Open Access

Anke Fluhrer

• The complexity of our Earth system makes the investigation of its critical variables indispensable in order to understand the interrelationships and interactions of different sub-systems and processes in detail. Various parameters play a key role in the global weather and climate system of our Earth, as they control interactions and exchange processes as well as link the water, carbon, and energy cycles worldwide. Accordingly, the global determination and long-term monitoring of important geophysical parameters, such as soil moisture and soil surface roughness, is of significant relevance, especially in terms of, for instance, improving weather and climate forecasts, quantifying net carbon fluxes, and monitoring the condition assessment of existing infrastructures (e.g., roads). In this PhD thesis, active and passive microwave remote sensing is exploited to estimate soil moisture and soil surface roughness. The associated methods developed herein and conducted analyses are of highThe complexity of our Earth system makes the investigation of its critical variables indispensable in order to understand the interrelationships and interactions of different sub-systems and processes in detail. Various parameters play a key role in the global weather and climate system of our Earth, as they control interactions and exchange processes as well as link the water, carbon, and energy cycles worldwide. Accordingly, the global determination and long-term monitoring of important geophysical parameters, such as soil moisture and soil surface roughness, is of significant relevance, especially in terms of, for instance, improving weather and climate forecasts, quantifying net carbon fluxes, and monitoring the condition assessment of existing infrastructures (e.g., roads). In this PhD thesis, active and passive microwave remote sensing is exploited to estimate soil moisture and soil surface roughness. The associated methods developed herein and conducted analyses are of high importance, since the recording of geophysical parameters makes an essential contribution to climate and ecosystem research. This study is divided into three sub-studies. It addresses the possibility to close a research and knowledge gap in remote sensing and proposes new approaches in the field of microwave remote sensing for soil- and vegetation-related parameter estimation. The innovation of this study lies specifically in the detailed analysis of longwave microwaves of low frequencies (L- to P-band). The first sub-study focused on a combined method of L-band radar and radiometer satellite observations for the simultaneous determination of soil surface roughness parameters on global and temporal continuous scales. It was found that the proposed covariation-based active-passive microwave retrieval algorithm for soil surface roughness estimation is independent of soil permittivity (soil moisture) inputs in non-arid areas (permittivity > 10 [-]). The determined surface roughness parameters correspond to local land surface conditions, e.g., rather smooth surfaces over non-vegetated, sandy and dry deserts, and rather rough surfaces at the edge of deserts, where smaller vegetation appears. Further, no correlations between roughness patterns and precipitation or soil texture could be found at the investigated scale. In the second sub-study, low frequency (P-band) synthetic aperture radar (SAR) data are employed to investigate the penetration and scattering behavior of active microwaves interacting with different vegetation covers and soil types. The investigation relies on a proposed hybrid decomposition technique for separating the total SAR signal into individual scattering mechanisms (soil, dihedral, and volume), and is subsequently used for lateral soil moisture estimation in the upper root zone (~20-30 cm) as well as P-band penetration depth calculations. The proposed moisture estimation approach is the first method (to the best of my knowledge) for estimating complex permittivity from microwave remote sensing. The method was validated at different monitoring sites across the United States and it was shown that P-band microwaves can penetrate up to 35 cm into the soil, depending on the local landcover and moisture conditions. In the third sub-study, a combined approach of active microwave remote sensing and soil hydrological modeling is proposed for the determination of vertically continuous soil moisture profiles. For that, airborne P-band SAR data are compared to soil hydrological model simulations based on the HYDRUS-1D for estimating the vertical discontinuity and variability of soil moisture with depth. With this combined approach, vertically continuous soil moisture profiles can be estimated with medium to high Pearson’s coefficients of determination (R^2 of 0.48 to 0.92). In summary, innovative retrieval approaches are proposed for soil moisture and surface roughness estimation from microwave remote sensing. High consistencies between retrieval results and auxiliary data (in situ, model and reanalysis) confirm the feasibility of the proposed approaches. These approaches can be extended for future research, for example, in the prospect of upcoming P-band satellite missions – the BIOMASS mission of the European Space Agency (ESA) or the SigNals Of Opportunity P-band Investigation (SNOOPI) mission of the National Aeronautics and Space Administration (NASA).…