Evaluation of near-surface soil water status through the inversion of soil–canopy radiative transfer models in the reflective optical domain

Knowledge of the spatial and temporal variability of surface soil water content (SWC) is important to understand the linkage between hydrological, ecological and geological processes in a region. Optical Earth observation (EO) data offer the pos- sibility to retrieve surface soil water information, since an overall decrease of soil reflectance corresponds to increasing SWC. Sensitivity analysis of the combined leaf (PROSPECT) and canopy (SAILH) reflectance models (PROSAIL) to soil reflectance variations was carried out, and remote sensing and ground data from different experimental agricultural sites (ESA Spectra Barrax Campaigns (SPARC) 2004, ESA Airborne SAR and Optic Campaigns (AgriSAR) 2006 and participatory multi-level EO-assisted tools for irrigation water management and agricultural decision-support (PLEIADeS) 2007) were exploited. A simple look-up table (LUT) inversion technique was implemented to estimate canopy and soil variables. High negative relationships (r = − 0.87) between the soil reflectance factor of the model and the measured SWC were found for several crop types and different locations exhibiting a low fractional vegetation cover (fCover). Even though quantification of SWC is difficult, the method could be useful to obtain relative SWC information, especially before the start and at the beginning of the growing season. Furthermore, the physically based estimation approach offers the possibility of getting information about soil and canopy characteristics concurrently from optical EO data. The methodology presented in this article may also represent a suitable complement in the retrieval of SWC from active microwave.