How John Monteith's Formulation of the Penman-Monteith Equation Helped to Standardize the World of Reference Evapotranspiration.

The Penman-Monteith (PM) equation is now a global standard equation for defining reference crop evapotranspiration, having been adopted by the United Nations Food and Agriculture Organization (FAO) in 1998.  The name is known and used in every country on the globe. The success of the PM equation and its adoption by FAO is because the physically-based PM equation lends itself very well to simulating the densely vegetated, well-watered surface represented by the reference condition.  In fact it is the well-watered condition, where air and surface temperature differences are small, that the combination equation adheres to the assumptions made in its development.  Monteith’s addition of bulk surface resistance to Penman’s combination equation provided the proper ‘braking’ of the ET estimate under conditions of high wind and large vapor pressure gradients, and causes the equation to match measured data over a wide range of conditions.

 In the adoption of the PM method by the FAO, a standardized set of parameters was developed, where specific values for surface resistance, albedo and roughness length were specified.  These same parameters have also been adopted by the American Society of Civil Engineers (ASCE) for the clipped cool season grass reference of ASCE.  In addition, a second, tall alfalfa reference was formulated and standardized by ASCE to create a reference condition that closely approximates a near maximum upper limit that is bounded by energy availability and aerodynamic transfer.  The FAO and ASCE standardizations on reference crop ET (ETref) calculation have become important milestones in operational and consistent estimation of ET and have reduced confusion over geographically varying reference methods and have reduced time and resource investments in measuring and establishing locally or nationally derived reference methods.  The reference basis represented by the PM method has propelled the use of the ‘two-step’ crop coefficient x reference crop ET approach to estimate ET, where the reference ET estimate incorporates most impacts of weather on ET and the crop coefficient (Kc) incorporates bulked impacts of crop type, phenology, physiology and architecture on ET.  The Kc x ETref approach lends itself to visual review and assessment of the Kc curve, which is useful for relatively rapid identification of computational error or impact of faulty measurement data or weather data used in the derivation.  This is particularly appealing for less-experienced users and promotes transfer of Kc information across wide geographical locations.

The FAO and ASCE standardizations have proposed means to approximate missing weather data, for example where only air temperature is measured so that the physically based PM method can be applied over a broad range of locations and time periods.  When weather data have been collected over dry surfaces having low ET, air temperature can be elevated and vapor pressure reduced, causing the PM method to overstate the true value for reference ET.  In those situations, a 'conditioning' procedure should be applied where scalar profiles are altered by simulating effects of changing surface characteristics and fluxes on the profile shapes via blending height theory.   

Challenges in applying the PM equation include its application to estimate ET from dry surfaces that have temperatures well above those of the air temperature at reference height.  Under those conditions, the proper application of the PM equation requires the iterative solution of surface temperature via the energy balance so that the PM equation essentially decomposes back to its original energy and radiation balance components, with the  PM equation, in essence, 'evaporating.'  Under those conditions, the ‘AFIB’ method (aerodynamic fluxes using an iterative energy balance) is probably a more useful and computationally more efficient approach than iterative solution with the PM method.

The PM method as applied with the ASCE standardization for the tall reference crop condition, is now used as the calibration basis for the METRIC satellite-based surface energy balance model.  The METRIC model and similar models are becoming widely used in the USA to develop maps of ET over large regions, especially for determination of ET from agricultural fields, where Landsat imagery provide the thermal information and sufficiently high resolution to identify ET from individual fields.  John Monteith’s thoughtful developments more than forty-five years ago continue to grow in breadth and depth of application.