Abstract
Global surface soil moisture products are derived from passive L-band microwave satellite observations. The applied retrieval algorithms include dielectric models (relating soil water content to relative permittivity) developed for mineral soils. First efforts to generate equivalent models for areas where organic surface layers are present such as in the high-latitude regions have recently been undertaken. The objective of this study was to improve our still insufficient understanding of L-band emission of organic substrates in prospect of enhancing soil moisture estimations in the high latitudes undergoing most rapid climatic changes. To this end, L-band relative permittivity measurements using a resonant cavity were carried out on a wide range of organic surface layer types collected at different sites. This dataset was used to evaluate two already existing models for organic substrates. Some samples from underlying mineral layers were considered for comparison. In agreement with theory the bulk relative permittivity measured in organic substrate was decreased due to an increased bound water fraction (where water molecules are rotationally hindered) compared to the measured mineral material and corresponding output of the dielectric model for mineral soils used in satellite algorithms. No distinct differences in dielectric response were detected in the measurements from various organic layer types, suggesting a generally uniform L-band emission behavior. This made it possible to fit a simple empirical model to the data obtained from all collected organic samples. Outputs of the two existing models both based on only one organic surface layer type were found to lie within the spread of our measured data, and in close proximity to the derived simple model. This general consensus strengthened confidence in the validity of all these models. The simple model should be suitable for satellite soil moisture retrieval applications as it is calibrated on a wide range of organic substrate types and the entire wetness range, and does not require any auxiliary input that may be difficult to obtain globally. This renders it generically applicable wherever organic surface layers are present.
Highlights
Due to significantly reduced microbial activity pronounced soil carbon reservoirs have been accumulating in the circumpolar northern cold climate zone, outweighing the vegetation and atmospheric carbon pools together [1,2]
We show simple empirical models for organic soil surface layers as well as sandy mineral soils that could be developed based on the acquired water content—relative permittivity couples
This station is situated in cropland without presence of a pronounced organic layer and represents average soil texture conditions of all 30 network stations deployed within the Soil Moisture and Ocean Salinity (SMOS) pixel chosen for validation purposes [34]
Summary
Due to significantly reduced microbial activity pronounced soil carbon reservoirs have been accumulating in the circumpolar northern cold climate zone (boreal forest/tundra), outweighing the vegetation and atmospheric carbon pools together [1,2]. The higher Northern latitudes are especially sensitive to climate change due to above-average rising temperatures (e.g., [3,4]). At present still mostly locked in permafrost, there is a high potential that on thawing, this storage is subject to release to the atmosphere in form of greenhouse gases or in a dissolved state to the hydrosphere (e.g., [5,6]). Significant knowledge gaps remain concerning the quantification of the impact of thawing ground on the global carbon cycle (i.e., magnitude, type and timing of greenhouse gas emissions, e.g., [7]). There is a strong need to monitor hydrologic states and water redistribution processes in these regions, where pronounced organic surface horizons are common standard (a soil horizon is a layer whose physical characteristics differ from the layers above and beneath)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
