This study has demonstrated that for tea canopies, thematic mapper data (Landsat 5 short wave and thermal infrared bands) can be used to produce regional estimates of important surface parameters like albedo, surface temperature, net radiation and instantaneous evapotranspiration. The study has also shown that the TERGRA model can be successfully used to convert instantaneous evapotranspiration to daily evapotranspiration for tea canopies. At Kericho, the combination of remotely sensed data and TERGRA model produced point daily evapotranspiration estimates that differed from ground-based estimates (Penman and Bowen ratio energy balance methods) by up to 7%. For field #12, the range of evapotranspiration estimates (0.3 mm/day, by use of the Bowen ratio, Penman and remote sensing methods) was much smaller than the spatial variability of evapotranspiration throughout the study area (2-5 mm/day) as estimated by the remote sensing technique. When a large number of pixel values is used to estimate regional evapotranspiration, the standard error of the estimate approaches zero. Consequently, mean aerial evapotranspiration can be estimated to a better accuracy than would be possible with point based models. The study also showed that regression analysis (whereby evapotranspiration was predicted by use of remotely sensed canopy temperature) could be used to estimate regional evapotranspiration. Finally, for the specific situation at Kericho, advection was found to be virtually non-existent over mature tea canopies.

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