Passive L-Band Microwave Remote Sensing of Organic Soil Surface Layers: A Tower-Based Experiment

Abstract

Organic soils play a key role in global warming because they store large amount of soil carbon which might be degraded with changing soil temperatures or soil water contents. There is thus a strong need to monitor these soils and, in particular, their hydrological characteristics using, for instance, space-borne L-band brightness temperature observations. However, there are still open issues with respect to soil moisture retrieval techniques over organic soils. In view of this, organic soil blocks with their vegetation cover were collected from a heathland in the Skjern River catchment in western Denmark and then transported to a remote sensing field laboratory in Germany where their structure was reconstituted. The controlled conditions at this field laboratory made it possible to perform tower-based L-band radiometer measurements of the soils over a period of two months. Brightness temperature data were inverted using a radiative transfer (RT) model for estimating the time variations in the soil dielectric permittivity and the vegetation optical depth. In addition, the effective vegetation scattering albedo parameter of the RT model was retrieved based on a two-step inversion approach. The remote estimations of the dielectric permittivity were compared to in situ measurements. The results indicated that the radiometer-derived dielectric permittivities were significantly correlated with the in situ measurements, but their values were systematically lower compared to the in situ ones. This could be explained by the difference between the operating frequency of the L-band radiometer (1.4 GHz) and that of the in situ sensors (70 MHz). The effective vegetation scattering albedo parameter was found to be polarization dependent. While the scattering effect within the vegetation could be neglected at horizontal polarization, it was found to be important at vertical polarization. The vegetation optical depth estimated values over time oscillated between 0.10 and 0.19 with a mean value of 0.13. This study provides further insights into the characterization of the L-band brightness temperature signatures of organic soil surface layers and, in particular, into the parametrization of the RT model for these specific soils. Therefore, the results of this study are expected to improve the performance of space-borne remote sensing soil moisture products over areas dominated by organic soils.

Vegetation Optical Depth and Soil Moisture Retrieved from L-Band Radiometry over the Growth Cycle of a Winter Wheat

“…The stronger anisotropy observed in this study for the wheat vegetation was also shown by the retrieved tt V parameter with a mean value of 3.82 in comparison to 2.54 for the mustard (see Figure 7d,h). Additionally, several studies have shown (e.g., [9,11,14,45]) that neglecting the polarization dependency of the ω parameter, as in this study, could also introduce potential errors in the τ retrieval, and therefore influence the estimation of the angle dependency especially at V polarization. Finally, the angular dependency may also be related to the amount of VWC and LAI.…”

“…The measurements were performed at horizontal (p = H) and vertical (p = V) polarization (p) using a 3 s integration time to provide an accuracy of about 1 K. The radiometer was attached to a dual-mode conical horn antenna (60 cm in diameter and 67 cm long) with symmetrical and identical beams and a −3 dB full beam width of 23 • in the far field. A detailed description of the technical specifications of ELBARA-II can be found in [11] and [29], as well as the internal and external calibrations used to retrieve the brightness temperatures (T B ) from the radiometer raw data. The radiometer was mounted at about 4 m height on a moving platform (aluminum bridge) ( Figure 1a), which was moved between the gridded and non-gridded plot (the two orange rectangles in Figure 1b depict the exact bridge positions).…”

“…The T B data consisted of mean values of the 5 measurements performed per day (n = 33 days) at each incidence angle and for each polarization. For the single-angle approach, τ H and τ V were simultaneously inverted within the parameter range [0-3], while for the multi-angle approach, τ NAD and tt V were estimated within the parameter ranges [0-3] and [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], respectively. The inversions were performed with the multi-level coordinate search (MCS) algorithm, being a global optimization tool as introduced by Huyer and Neumaier [38].…”

“…Unfortunately, satellite missions are still limited in their spatial resolution (>10 km), and therefore, process studies looking at specific effects such as the effect of soil surface roughness, topography, or the vegetation canopy on spaceborne brightness temperature observations are hardly feasible. To overcome this limitation, tower-based radiometer measurements are still needed to improve our process understanding and to develop and validate radiative transfer models (e.g., [9][10][11]). In particular, the effect of the vegetation layer on the L-band emissions is of uppermost importance to improve the soil moisture retrieval and enable vegetation observations from L-band brightness temperature data.…”

“…However, their respective influence on the microwave emissions is not yet fully understood. To investigate their influence on the soil microwave emission in order to improve the soil moisture retrieval, several studies have already been performed using field-scale (e.g., [11,[13][14][15]) or air-and spaceborne L-band radiometer measurements (e.g., [16][17][18][19][20]).…”