Vegetation Optical Depth and Soil Moisture Retrieval Using L-Band Radiometry Over the Entire Growing Season of a Winter Wheat Stand
This paper describes the first results of a field experiment conducted at the Selhausen remote sensing field laboratory (Germany) over the entire growing season of a winter wheat. Brightness temperature measurements were performed with the passive microwave L-band radiometer ELBARA-II. The data were collected above two different footprints within a homogeneous winter wheat stand in order to disentangle between the radiation originating from the soil and the radiation originating from the vegetation. In a first step, the brightness temperature $(T_{\mathrm{B}})$ data collected above the first plot with the soil surface covered by a metal grid (i.e., blocking the radiations from the ground) were used to retrieve the vegetation optical depth ( $\tau$ ). In a second step, $T_{\mathrm{B}}$ data collected above the second plot without the metal grid on the soil surface were used to retrieve the soil surface moisture using several inverse modeling approaches. All modeling investigations were performed with the simple zero-order $\tau-\omega$ model. The results show that using $\tau$ data derived from the first plot as a priori information, which showed to be time, polarization, and angle dependent, allows us to improve the soil moisture retrieval, due to a better representation of the vegetation canopy effect on the measured $T_{\mathrm{B}}$ . Furthermore, the correlations between $\tau$ and different vegetation indices were analyzed and highlight the potential of $\tau$ in terms of vegetation monitoring for, e.g., all-weather measurements compared to optical measurements.