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Open AccessArticle

Assessment of Multi-Scale SMOS and SMAP Soil Moisture Products across the Iberian Peninsula

1
CommSensLab–UPC Unidad de Excelencia María de Maeztu, Department of Signal Theory and Communications, Universitat Politècnica de Catalunya (UPC) and IEEC-CTE/UPC, Jordi Girona 1-3, 08034 Barcelona, Spain
2
Barcelona Expert Center (BEC), Passeig Marítim de la Barceloneta 37-47, 08003 Barcelona, Spain
3
Microwave and Radar Institute, German Aerospace Center (DLR), Münchener Strasse 20, 82234 Weßling, Germany
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Institute of Marine Sciences, Spanish National Research Council (ICM-CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain
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Department of Civil and Environmental Engineering, Massachusetts Institute of Technology (MIT), 15 Vassar Street, Cambridge, MA 02139, USA
6
Image Processing Laboratory, Universitat de València (UV), Catedrático José Beltrán 2, 46010 València, Spain
*
Author to whom correspondence should be addressed.
Remote Sens. 2020, 12(3), 570; https://doi.org/10.3390/rs12030570
Received: 20 December 2019 / Revised: 4 February 2020 / Accepted: 6 February 2020 / Published: 8 February 2020
(This article belongs to the Special Issue Ten Years of Remote Sensing at Barcelona Expert Center)
In the last decade, technological advances led to the launch of two satellite missions dedicated to measure the Earth’s surface soil moisture (SSM): the ESA’s Soil Moisture and Ocean Salinity (SMOS) launched in 2009, and the NASA’s Soil Moisture Active Passive (SMAP) launched in 2015. The two satellites have an L-band microwave radiometer on-board to measure the Earth’s surface emission. These measurements (brightness temperatures TB) are then used to generate global maps of SSM every three days with a spatial resolution of about 30–40 km and a target accuracy of 0.04 m3/m3. To meet local applications needs, different approaches have been proposed to spatially disaggregate SMOS and SMAP TB or their SSM products. They rely on synergies between multi-sensor observations and are built upon different physical assumptions. In this study, temporal and spatial characteristics of six operational SSM products derived from SMOS and SMAP are assessed in order to diagnose their distinct features, and the rationale behind them. The study is focused on the Iberian Peninsula and covers the period from April 2015 to December 2017. A temporal inter-comparison analysis is carried out using in situ SSM data from the Soil Moisture Measurements Station Network of the University of Salamanca (REMEDHUS) to evaluate the impact of the spatial scale of the different products (1, 3, 9, 25, and 36 km), and their correspondence in terms of temporal dynamics. A spatial analysis is conducted for the whole Iberian Peninsula with emphasis on the added-value that the enhanced resolution products provide based on the microwave-optical (SMOS/ERA5/MODIS) or the active–passive microwave (SMAP/Sentinel-1) sensor fusion. Our results show overall agreement among time series of the products regardless their spatial scale when compared to in situ measurements. Still, higher spatial resolutions would be needed to capture local features such as small irrigated areas that are not dominant at the 1-km pixel scale. The degree to which spatial features are resolved by the enhanced resolution products depend on the multi-sensor synergies employed (at TB or soil moisture level), and on the nature of the fine-scale information used. The largest disparities between these products occur in forested areas, which may be related to the reduced sensitivity of high-resolution active microwave and optical data to soil properties under dense vegetation. View Full-Text
Keywords: soil moisture; moisture variability; temporal dynamics; moisture patterns; spatial disaggregation; Soil Moisture Active Passive (SMAP); Soil Moisture and Ocean Salinity (SMOS); REMEDHUS soil moisture; moisture variability; temporal dynamics; moisture patterns; spatial disaggregation; Soil Moisture Active Passive (SMAP); Soil Moisture and Ocean Salinity (SMOS); REMEDHUS
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MDPI and ACS Style

Portal, G.; Jagdhuber, T.; Vall-llossera, M.; Camps, A.; Pablos, M.; Entekhabi, D.; Piles, M. Assessment of Multi-Scale SMOS and SMAP Soil Moisture Products across the Iberian Peninsula. Remote Sens. 2020, 12, 570.

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