Error in Radar-Derived Soil Moisture due to Roughness Parameterization: An Analysis Based on Synthetical Surface Profiles

Hans Lievens,Bernard De Baets,Niko E.C Verhoest,Jesús Álvarez-Mozos,Hilde Vernieuwe

doi:10.3390/s90201067

Hans Lievens, Bernard De Baets + Show 3 more

Open Access

https://doi.org/10.3390/s90201067

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Abstract

In the past decades, many studies on soil moisture retrieval from SAR demonstrated a poor correlation between the top layer soil moisture content and observed backscatter coefficients, which mainly has been attributed to difficulties involved in the parameterization of surface roughness. The present paper describes a theoretical study, performed on synthetical surface profiles, which investigates how errors on roughness parameters are introduced by standard measurement techniques, and how they will propagate through the commonly used Integral Equation Model (IEM) into a corresponding soil moisture retrieval error for some of the currently most used SAR configurations. Key aspects influencing the error on the roughness parameterization and consequently on soil moisture retrieval are: the length of the surface profile, the number of profile measurements, the horizontal and vertical accuracy of profile measurements and the removal of trends along profiles. Moreover, it is found that soil moisture retrieval with C-band configuration generally is less sensitive to inaccuracies in roughness parameterization than retrieval with L-band configuration.

Highlights

Surface soil moisture plays a crucial role in various hydrological and agronomical processes: the top layer moisture content controls the infiltration rate during precipitation events and largely influences the amount of surface runoff, it drives the crop development, and affects the evapotranspiration rate and the micro-climate and -meteorology.The retrieval of soil moisture content from Synthetic Aperture Radar (SAR) relies on the dependency of the backscattered radar signal on the surface reflection coefficients of the sensed target [1]
Apart from soil moisture, the backscattered radar signal shows to be extremely dependent on the roughness state of the sensed surface, in most backscatter models described by the surface root mean square (RMS) height s, the correlation length l and an autocorrelation function (ACF) [4]
The present paper focuses on the influence of standard measurement techniques on the parameterization of roughness and its impact on soil moisture retrieval

Summary

Introduction

Surface soil moisture plays a crucial role in various hydrological and agronomical processes: the top layer moisture content controls the infiltration rate during precipitation events and largely influences the amount of surface runoff, it drives the crop development, and affects the evapotranspiration rate and the micro-climate and -meteorology. Discusses the sensitivity of this soil moisture retrieval to RMS height and correlation length, further, in Section 4., the generation of synthetical 1-dimensional roughness profiles is explained, and subsequently, a theoretical study on these synthetical profiles is performed in order to assess the influence of roughness parameterization techniques on soil moisture retrieval. Based on the results of Section 3., one can deduce that even small errors in roughness parameterization, of RMS height, may have a large impact on the soil moisture retrieval. To 4.6., different experiments are set up to demonstrate respectively the influences of the following parameterization aspects: the profile length, the number of profiles over which s and l are averaged, the spacing between height points, instrument accuracy and the trend removal technique applied. In order to assess the influence of roughness parameterization techniques on the soil moisture retrieval, a series of synthetical rough surface profiles is generated. These scaling properties are well known, the impact of using different profile lengths on the soil moisture retrieval has not been reported yet [14]

Experimental setup

Findings

Conclusions