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Abstract
Abstract. The purpose of this study was to develop an approach to estimate soil surface parameters from C-band polarimetric SAR data in the case of bare agricultural soils. An inversion technique based on multi-layer perceptron (MLP) neural networks was introduced. The neural networks were trained and validated on a noisy simulated dataset generated from the Integral Equation Model (IEM) on a wide range of surface roughness and soil moisture, as it is encountered in agricultural contexts for bare soils. The performances of neural networks in retrieving soil moisture and surface roughness were tested for several inversion cases using or not using a-priori knowledge on soil parameters. The inversion approach was then validated using RADARSAT-2 images in polarimetric mode. The introduction of expert knowledge on the soil moisture (dry to wet soils or very wet soils) improves the soil moisture estimates, whereas the precision on the surface roughness estimation remains unchanged. Moreover, the use of polarimetric parameters α1 and anisotropy were used to improve the soil parameters estimates. These parameters provide to neural networks the probable ranges of soil moisture (lower or higher than 0.30 cm3 cm−3) and surface roughness (root mean square surface height lower or higher than 1.0 cm). Soil moisture can be retrieved correctly from C-band SAR data by using the neural networks technique. Soil moisture errors were estimated at about 0.098 cm3 cm−3 without a-priori information on soil parameters and 0.065 cm3 cm−3 (RMSE) applying a-priori information on the soil moisture. The retrieval of surface roughness is possible only for low and medium values (lower than 2 cm). Results show that the precision on the soil roughness estimates was about 0.7 cm. For surface roughness lower than 2 cm, the precision on the soil roughness is better with an RMSE about 0.5 cm. The use of polarimetric parameters improves only slightly the soil parameters estimates.
Highlights
Soil moisture content is an important parameter in hydrology, agronomy and as a boundary condition for land surface atmospheric interaction
The objective of this study is to develop an inversion technique based on neural networks to estimate soil surface parameters over bare agricultural areas from fully polarimetric RADARSAT-2 C-band SAR data
The neural networks developed above have been tested for the evaluation of the precision on soil moisture and surface roughness estimates
Summary
Soil moisture content is an important parameter in hydrology, agronomy and as a boundary condition for land surface atmospheric interaction. The radar backscatter is a function of soil moisture and surface roughness and sensor configuration (radar wavelength, incidence angle, polarization). The possibility of retrieving these soil parameters is insufficiently investigated from Cband polarimetric SAR (synthetic aperture radar) data. Extensive studies have been conducted to retrieve soil moisture by using mono- or multi-polarization C-band SAR data The availability of RADARSAT-2 data (C-band, ∼5.3 GHz) should enable improvements and increase the ability to retrieve soil parameters, based on RADARDAT’s capability of providing images in full polarization. When using only one radar channel (one incidence angle and one polarization), a better estimate of soil moisture is obtained for a SAR configuration that minimizes the effects of surface roughness (low incidence angle)
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