours) of the month being calculated

<math>\alpha = (6.75 \times 10^{-7}) I^3 - (7.71 \times 10^{-5}) I^2 + (1.792 \times 10^{-2}) I + 0.49239</math>

<math>I = \sum_{i=1}^{12} \left(\frac{T_{m_{i}}}{5}\right)^{1.514}</math> is a [[heat index]] which depends on the 12 monthly mean temperatures <math>T_{m_{i}}</math>.<ref name = thornthwaite48>{{Cite journal
  | doi = 10.2307/210739
  | last = Thornthwaite
  | first = C. W.
  | title = An approach toward a rational classification of climate
  | journal = Geographical Review
  | volume = 38
  | issue = 1
  | pages = 55–94
  | year = 1948
  | jstor = 210739
  | bibcode = <!-- do not add one, unless you manually verify it is exactly the same.  Citation bot finds one for a reference to this -->
 }}</ref>

Somewhat modified forms of this equation appear in later publications (1955 and 1957) by [[C. W. Thornthwaite]] and Mather.<ref name=black07>{{Cite journal
 |last        = Black
 |first       = Peter E.
 |title       = Revisiting the Thornthwaite and Mather water balance
 |journal     = Journal of the American Water Resources Association
 |volume      = 43
 |issue       = 6
 |pages       = 1604–1605
 |year        = 2007
 |doi         = 10.1111/j.1752-1688.2007.00132.x
 |bibcode     = 2007JAWRA..43.1604B
 }}</ref>