Seven Years of Advanced Synthetic Aperture Radar (ASAR) Global Monitoring (GM) of Surface Soil Moisture over Africa

Abstract

Abstract:A surface soil moisture (SSM) product at a 1-km spatial resolution derived from the Envisat Advanced Synthetic Aperture Radar (ASAR) Global Monitoring (GM) mode data was evaluated over the entire African continent using coarse spatial resolution SSM acquisitions from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) and the Noah land surface model from the Global Land Data Assimilation System (GLDAS-NOAH). The evaluation was performed in terms of relative soil moisture values (%), as well as anomalies from the seasonal cycle. Considering the high radiometric noise of the ASAR GM data, the SSM product exhibits a good ability (Pearson correlation coefficient (R) = ~0.6 for relative soil moisture values and root mean square difference (RMSD) = 11% when averaged to 5-km resolution) to monitor temporal soil moisture variability in regions with low to medium density vegetation and yearly rainfall >250 mm. The findings agree with previous evaluation studies performed over Australia and further strengthen the understanding of the quality of the ASAR GM SSM product and its potential for data assimilation. Problems identified in the ASAR GM algorithm over arid regions were explained by azimuthal effects. Diverse backscatter behavior over different soil types was identified. The insights gained about the quality of the data were used to establish a reliable masking OPEN ACCESS Remote Sens. 2014, 6 7684 of the existing ASAR GM SSM product and the identification of areas where further research is needed for the future Sentinel-1-derived SSM products.

Monitoring of the Lac Bam Wetland Extent Using Dual-Polarized X-Band SAR Data

“…Alternative SAR remote sensing techniques for soil moisture retrieval or water level measurements can be of relevance for wetland monitoring as well. Data from SAR sensors have, among other remote sensing sensors, also been used for surface soil moisture estimation in Africa on a medium resolution scale (around 1 km) [46]. Interferometric synthetic aperture radar (InSAR) has been used to detect water level changes in wetlands [47][48][49][50].…”

Modelling and correcting azimuthal anisotropy in Sentinel-1 backscatter data

“…Bartalis, Scipal, and Wagner (2006) investigated the azimuthal anisotropy of scatterometer backscatter measurements over all global land areas and localized regions which show a strong azimuthal dependency. Problems which were explained by azimuthal effects were also identified by Envisat ASAR (Advanced Synthetic Aperture Radar) GM (global monitoring) backscatter time series over arid regions (Dostálová et al 2014). In such areas, the ASAR backscatter from the descending and ascending orbits revealed a difference that adds a systematic bias in time series.…”

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

“…Given the current large data volumes being generated by Sentinel-1A, and the significant increase when Sentinel-1B is launched in 2016, soil moisture change detection techniques such as the TU Wien approach adapted for ASAR data are likely to be applicable and improved for Sentinel-1 data (Hornacek et al, 2012). The benefits of using SAR data with a higher radiometric resolution has been demonstrated by Dostálová et al (2014) and this new dataset could be used not only as another validation source for coarse-scale soil moisture products but also for applications which require finer spatial resolution soil moisture data such as hydrological or runoff modelling.…”

“…This technique was originally developed for ERS scatterometer and Advanced Scatterometer (ASCAT) data but has been successfully adapted to both ASAR WS and GM data (e.g. Pathe et al, 2009;Mladenova et al, 2010;Doubková et al, 2012;Peters et al, 2012;Dostálová et al, 2014;Zribi et al, 2014). The TU Wien change detection approach derives relative changes in surface soil moisture and indirectly accounts for surface roughness and vegetation by assuming changes in these parameters will generally occur at longer temporal scales than soil moisture changes.…”

“…The Advanced Synthetic Aperture Radar (ASAR) instrument onboard the ENVISAT satellite was capable of providing global measurements at 1km and 150m spatial resolution every four to seven days, depending on the acquisition plan (Desnos et al, 2000). Although there are certain technical limitations in retrieving surface soil moisture from SAR data, significant progress has been made in recent years to the point that SAR data could be used not only as another validation source for coarse-scale soil moisture products but also for applications which require finer spatial resolution soil moisture data such as hydrological or runoff modelling (Dostálová et al, 2014;Pratola et al, 2014). Furthermore, the regular temporal coverage and higher spatial resolution of current C-band sensors such as Sentinel-1 will provide greater opportunities to characterise surface soil moisture within the large areas covered by coarse-scale product cells and help strengthen the understanding of such products.…”