Remote sensing of plant stresses and its use in irrigation management

This paper outlines the ways in which thermal and spectral remote sensing can be used to monitor the development of water deficits in horticultural crops and to schedule irrigation. The first part outlines the basic principles involved in the use of remote sensing for monitoring plant responses to water deficits. Thermal sensing is shown to be particularly sensitive to stomatal closure, while spectral sensing can be used to study both the development of leaf area (using standard vegetation indices) and the responses of processes such as photosynthesis (using chlorophyll fluorescence and the photochemical reflectance index). Such remote sensing methods may be applied both within-field and from airborne or even satellite-mounted sensors. The specific opportunities and problems of applying this technology to a range of horticultural crops will then be addressed and recommendations made, particularly relating to its application to irrigation scheduling. The problems involved with scaling up remote sensing from the leaf scale to the canopy scale are discussed with particular emphasis on the large potential errors involved when inverting a remotely sensed index (whether CWSI or any other vegetation index) to estimate a biophysical parameter such as leaf area index, stomatal conductance or photosynthesis rate.