Abstract
In the subarctic tundra, soil moisture information can benefit permafrost monitoring and ecological studies, but fine-scale remote-sensing approaches are lacking. We explore the suitability of C-band SAR, paying attention to two challenges soil moisture retrieval faces. First, the microtopography and the heterogeneous organic soils impart unique microwave scattering properties, even in absence of noteworthy shrub cover. Empirically, we find the polarimetric response is highly random (entropies >0.7). The randomness limits the applicability of purely polarimetric approaches to soil moisture estimation, as it causes a tailor-made decomposition to break down. For comparison, the L-band scattering response is more surfacelike, also in terms of its angular characteristics. The second challenge concerns the large spatial but small temporal variability of soil moisture observed at our site. Accordingly, the Radarsat-2 C-band backscatter has a limited dynamic range (~2 dB). However, contrary to polarimetric indicators, it shows a clear surface soil moisture signal. To account for the small dynamic range while retaining a 100-m spatial resolution, we embed an empirical time-series model in a Bayesian framework. This framework adaptively pools information from neighboring grid cells, thus increasing the precision. The retrieved soil moisture index achieves correlations of 0.3–0.5 with in situ data at 5 cm depth and, upon calibration, root-mean-square errors of <0.04 m3m−3. As this approach is applicable to Sentinel-1 data, it can potentially provide frequent soil moisture estimates across large regions. In the long term, L-band data hold greater promise for operational retrievals.
Highlights
Subarctic tundra ecosystems are responding rapidly to climate change, as evidenced by widespread changes in their vegetation cover and permafrost conditions [1], [2]
Soil moisture varies on a range of scales, as its distribution is shaped by the microtopography, topographic position and vegetation cover
The angular dependence of the backscatter is another important indicator of the scattering behavior, and it is useful for distinguishing surface from volume scattering
Summary
Subarctic tundra ecosystems are responding rapidly to climate change, as evidenced by widespread changes in their vegetation cover and permafrost conditions [1], [2]. The microtopography and the organic soils likely impart unique scattering characteristics (polarimetry, angular dependence), but these have not been quantified, and dedicated scattering models are lacking In light of these complexities, the most expedient approach to soil moisture estimation remains an open question. Numerous such approaches have been applied with some success in agricultural regions [16], [26], but most approaches cannot account for the variable microtopography In light of their simplicity, polarimetric methods and simple empirical methods seem to constitute a promising first step, which could lead to bespoke backscatter-based models. To address these open questions, and more generally provide a first overview of the limitations and opportunities of soil moisture estimation in the subarctic tundra, we pursue 3 closely linked objectives. We assess the soil moisture retrievals at two instrumented plots in the open tundra, for both the VV backscatter (available from Sentinel-1) and the surface backscatter extracted using the MTV decomposition from objective 1
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