Inversion Of Soil Moisture Based On Dual-polarization RADARSAT-2 Data

Soil moisture is one of the most important parameters on the land surface.It plays an important role in the global water cycle,energy balance and climatic changes.Its spatial distribution and dynamic changes play a decisive role in the evolution of the ecological environment.The arid and semi-arid regions in western China are vast in area and have poor natural conditions.Due to the combined effects of climate change and human activities,the river flowed off,the lake disappeared,the vegetation degraded,and the oasis shrank.For a long time,the lack of water resources in the arid areas,the problems of land desertification and salinization are very prominent.Soil moisture is one of the most active factors in the arid and semi-arid ecosystems,which restricts the evolution of the ecological environment and the stability of the ecosystem.Therefore,quantitative inversion of soil moisture in arid and semi-arid regions has very important practical significance for understanding the environmental evolution and ecological environment protection in arid and semi-arid areas.Due to its unique advantages such as all-weather,all-day,penetrating,and sensitivity to surface roughness,feature geometry and dielectric properties,microwave remote sensing technology has been widely used to monitor large areas of soil moisture.In the process of retrieving soil moisture by microwave remote sensing,surface roughness is an important factor affecting backscattering.The commonly used solution is to use the combination roughness to combine the root mean square height S and the correlation length L to reduce the number of parameters in the soil moisture inversion model.Although the method reduces the number of parameters of the model and simplifies the inversion model,its form is not convenient for quantitative inversion of the surface roughness parameters S and L in the subsequent work.Therefore,in this study,the Juyanze,a dried-up lake located to the northeast of Ejina Banner,Inner Mongolia China,was used as the study area.The backscattering characteristics of the bare surface in arid area were firstly explored using the single-scattering AIEM model.based on this,the optimal application scope of the single-scattering AIEM model was analyzed.Then,using the simulated data of the single-scattering AIEM model,the experiential relationship between the backscattering coefficients,the mean values of co-polarized backscattering coefficient and the surface RMS height,the correlation length,and the Normal Fresnel reflection coefficient was directly discussed,and the experiential relationship between the Normal Fresnel reflection coefficient and soil moisture was directly discussed.Combined with field measured data,this article establishes an experiential model that is suitable for inversion of soil moisture in this study area.Finally,combined with RADARSAT-2 remote sensing images,the soil moisture of the study area(0-5cm)was quantitatively inverted.The main findings are as follows:(1)In the extremely arid Juyanze area,the backscattering coefficients increases gradually with the decrease of the incidence angle,the increase of soil water content and root mean square height,as well as the decrease of the correlation length.Meanwhile,with the increase of the incidence angle,the soil moisture content and root mean square height,as well as correlation length,the sensitivity of backscattering coefficient to all the above parameters decreases slowly over the bare surface.(2)The surface parameters have a great influence on the backscattering coefficients of the bare surface in the study area.Among them,the roughness parameters S and L have greater influence on the backscattering characteristics than the soil moisture Mv.The backscattering characteristics of the bare surface of the study area show strong regularity.The empirical and semi-empirical model between the backscattering coefficients and the surface parameters can be used to quantify the surface parameters.(3)Based on the analysis of single-scattering AIEM model simulation data,the single-scattering AIEM model can accurately simulate the roughness range of the backscatter coefficient as S?(0.2cm,1.4cm),L?(12cm,33cm).(4)Based on the simulation database of AIEM model,this paper constructed an empirical model for soil moisture inversion with a wide range of incident angles,and combined with field measured data to establish an empirical model for soil moisture retrieval in the study area.The analysis shows that the correlation coefficient between the soil water content retrieved from the empirical model and the soil water content measured in the field reaches 0.826,and is significantly correlated at the 0.01 level.The empirical model has high inversion accuracy.(5)Based on the RADARSAT-2 SAR image of HH and VV polarization,the soil moisture inversion model was used to quantitatively invert the soil moisture in Juyanze area.The results showed that the soil moisture content in the study area was overall low,and the soil moisture content in most areas was less than 20%.The soil moisture content of the model inversion was consistent with the actual area of the study,indicating that the empirical model has high accuracy and is suitable for quantitative inversion of soil moisture in the study area.