Analysis of temporal backscattering of cotton crops using a semiempirical model (original) (raw)

To develop an operational methodology for estimating soil moisture and crop biophysical parameters and to generate a crop cover map, backscattering signatures of vegetation canopies are investigated using multitemporal Radarsat synthetic aperture radar (SAR) data over a predominantly cotton-growing area in India during low to peak crop growth stage. A simple parameterization of the water-cloud model with volumetric soil moisture content (m ) and leaf area idex (LAI) is used to simulate the microwave backscattering coefficient ( 0 ), as it is found to be a good candidate for operational purposes as demonstrated by several workers in past. The influence of crop height ( ), LAI, and m on 0 is investigated during peak crop growth stage. A linear relationship between LAI and crop height is derived semiempirically, and a linear zone is chosen for analysis during the peak crop-growing stage. Estimation of average volume fraction of leaves ( ) and attenuation factor ( ) by two different approaches is discussed: 1) using linear relationship between LAI versus crop height and 2) from the water-cloud model parameter ( ) estimation by iterative minimum least square error approach. It is observed that model-estimated parameters agree well with the measured values within an acceptable error limit. At lower soil moisture, m = 0 02 (cm 3 cm 3 ), the dynamic range of 0 is found to be about +5 dB for 0-70 cm of crop height but monotonously decreases to null at a transition point, having m 0 38 (cm 3 cm 3 ). A positive correlation is found between backscattering coefficient and crop height till this transition point but shows a negative correlation beyond that, signifying the predominant attenuation by vegetation over soil. Differential moisture sensitivity (d 0 dm ) of the backscattering coefficient decreases by half from 20.55 dB (cm 3 cm 3 ) for dry and bare-field conditions to 10.68 dB (cm 3 cm 3 ) for wet and crop-covered fields (m = 0 38 cm 3 cm 3 , = 70 cm), whereas differential crop height sensitivity (d 0 d ) varies from 0.22-0.03 dB/cm for bare-field conditions to crop-covered fields with crop height 70 cm. It is found that the percentage of relative error is smallest (2.27%) for LAI and attenuation factor estimation using the value of , from LAI models, whereas it is 4.25% when estimating from the attenuation coefficient ( ) from the model. Index Terms-Crop attenuation, differential moisture and crop height sensitivity, leaf area index (LAI), synthetic aperture radar (SAR) backscatter, water-cloud model.