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Temporal Stability of Soil Moisture and Radar Backscatter Observed by the Advanced Synthetic Aperture Radar (ASAR)

View Article: PubMed Central - PubMed

ABSTRACT

The high spatio-temporal variability of soil moisture is the result of atmospheric forcing and redistribution processes related to terrain, soil, and vegetation characteristics. Despite this high variability, many field studies have shown that in the temporal domain soil moisture measured at specific locations is correlated to the mean soil moisture content over an area. Since the measurements taken by Synthetic Aperture Radar (SAR) instruments are very sensitive to soil moisture it is hypothesized that the temporally stable soil moisture patterns are reflected in the radar backscatter measurements. To verify this hypothesis 73 Wide Swath (WS) images have been acquired by the ENVISAT Advanced Synthetic Aperture Radar (ASAR) over the REMEDHUS soil moisture network located in the Duero basin, Spain. It is found that a time-invariant linear relationship is well suited for relating local scale (pixel) and regional scale (50 km) backscatter. The observed linear model coefficients can be estimated by considering the scattering properties of the terrain and vegetation and the soil moisture scaling properties. For both linear model coefficients, the relative error between observed and modelled values is less than 5 % and the coefficient of determination (R2) is 86 %. The results are of relevance for interpreting and downscaling coarse resolution soil moisture data retrieved from active (METOP ASCAT) and passive (SMOS, AMSR-E) instruments.

No MeSH data available.


Sensitivity (left) and dry backscatter reference (right). The unit of both parameters is decibels.
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f8-sensors-08-01174: Sensitivity (left) and dry backscatter reference (right). The unit of both parameters is decibels.

Mentions: Figure 8 shows that the sensitivity ranges from about 3 dB for forests and settlements to values exceeding 12 dB for some agricultural fields. Disregarding settlements, the spatial pattern of Sl thus conveys a good impression of the spatial distribution of vegetation in the study area. Despite being more strongly influenced by surface roughness, also the dry backscatter reference reflects vegetation patterns well, with values ranging from −16 dB for agricultural fields up to -8 dB for forests and settlements. Based on these two parameters, the backscatter scaling coefficients a and b can be calculated according to models (26) and (27). The modelled values are compared to the observed ones in Figure 7. One can see that the spatial patterns of both scaling coefficients are very well reproduced by this model, although the dynamic range of the modelled a and b images is somewhat smaller.


Temporal Stability of Soil Moisture and Radar Backscatter Observed by the Advanced Synthetic Aperture Radar (ASAR)
Sensitivity (left) and dry backscatter reference (right). The unit of both parameters is decibels.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3927501&req=5

f8-sensors-08-01174: Sensitivity (left) and dry backscatter reference (right). The unit of both parameters is decibels.
Mentions: Figure 8 shows that the sensitivity ranges from about 3 dB for forests and settlements to values exceeding 12 dB for some agricultural fields. Disregarding settlements, the spatial pattern of Sl thus conveys a good impression of the spatial distribution of vegetation in the study area. Despite being more strongly influenced by surface roughness, also the dry backscatter reference reflects vegetation patterns well, with values ranging from −16 dB for agricultural fields up to -8 dB for forests and settlements. Based on these two parameters, the backscatter scaling coefficients a and b can be calculated according to models (26) and (27). The modelled values are compared to the observed ones in Figure 7. One can see that the spatial patterns of both scaling coefficients are very well reproduced by this model, although the dynamic range of the modelled a and b images is somewhat smaller.

View Article: PubMed Central - PubMed

ABSTRACT

The high spatio-temporal variability of soil moisture is the result of atmospheric forcing and redistribution processes related to terrain, soil, and vegetation characteristics. Despite this high variability, many field studies have shown that in the temporal domain soil moisture measured at specific locations is correlated to the mean soil moisture content over an area. Since the measurements taken by Synthetic Aperture Radar (SAR) instruments are very sensitive to soil moisture it is hypothesized that the temporally stable soil moisture patterns are reflected in the radar backscatter measurements. To verify this hypothesis 73 Wide Swath (WS) images have been acquired by the ENVISAT Advanced Synthetic Aperture Radar (ASAR) over the REMEDHUS soil moisture network located in the Duero basin, Spain. It is found that a time-invariant linear relationship is well suited for relating local scale (pixel) and regional scale (50 km) backscatter. The observed linear model coefficients can be estimated by considering the scattering properties of the terrain and vegetation and the soil moisture scaling properties. For both linear model coefficients, the relative error between observed and modelled values is less than 5 % and the coefficient of determination (R2) is 86 %. The results are of relevance for interpreting and downscaling coarse resolution soil moisture data retrieved from active (METOP ASCAT) and passive (SMOS, AMSR-E) instruments.

No MeSH data available.