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 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.
Highlights
The ability of coarse resolution (~25–50 km) microwave remote sensing products from both passive and active satellites to capture the variability of soil moisture was demonstrated by numerous studies (e.g., [1,2,3,4])
If there is no variation in the real soil moisture content, there may not be any linear correlation between soil moisture datasets, even though the datasets may be accurate in absolute terms
The results suggest that the Advanced Synthetic Aperture Radar (ASAR) Global Monitoring (GM) Surface Soil Moisture (SSM) product can capture the seasonal cycle of soil moisture well, whereas its ability to represent single precipitation events is lower
Summary
The ability of coarse resolution (~25–50 km) microwave remote sensing products from both passive and active satellites to capture the variability of soil moisture was demonstrated by numerous studies (e.g., [1,2,3,4]) Their benefits in many research fields, such as numerical weather forecasting [5,6], runoff modeling [7,8], agricultural drought monitoring [9], land data assimilation [10] or studies of land atmospheric feedbacks [11], have been demonstrated. The Advanced Synthetic Aperture Radar (ASAR) sensor onboard the Envisat satellite was an active microwave system operating at a central frequency of 5.331 GHz (C-band) It offers multiple acquisition modes employing both the conventional stripmap SAR, as well as the ScanSAR technique.
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