Practical and Theoretical Benefits of an Alternative to the Penman‐Monteith Evapotranspiration Equation
Kaighin A. McColl
Abstract
Abstract The Penman‐Monteith equation is used widely to estimate evapotranspiration ( E ) and to understand its governing physics. I present an alternative to the Penman‐Monteith equation that has both practical and theoretical advantages, at no appreciable cost. In particular, the new equation requires no additional assumptions, empiricism, or computational cost compared with the Penman‐Monteith equation. Practically, the new equation is consistently more accurate over a wide range of conditions when compared with eddy covariance observations: The new equation has lower errors compared with Penman‐Monteith estimates of ET at all of the 79 eddy covariance sites available for the analysis. Using the new equation reduces errors, on average, by 67 % , from 8.55 to 2.81 [W m −2 ]. At night, the improvement is even greater (92% reduction in error; from 1.26 to 0.097 [W m −2 ]). This improvement is achieved without calibration. Theoretically, the new equation corrects a conceptual error in the Penman‐Monteith equation, in which the Penman‐Monteith equation incorrectly implies that E from a saturated surface into a saturated, turbulent atmosphere (“equilibrium” E ) is exactly equivalent to E from an unsaturated surface into an unsaturated, laminar atmosphere. The conceptual error is traced back to the failure of the Penman‐Monteith equation in important limiting cases; these errors are eliminated by the new equation. I use the new equation to revise an existing theory of land‐atmosphere coupling affected by the conceptual error in the Penman‐Monteith equation and to reassess several common but incorrect definitions of equilibrium E .