Direct Penman-Monteith and Aerodynamic Energy Balance Equations

The main advantage of the Penman-Monteith (PM) equation and other combination equations over direct aerodynamic approaches is that air temperature and humidity measurements are required at only one reference height. However, specifying these parameters over time can be challenging. Many land surfaces have vegetation canopies that do not completely cover the ground surface or understory, including forests, cultivated row crops, orchards, vineyards, and natural systems. The chapter describes three resistance-based methods for estimating evapotranspiration (ET) from sparse canopies: the standard PM method, the multilayer method, and the two-source method. Aerodynamic roughness length and displacement of the zero plane of the wind profile are common parameters of many physically based models. Finally, the chapter summarizes several example applications of the single-layer PM method for estimating ET directly, without the use of crop coefficients.