ESTIMATING EVAPORATION FROM PASTURE USING INFRARED THERMOMETRY - EVALUATION OF A ONE-LAYER RESISTANCE MODEL

A one-layer resistance model has been used with infrared thermometry to estimate sensible and latent heat flux in pastures near Goulburn, New South Wales. The model compares reasonably well with energy balance-Bowen ratio measurements. However, the relative error in the evaporation estimates becomes significant in very dry conditions and at low net radiation values. An aerodynamic surface temperature may be computed from independent sensible heat flux measurements, air temperature measurements and estimates of aerodynamic resistance. The differences between computed and observed surface temperatures show characteristic diurnal trends and vary between days of measurement. These differences may be caused by errors in measuring the sensible heat flux and the surface temperature and in estimating the aerodynamic resistance. The pasture data obtained in this study are used to assess such uncertainties. The major cause for differences between the computed and observed surface temperatures is considered to be the incompleteness of the vegetative cover during each of the experimental periods. The measured Ts values apply to both the vegetation and the soil surface, whereas the observed T0 values are based on a model which assumes complete cover and only considers foliage temperature. Finally, the two-layer model of Shuttleworth and Wallace is used to show that the relationship between the measured infrared surface temperature and the canopy air temperature depends on the aerodynamic resistances, the fractional vegetated area and the temperature differences between foliage, canopy air and soil surface. © 1990.