Combining ENVISAT ASAR and spectral vegetation indices to evaluate grass properties in Otway, Australia

SAR has advantage over optical due to its all-weather working ability and penetration capability. The sensitivity of ENVISAT ASAR HH backscatter to multiple pasture properties (particularly biomass) has been examined over pasture area in Otway, Australia. First of all, decision-tree classification was performed using two MODIS NDVI images to extract grass areas from the study area. Then, over the classified grass area, by relating ENVISAT ASAR HH dB to MODIS NDVI and M.I (soil moisture index, calculated from climate data), it has been proved that the incidence angle of 17° is better than 33° for ASAR HH to detect temporal changes of grass properties and soil moisture, with slightly higher sensitivity to grass biomass than soil moisture. With MODIS NDVI describing the whole study area as reference for specific paddock, Landsat TM was used to assist understanding SAR signal at paddock scale. It was found that HH dB is moderately correlated to the TM NDVI, NDWI and EVI, with priority over NDWI (plant water content).