Assessing Seasonal Backscatter Variations with Respect toUncertainties in Soil Moisture Retrieval in SiberianTundra Regions

• Published in: Remote Sensing
• Main Authors: Hoegstrom, E., Trofaier, A.M., Gouttevin, I., Bartsch, A.
• Other Authors: VIENNA UNIVERSITY OF TECHNOLOGY VIENNA AUT, IRSTEA LYON UR HHLY FRA
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: HYDROLOGIE; INCERTITUDE; TELEDETECTION; HYDROLOGY; REMOTE-SENSING; HIGH-LATITUDES; REMOTE SENSING; STATISTICAL UNCERTAINTY; Arctic; Asar; Ice; permafrost; Tundra
• Online Access: https://irsteadoc.irstea.fr/cemoa/PUB00043413

Knowledge of surface hydrology is essential for many applications, including studies that aim to understand permafrost response to changing climate and the associated feedback mechanisms. Advanced remote sensing techniques make it possible to retrieve a range of land-surface variables, including radar retrieved soil moisture (SSM). It has been pointed out before that soil moisture retrieval from satellite data can be challenging at high latitudes, which correspond to remote areas where ground data are scarce and the applicability of satellite data of this type is essential. This study investigates backscatter variability other than associated with changing soil moisture in order to examine the possible impact on soil moisture retrieval. It focuses on issues specific to SSM retrieval in the Arctic, notably variations related to tundra lakes. ENVISAT Advanced Synthetic Aperture Radar (ASAR) Wide Swath (WS, 120 m) data are used to understand and quantify impacts on Metop (AAdvanced Scatterometer (ASCAT, 25 km) soil moisture retrieval during the snow free period. Sites of interest are chosen according to ASAR WS availability, high or low agreement between output from the land surface model ORCHIDEE and ASCAT derived SSM. Backscatter variations are analyzed with respect to the ASCAT footprint area. It can be shown that the low model agreement is related to water fraction in most cases. No difference could be detected between periods with floating ice (in snow off situation) and ice free periods at the chosen sites. The mean footprint backscatter is however impacted by partial short term surface roughness change. The water fraction correlates with backscatter deviations (relative to a smooth water surface reference image) within the ASCAT footprint areas . Backscatter deviations of up to 5 dB can occur in areas with less than 50% water fraction and an assumed soil moisture related range (sensitivity) of 7 dB in the ASCAT data. The sensitivity is also positively correlated with water fraction in regions with low land-surface model agreement (R = 0.68). A precise quantification of the impact on soil moisture retrieval would, however, need to consider actual soil moisture changes and sensor differences. The study demonstrates that the usage of higher spatial resolution data than currently available for SSM is required in lowland permafrost environments. / La connaissance de l'hydrologie de surface est essentielle mais difficile aux hautes latitudes du fait de la rareté des observations. Les observations satellites offrent le moyen d'y pallier, mais encore faut-il s'assurer de la validité de ces approches dans ces régions. Par l'intermédiaire de deux jeux de données satellite de résolutions différentes, on étudie ici la sensibilité du signal de micro-ondes actives rétrodiffusées à des facteurs autres que l'humidité superficielle des sols. Les zones d'étude ont été ciblées pour correspondre à des régions où les modèles hydrologiques divergent des observations. Dans la plupart de ces zones, l'utilisation d'un produit satellite à fine résolution met en évidence l'impact des lacs sur le signal micro-onde observé, et utilisé pour inférer l'humidité superficielle dans les produits satellites. Il est donc nécessaire d'apporter des corrections aux produits satellitaires dans les régions où la forte concentration en étendues d'eau libre et l'occurrence de vents violents en limitent la validité pour l'hydrologie de surface.