Retrieval of leaf water content of winter wheat from canopy spectral reflectance data using a position index (λmin) derived from the 1200 nm absorption band

Abstract

Accurate estimation of leaf water content (LWC) using remote sensing can assist in determining the vegetation physiological status and has important implications for drought monitoring and fire risk evaluation. This letter focuses on retrieving LWC from canopy spectra of winter wheat measured with an Analytical Spectral Device (ASD) FieldSpec Pro Spectroradiometer. The experimental plots were treated with five levels of irrigation (0, 200, 300, 400, 500 mm), and each treatment had three replications. Winter wheat canopy spectra and LWC were collected at three growth stages (booting, blooming and milking). The temporal variations of LWC and spectral reflectance were analysed. A wavelength position index (denoted by λmin) describing the general shape features of the 1200 nm water absorption band was extracted by fitting a quadratic line. The correlations between λmin and LWC at three different growth stages were analysed. It was found that when LWC increased or winter wheat approached maturation, λmin tended to shift to longer wavelengths. The physical base behind such phenomenon is that λmin relates to canopy water content (CWC), which was confirmed by the simulation results obtained with the PROSAIL model. Compared with two water indices (plant water index and normalized difference water index), λmin was a better indicator of LWC at booting and milking stages when soil exposure was relatively large, whereas it was not effective at blooming stage probably due to saturation problems.