An Alternative Roughness Parameterization for Soil Moisture Retrieval from Passive Microwave Observations
Martens, Brecht1; Lievens, Hans1; Walker, Jeff2; Panciera, Rocco3; Tanase, Mihai3; Monerris, Alessandra2; Gao, Ying2; Wu, Xiaoling2; Verhoest, Niko1
1Ghent University - Laboratory of Hydrology and Water Management, BELGIUM; 2Monash University - Department of Civil Engineering, AUSTRALIA; 3University of Melbourne - Department of Infrastructure Engineering, AUSTRALIA
ABSTRACT
The usefulness of L-band radiometer observations for retrieval of near-surface soil moisture has been demonstrated for many years during different ground- and airborne experiments (e.g. Jackson et al. 1982; 1983). The retrieval of soil moisture is based on the inversion of radiative transfer models such as the L-band Microwave Emission from the Biosphere model (LMEB). Together with soil moisture, vegetation parameters, such as the vegetation opacity, and soil roughness also have a significant effect on the measured brightness temperatures from L-band radiometers. However, the quality of the retrievals is highly influenced by the lack of knowledge on the soil roughness. In the LMEB-model, soil roughness is being characterized by a single parameter. Unfortunately, the relation between this single roughness parameter and physical roughness parameters such as the correlation length or the RMS-height, is not well known. Moreover, measurements of physical roughness characteristics in the field are highly uncertain and time consuming. In order to circumvent the potential problems inherent to roughness measurements in the field, and the problem of relating these observations to the parameter used in the LMEB-model, roughness parameters can be calibrated, resulting in effective roughness parameters. For the retrieval of near-surface soil moisture from active microwave observations, this technique has meanwhile shown its large potential. However, it has also been shown that assuming temporally constant roughness parameters for soil moisture retrieval from SAR observations may not be justified (Baghdadi et al., 2004; 2006). To accommodate for temporal variability in roughness parameters, Lievens et al. (2011) developed a linear model for deriving effective roughness parameters from radar backscatter and incidence angle. Similar to active microwave observations, many studies investigated the potential of the effective roughness parameter technique for soil moisture retrieval from passive microwave observations. Most of the studies found that the effective roughness parameter used in the LMEB-model is impacted by the soil moisture content in a linear way (e.g. Wigneron et al., 2001; Escorihuela et al., 2007; Saleh et al., 2007). A similar linear relationship is used in the operational Level 2 soil moisture processor of the Soil Moisture and Ocean Salinity (SMOS) mission. This dependence of the roughness parameter on soil moisture has been explained by a dielectric roughness induced by the spatial heterogeneity of moisture in the soil, which causes local variations in the dielectric properties. However, the linear relationship between the roughness parameter and soil moisture is not yet well understood and its robustness, especially under vegetated surfaces, needs to be further assessed. This study will further investigate the potential of the effective roughness parameter technique for both active and passive microwave observations. Given the similarities between the problems of roughness parameterization from both active and passive microwave observations, the transferability of the technique introduced by Lievens et al. (2011) to passive microwave observations will be tested. To this end, an extensive data set of both airborne passive and active microwave data, together with intensive ground observations of soil moisture and vegetation, collected during the SMAPEx-campaigns in Australia (Panciera et al., 2010a; 2010b; Monerris et al., 2011), will be used. This will allow a further investigation of the robustness of the linear model for deriving effective roughness parameters for active microwave observations introduced by Lievens et al. (2011). Furthermore, it will be possible to test the transferability of the technique to passive microwave data. It is clear that the development of a simple and robust technique for deriving effective roughness parameters for the retrieval of near-surface soil moisture from as well active and passive microwave data will be useful considering the data that will become available from the planned SMAP mission.
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