Bruce Wickham
Estimating Evapotranspiration from a Vegetated Surface Based on the Three-Temperatures Method
Introduction
The Three-Temperatures (3-T) method is based on the simplified surface energy balance principle, which describes the net result of exchanges between energy flux densities through radiation, convection, and conduction across the soil-plant-atmosphere interface in the vertical profile. The 3-T method estimates the latent heat energy flux density component, which is regarded as the energy available to drive the evapotranspiration (ET) process, by quantifying five key parameters and utilising an imitation surface (e.g. artificial vegetation that does not transpire), as outlined in the equation below:
Aims
One of the aims of this project is to demonstrate the successful application of the 3-T method for small vegetated surfaces (e.g. < 5 m²) to produce continuous records of ET estimates at sub-daily time steps and potentially derive a set of crop coefficients. Furthermore, the project seeks to gain a better understanding of the method’s assumptions, limitations, and performance in estimating ET from vegetated surfaces in the field (e.g. Green Infrastructure and Sustainable Drainage Systems, located in urban environments).
Results so far
A preliminary study of the 3-T method has been established, collecting data at one-minute time steps for a vegetated system located on a roof at Sheffield University, in the city centre of Sheffield, as shown in Figure 1. Figure 2 shows the results for 10 different approaches to applying the 3-T method (based on differences between using constant and variable albedo and emissivity values, and accounting for soil heat flux density values) and the corresponding reference ET (ETo), calculated by the Penman-Monteith method (FAO-56). Early analysis of the results shows promise for the 3-T method to produce continuous ET estimates during the daytime.
