J. Penuelas et al., ESTIMATION OF PLANT WATER CONCENTRATION BY THE REFLECTANCE WATER INDEX WI (R900 R970)/, International journal of remote sensing, 18(13), 1997, pp. 2869-2875
Water Index WI (R900/R970) was used for the estimation of plant water
concentration (PWC) by ground-based, reflectance measurements. Reflect
ance and PWC were measured for adult plants growing in the field throu
ghout an annual cycle and in potted seedlings submitted to progressive
desiccation. The species studied were characteristicly Mediterranean:
Pinus halepensis, Quercus ilex, Quercus coccifera, Arbutus unedo, Cis
tus albidus, Cistus monspeliensis, Phillyrea angustifolia, Pistacia le
ntiscus and Brachypodium retusum. WI was significantly correlated with
PWC when all the species were considered together, and with almost al
l the species considered individually, especially when a wider range o
f PWC was obtained by extreme dessication of experimental plants. The
correlations increased when normalizing WI by NDVI. The wavelength of
the trough corresponding to water absorption band tended to shift from
970-980 nm to lower wavelengths 930-950 nm with decreasing PWCs. Infr
ared measurement of plant temperature and T-leaf-T-air provided worse
assessment of PWC. A simple radiometer measuring plant reflectance at
680, 900, and 970 nm could speed up the measurement of PWC, and be use
ful in wildfire risk evaluation and drought assessment.