Biophysical parameters retrieval and sensitivity analysis of rabi crops (Mustard and wheat) from structural perspective

Abstract

The sensitivity of dual-polarized Sentinel-1 backscatter towards biophysical parameters (height and biomass) of wheat and mustard crops was investigated. The plant height and biomass observations categorized into three groups were useful in understanding the sensitivity across a particular biomass and height range whose significance was determined using a statistical measure (student’s t-test). The crop parameters were retrieved only for the C-band sensitive biomass (< 5 kg m−2) and height (< 160 cm for mustard and < 80 cm for wheat) range considering the saturation of signals at advanced crop stages and based on the detailed investigation. The sensitivity towards the mustard plant height becomes very weak as the crop proceeds to a height > 190 cm. A low RMSE (11.50 cm) was observed when the retrieval was done for height < 160 cm. The cross-polarized responses were more sensitive to crop biomass than co-polarized responses mainly due to the dominant depolarization of the transmitted power. An early saturation was found at co-polarized V V (4 kg m−2) as compared to cross-polarized V H (6 kg m−2) particularly for planophiles like mustard and little later in the case of erectophile such as wheat. The backscatter response was found to be sensitive at early crop stages for both the crop geometry, and hence retrieval of biophysical parameters at these stages can yield better accuracy than the overall retrieval. The retrieval of wheat height resulted in a low RMSE of 9.25 cm when the retrieval was carried out for crop height < 80 cm. Retrieval was attempted using the simplistic logarithmic model which can find ways in the operational application using wide swath dual-polarized datasets.