Detecting management practices of improved grasslands
using ERS-1 SAR data
3. Satellite data
For the analysis, Landsat TM and ERS-1 SAR data (Level 2.1) acquired during the growing seasons in 1994 and 1995 were used. ERS-1 SAR data were processed by NASDA (National Space Development Agency of Japan). The data are three-look precision images with pixel sizes of 12.5 m in both azimuth and range directions. The TM-SAR pairs for this study on backscattering signatures of grasslands are (1) TM (17 May 1994)-SAR (1 June 1994), and (2) TM (13 July 1994)-SAR (8 July 1994), and (3) TM (5 August 1994)-SAR (14 August 1994) and (4) TM (17 August 1995)- SAR (21 August 1995).
Results and discussion
1. Backscattering signatures of unmown, mown and renovated grasslands
The backscattering coefficients per field derived from ERS-1 SAR data for unmown, mown and renovated grasslands were shown in figures 3. An extremely high backscatter coefficient for renovated grasslands on all ERS-1 SAR data can be identified. Unmown and mown grasslands showed lower backscattering coefficient.
Figure 3. Backscattering signatures off unmown, mown and renovated grasslands on ERS-1 SAR data.
Renovated grasslands have bare soil surface. Their backscattering signatures are dominated by soil moisture and surface roughness (Major et. al. 1993). Increased backscatter from renovated treatment considered to be due partly to higher moisture content and partly to higher surface roughness. However, renovated grasslands never have extremely higher moisture contents
rather than adjacent grasslands. Renovated grasslands have ground surfaces with high roughness due to ploughing approximately 25 cm in depth Unmown and mown grasslands do not have high surface roughness. Major et. al. (1993) and many other studies have suggested that ploughed bare soil was an important sources of scattering in agricultural fields. Thus, high backscattering coefficients in renovated grasslands were considered to be mainly due to the high roughness of the ground surface.
On the other hand, unmown and mown grasslands showed relatively low backscatter coefficients and have almost the same signatures. Although unmown grasslands covered by grass approximately have 100 cm in height in the growing season, canopies of small leaf crops such as grass was not a significant component for backscattering signatures in the C-band. Success in ERS-1 SAR monitoring of biomass change in sugar beet (Xu et. al. 1997) which radar backscatter results mainly from the vegetation and water has been reported. However, in Hoekman and Bouman (1993), among various crops, grass had the lowest backscatter in the C-band. Lin et al. (1994) reported that the presence of the short grass canopy does not have a significant effect on C-band backscattering signatures. In conclusion, identifying whether grass has been mown or not is still difficult using ERS-1 SAR in the C-band alone. Higher frequencies (X- and K-bands) should be used to monitor changes in grass biomass.
