Modelling evapotranspiration from a barley field over the growing season

Abstract

This study tested a modified version of the two-layer evaporation energy combination scheme of Shuttleworth–Wallace (S–W). The first modification was the application of a two-layer soil module, which included the soil surface resistance for evaporation and soil moisture in the rooting layer controlling the canopy resistance. Another modification was the inclusion of the surface temperature in the calculation of the aerodynamic resistance. The surface temperature controls both the stability and the canopy resistance. The modelled total evapotranspiration was compared with evaporation calculated from the Bowen-ratio energy-balance technique over two growing seasons of a barley field, which provided a wide range of canopy densities and variation of driving factors with rainy, dry, warm and cold periods. The S–W scheme result on a daily and hourly basis was generally in good agreement with the measured evapotranspiration. The largest deviation between the measured and simulated evaporation occurred in a late phase of the growing season with a yellow non-transpiring canopy and, after harvesting, with stubble. The simulated surface temperature was mainly within ±2 K of direct measurements, both in bare soil and dense canopy situations. The study showed that the scheme is applicable for practical use, since the weather input parameters required are limited to those which are available as normal synoptic observations.