An Historic Overview of Penman-Monteith Equation and Important Applications in Water Management.

The Penman-Monteith (PM) equation has become a de-facto global standard for defining reference crop evapotranspiration, having been adopted by the UN Food and Agriculture Organization (FAO) in 1998 and by the American Society of Civil Engineers (ASCE) in 2005.  The success and adoption of the combination equation is due to the physical basis in simulating the surface energy balance.  Monteith’s addition of bulk surface resistance to Penman’s equation provided the proper ‘braking’ of the ET estimate under high wind and large vapor pressure gradients.

The adoption of the PM method by FAO ASCE as a reference method produced a standardized set of parameters for surface resistance, albedo and roughness length for both the traditional clipped, cool season grass reference and a second tall alfalfa reference that closely approximates a near maximum upper limit on ET that is bounded by energy availability and aerodynamic transfer.  The PM reference basis 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 ASCE PM method is used in the calibration of the METRIC satellite-based surface energy balance model to develop maps of ET over large regions, where Landsat imagery provides thermal information and sufficiently high resolution to identify ET from individual fields.

It is the well-watered condition, where air and surface temperature differences are small, that the PM combination equation adheres to assumptions made in its development. When weather data are collected over dry surfaces, the PM equation requires extra computations and considerations, including the iterative solution of surface temperature via energy balance to correctly estimate a) the slope of the saturation vapor pressure curve; b) buoyancy effects on aerodynamic resistance; c) increased emitted long wave radiation and d) increased soil heat flux density as compared to the 'reference surface'.  When this is done, the PM equation essentially decomposes back to its original energy and radiation balance components, and the PM equation, in essence, 'evaporates.'  Under those conditions, an ‘AFIB’ method (aerodynamic fluxes using an iterative energy balance) is a more useful and computationally more efficient approach than iterative solution with the PM method.