Comparison of Different Approaches to Retrieve Plant Water Content of Summer Barley Canopies from Spectroradiometric Measurements

Abstract

Different empirical and physically based methods were employed to derive the vegetation water content of summer barley plots (n=22) from spectroradiometric measurements (ASD FieldSpec II). Data were acquired for two different phenological stages in May and June 2005. For the empirical approaches, a ratio index using the reflectances at 1355 and 710 nm and the partial least squares regression provided the best estimation results (r¡ sup¿ 2¡/sup¿ ¿ 0.90). Canopy radiative transfer modeling was performed by coupling the PROSPECT and SAIL models. The retrieved values for C¡ sub¿ w¡/sub¿ (equivalent leaf water thickness) × LAI were highly correlated (r¡ sup¿ 2¡/sup¿ = 0.86) with the measured canopy water contents, but showed distinct underestimates. For the vegetative phenological stage investigated in May, the PROSAIL results were very close to the measured water contents of the leaf fraction, but this was not valid for the data collected in June. Obviously, different phenological stages need specific model calibration, as the presence of undetectable water in non-leaf tissues is variable. All approaches were applied to synthetic HyMap data generated by resampling the spectroradiometer readings. Estimation results did not differ significantly; thus, by neglecting spatial scaling effects, the pure spectral information provided by both data sets is almost equivalent.