Field-Scale Assessment of Multi-Sensor Soil Moisture Retrieval Under Grassland

Soil moisture under grassland is assessed at field scale using multiple sensing techniques: in situ soil moisture network measurements (SoilNet), rover-based cosmic ray neutron sensing (CRNS rover) and airborne polarimetric SAR acquisitions (PolSAR) at L-band. The three interdisciplinary techniques acquire on different spatial scales from meters to hectometers. In this study, the methods are blended at the field scale to estimate soil moisture under grassland in a synergistic as well as a stand-alone approach. Data from the TERENO Fendt test site near Weilheim (Germany) were recorded concurrently within the ScaleX campaign on 10th of July, 2015. The multisensor assessment reveals that PolSAR estimates benefit fundamentally from the in situ techniques to effectively remove the vegetation scattering component leading to very accurate permittivity estimates (RMSE < 1 [-]). The PolSAR analyses verified the full applicability of the low-parameterized vegetation scattering model to sufficiently represent grassland cover. Moreover, the comparison of all moisture products indicates the constraint of PolSAR to assess only the surface moisture at L-band, while the other two techniques are able to assess also soil moisture of deeper layers, reaching down to the root zone.