Developing a method for integrating canopy measurements into evapotranspiration predictions

Abstract

The availability of affordable instruments that measure surface radiative temperature and canopy cover have made it possible, in principle, to extend and improve existing methods of estimating potential evapotranspiration (ET) to predictions where assumptions of ‘full cover’ and ‘well-watered’ are violated. The aim of this study was to develop a model for ET whose input data requirements were not large, and would make use of surface temperature and cover measurements to predict ET in both potential and sub-potential conditions. A randomised experiment was conducted with two cultivars of barley (‘Dash’ and ‘Omaka’, erect and prostrate leaved, respectively) in combination with three irrigation treatments (High, Mid and Low) in a rain-out shelter. The treatments produced a wide range of surface temperature, cover and ET. Water fluxes in the soil were measured by methods sensitive enough to detect changes on the timescale of hours. Meteorological variables, short wave radiation albedo, ground heat flux density and surface temperature were logged on a sub-hour timescale. The Priestley and Taylor potential ET equation is modified and additional equations are derived to allow the inclusion of a crop coefficient to account for incomplete cover and to vary the alpha coefficient based on surface temperature to account for soil drying. Equations were also derived for estimating short-wave radiation, albedo and soil heat flux from canopy cover and meteorological variables. The resulting equations were able to quantify a wide range of ET values measured by detailed water balance in barley crops that often had incomplete canopy cover and encountered differing degrees of water stress. We conclude the approach presented has promise but requires broader testing. The successful integration of canopy measurements with an ET model will enable the production of ET maps which will have broad applications in research and irrigation management applications.