Assessing Seasonal Backscatter Variations with Respect to Uncertainties in Soil Moisture Retrieval in Siberian Tundra Regions

Abstract

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, Remote Sens. 2014, 6 8719 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 (R = 0.91−0.97). 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.

Can C-band synthetic aperture radar be used to estimate soil organic carbon storage in tundra?

“…Potential backscatter increase due to snow property changes is also lower than the observed range for SOC with about 2-3 dB (Naeimi et al, 2012). Soil moisture variation (dry to saturated) in tundra regions without significant proportion of water bodies causes about 5 dB variation during summertime (Högström et al, 2014).…”

“…The sensitivity to saturation levels also varies by vegetation coverage . Such approaches furthermore rely on the assumption that roughness and scatting mechanisms do not change over time, which is not the case over many areas in the high latitudes (Högström et al, 2014). C-band winter backscatter can be shown to be used as an alternative, as a proxy for wetness levels as well as soil organic carbon storage.…”

Intercomparison of Soil Moisture Retrievals From In Situ, ASAR, and ECV SM Data Sets Over Different European Sites

“…These poor results were likely due to the dense forest cover in these areas and also the reduced capability of the ASAR soil moisture retrieval algorithm for data acquired above 60° latitude. The challenges for deriving soil moisture in high latitudes has been documented by Bartsch et al (2011) and further explored by Gouttevin et al (2013) and Högström et al (2014). The presence of permanenet open water surfaces within the ECV cell size may be an additional source of disagreement between the satellite derived and in-situ soil moisture values.…”

Surface State across Scales; Temporal and Spatial Patterns in Land Surface Freeze/Thaw Dynamics

“…The backscatter over lakes drops after melt and does not increase afterwards as for the surrounding land area. Wind action over the lakes can increase the average backscatter within the ASCAT footprint on a short-term basis [21], which may lead to misclassifications. The regional results also show linear artifacts, which are caused by SAR processing issues with the NEST software [41].…”

“…They conclude that the canopy plays a significant role. Högström et al [21] quantify the impact of lake backscatter variations within the ASCAT footprint using ASAR wide swath data (150 m) for soil moisture retrieval, e.g., wind can cause an increase of 5 dB in areas with 50% water fraction.…”