Alternative models for predicting the foliage—Air temperature difference of well irrigated wheat under variable meteorological conditions. I. Derivation of parameters II. Accuracy of predictions

Summary

One means of using infrared measurements of foliage temperature (T _f ) for scheduling irrigations requires the use of meteorological data to predict the foliage-air temperature difference for a comparable well-watered crop (T ^* _f − T _a ). To determine the best method for making this prediction, parameters for models of increasing complexity for predicting (T ^* _f − T _a ) were derived for wheat using two sets of field data collected in 1982 and 1983.

The simplest model with vapor pressure deficit (VPD) as the sole predictor accounted for 64% of observed variance in (T ^* _f − T _a ). The next model with both VPD and net radiation (R _n ) as predictors accounted for 74%. The most complex model predicted (T ^* _f − T _a ) from the crop energy balance. In addition to VPD and R _n it included parameters for the effects of air temperature (T _a ), aerodynamic resistance (r _a ) and the canopy resistance of a well-watered crop (r _cp ) and accounted for 70% of the variance.

Accuracy of these alternative models was tested against an independent set of field data collected in 1984. The single variable model with VPD as sole predictor accounted for 17% of the variance in observed values of (T ^* _f − T _a ). This increased to 47% when the effect of R _n was included by using the two variable model and was increased further to 65% when the additional variables of T _a , r _a and r _cp were included by use of the energy balance model. When the complexity of the model was measured by its number of variables there was a close relationship between complexity and the accuracy of the predictions. Reasons for the residual variability are discussed. The need for improved instrumentation for meteorological measurements was indicated.