Predicting vegetation water content in wheat using normalized difference water indices derived from ground measurements.

Vegetation water content (VWC) is an important variable for both agriculture and forest fire management. Remote sensing technology offers an instantaneous and non-destructive method for VWC assessment provided we can relate in situ measurements of VWC to spectral reflectance in a reliable way. In this paper, based on radiative transfer models, three new normalized difference water indices (NDWI) are proposed for VWC [fuel moisture content (FMC), and equivalent water thickness (EWT)] estimation, taking both leaf internal structure and dry matter content into account. Reflectance at 1,200, 1,450 and 1,940 nm were selected and normalized with reflectance at 860 nm to establish three water indices, NDWI(1200), NDWI(1450) and NDWI(1940). Good correlations were observed between FMC (R (2) = 0.65-0.80) and EWT (both at the leaf scale, R (2) = 0.75-0.81 for EWT(L) and at the canopy scale, R (2) = 0.80-0.83 for EWT(C)) at various stages of wheat crop development.