Radiation, heat and mass fluxes through the terrestrial surface are affected by the vegetation layer. Therefore, an accurate characterization of the vegetation above the land surface is particularly relevant in order to estimate the exchange of water between the land and the atmosphere and their feedbacks. In this context, the vegetation influence on the L-band radiometer signal was analyzed and new insight were gathered setting up and operating a controlled field laboratory to monitor surface soil moisture and vegetation properties.

Firstly, ground-based L-band radiometer measurements were performed at the Selhausen remote sensing field laboratory (Germany) over the growing season of a winter wheat as well as a mustard vegetation stand. L-band microwave observations were collected over two different footprints within the homogenous vegetation stands in order to disentangle the emissions originating from the soil and from the vegetation. Based on brightness temperature measurements, the vegetation optical depth (VOD) was retrieved using the τ-ω radiative transfer model. The VOD showed to be time, polarization and angle dependent and varied between the two investigated vegetation types (winter wheat & mustard). In addition, a clear relationship with in situ measurements of vegetation properties was shown and the soil water content (SWC) retrieval was greatly improved by accounting for the vegetation effect in comparison to in situ SWC measurements.

Secondly, we addressed the retrieval of vegetation water content from VOD. Knowledge of the vegetation water status and its spatio-temporal dynamics is essential to monitor changes in ecosystem health and to assess the vegetation component of the water budget. In this study, we developed and validated an approach to estimate the gravimetric vegetation water content (mg). mg is defined as the amount of water [kg] per wet biomass [kg]. The validation is based on the comparison between mg from L-band radiometer observations and from in situ measurements. The mg estimates showed a good agreement with in situ measurements. This demonstrates the potential of L-band radiometer measurements for plant water status monitoring.

Overall, the results highlight the potential of passive microwave remote sensing for the characterization of soil surface and vegetation canopy properties. In addition, field-scale radiometer measurements at L-band shows great potential for improving spaceborne microwave products and for increasing the spatial coverage of ground-based soil moisture sensing beyond point-based networks.