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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.


Comparison of observed (left) and modelled (right) backscatter scaling coefficients a (top) and b (bottom). The parameter b is unitless and a is expressed in decibels.
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f7-sensors-08-01174: Comparison of observed (left) and modelled (right) backscatter scaling coefficients a (top) and b (bottom). The parameter b is unitless and a is expressed in decibels.

Mentions: As expected, the backscatter scaling parameters a and b derived from the ASAR WS images show a pronounced spatial pattern (Figure 7 left). Forests, shrubs and more dense herbaceous plants in the vicinity of small streams can be distinguished from cropland. Settlements show a similar behaviour as dense vegetation. To check the validity of the backscatter scaling model as developed in section 2.2., the sensitivity and dry backscatter reference were calculated according to equations (22) and (23) based on all 73 ASAR images. The assumption is that also S and σ0dry can in a first approximation be treated as constants at both scales. Equations (24) and (25) thus simplify to(27)a(x,y)≈σdry,l0(x,y)−Sl(x,y)Srσdry,r0(28)b(x,y)≈Sl(x,y)Sr


Temporal Stability of Soil Moisture and Radar Backscatter Observed by the Advanced Synthetic Aperture Radar (ASAR)
Comparison of observed (left) and modelled (right) backscatter scaling coefficients a (top) and b (bottom). The parameter b is unitless and a is expressed in decibels.
© Copyright Policy
Related In: Results  -  Collection

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

f7-sensors-08-01174: Comparison of observed (left) and modelled (right) backscatter scaling coefficients a (top) and b (bottom). The parameter b is unitless and a is expressed in decibels.
Mentions: As expected, the backscatter scaling parameters a and b derived from the ASAR WS images show a pronounced spatial pattern (Figure 7 left). Forests, shrubs and more dense herbaceous plants in the vicinity of small streams can be distinguished from cropland. Settlements show a similar behaviour as dense vegetation. To check the validity of the backscatter scaling model as developed in section 2.2., the sensitivity and dry backscatter reference were calculated according to equations (22) and (23) based on all 73 ASAR images. The assumption is that also S and σ0dry can in a first approximation be treated as constants at both scales. Equations (24) and (25) thus simplify to(27)a(x,y)≈σdry,l0(x,y)−Sl(x,y)Srσdry,r0(28)b(x,y)≈Sl(x,y)Sr

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.