Noninvasive Water Stress Assessment Methods in Orchards

Abstract

In large orchards up-to-date information is needed on water capacity and possible water stress of the fruit trees because the physiological reactions against the changing water capacity appear much earlier than the water stress. These physiological changes cannot be observed visually in the early period of water stress. One of the most sensitive indicator of photosynthetic activity, mutations, stress the chlorophyll content of the vegetation, which can be monitored by photometry. Thus, combining airborne remote-sensing technology and reflectance measurements of leaves can be the solution for detection of changing water content. With the use of remote-sensing technology, a large amount of agroecologic information can be achieved on sites such as orchards. The examined intensive apple and the extensive pear orchards, established on sandy soils, are situated in the northeastern part in Hungary. The apple orchard has a microirrigation system. Hyperspectral image was taken by AISA Dual airborne hyperspectral camera. The full spectra range of this system is 400–2450 nm, which was set to the maximum 498 spectral channels at the 2.25-m² spatial resolution. Parallel to the flight campaign, leaf samples were also taken. Samples of different kind of apple species were analyzed by ALTA II and AvaSpec 2048 hand portable spectrometers. Based on the hyperspectral image, vegetation indices were also calculated to detect the effect of water stress. The normalized difference vegetation index (NDVI), which is sensitive to changes in biomass, and the water band index (WBI), which is sensitive to changes in canopy water status, were calculated to measure the water supply of the leaves. As a result, the reflectance of 886 and 937 nm can result more accurate WBI (WBI = δ₈₈₆/δ₉₃₇) in the case of pear trees. The water-stressed samples showed a greater reflectance in near-infrared spectrum and a lower reflectance in the red spectrum.

Keywords:

• Airborne hyperspectral analisys
• laboratory reflectance measurements
• leaf reflectancy

Acknowledgment

This research was funded by OM-00265/2008 and OM-00270/2008 projects.