Inversion Of Soil Moisture Content In Fire Scars Of The Genhe Forest Area By Combing Of Optical And Microwave Remote Sensing Data

Forest fire is an important ecological disturbance for forest ecosystems.Forest fires have caused a large number of emission of greenhouse gases,soot and other noxious gas,serious soil erosion and environmental pollution,at the same time global warming further aggravated the generation of forest fires;On the other hand,forest fires can promote the renewal of forest ecosystems,regulate hydrological processes on the landscape scale,and affect global surface energy balance.Soil as the important environmental factor that affects the regeneration of forest ecosystem is the main object for forest monitoring after fire.Therefore,the continuous monitoring of soil properties in the fire scars can provide a basis for forest renewal evaluation,forest resource restoration of management and soil and water conservation,especially the monitoring of soil moisture content.Remote sensing technology can overcome the shortcomings of ground survey to continuously monitor the whole fire scars.The historical data of optical remote sensing is abundant and the method is mature,but it is greatly affected by the weather.Synthetic aperture radar(SAR)is a potential tool of remote sensing,because it can observe all day and night regardless of weather condition such as clouds and rain.However,there has been no research on the monitoring of water content of fire scars in large areas based on the combined optical and microwave remote sensing data and the microwave remote sensing model.This study had fire scars of the Genhe Forest Farm in Inner Mongolia as the study area.Based on Seninel-1 SAR data,optical data,CLDAS and other auxiliary data,as well as a coupling of advanced integrated equation model(AIEM)and water cloud model(WCM),this study set up a retrieval method of soil water content suitable for fire scars.Then soil water content in fire scars of Genhe Forest Farm has been analyzed to explore the effects of terrain and rainfall.The main research contents and conclusions are as follows:(1)Establishing the map of WCM coefficients around Genhe forest regionBased on the concept of effective surface roughness,using AIEM to simulate the surface backscattering coefficient by taking the CLDAS soil moisture time series data as training data,and coefficients of the water cloud model were properly calibrated.(2)Accuracy evaluation of the coupling inversion method of soil moisture content by AIEM and WCMThe accuracy of the inversion of soil moisture content was evaluated by different data set.Through the inversion of soil water content in the sample sites of different years of fire scars,the result showed a significant linear correlation with the measured value(R~2=0.87).Through the inversion in different years of heavily burned scars,the normalized difference water index(NDWI)was highly correlated to soil moisture content of fire scars(R:0.83?0.91),and the spatial distribution features were highly similar.Through inversion of soil moisture content of all Sentinel-1 SAR data from June to August 2018 in heavily burned scars,it was consistent with the change trend of accumulated precipitation of CLDAS.(3)Difference analysis of soil average moisture content under different topographic factors in fire scarsThe influences of elevation,slope and aspect on different severely burned areas were different.Firstly,when the recovery period reached 8 years.In the lightly burned areas,the soil moisture content was less affected by topographic factors.In the moderate burned areas,the soil moisture content was less affected by the slope and aspect,but decreased with the elevation.In the heavily burned areas,the soil moisture content decreased with the increase of elevation,increased with the increase of slope,and was lower than that in lightly burned areas in all aspects;When the recovery period reached 15 years.In the lightly burned areas,The influences of topographic factors on the soil moisture content were still not obvious.In the moderate burned areas,the soil moisture content was lower than that in lightly burned areas in poor water and heat conditions with steep slope(>25°)or north aspect.In the heavily burned areas,the soil moisture content exceeded the moderate burned areas and reached the lightly burned areas level under steep slope(>25°)and north aspect with poor water and heat conditions.Generally,the effect of topographic factors on lightly burned areas was small and vegetation recovered fast.When the habitat condition is good,moderate burned areas were less affected by topographic factors and vegetation recover fast.The vegetation destruction in heavily burned areas was serious but the surface was less affected by the topographic factors and renewed rapidly.(4)Analysis of the distribution characteristics of the moisture content in the soil under different precipitation conditionsIn the recovery period of 8 years,lightly,moderate and heavily burned areas was still in the stage of vegetation recovery.The soil moisture content was significantly affected by precipitation conditions in some of lightly burned areas and total moderate and heavily burned areas.After short-term precipitation,the soil moisture content was relatively high,while in the long-term non-precipitation,the soil moisture content was generally low.When the recovery period reached 15 years,the lightly burned areas recovered well,and there was no significant dominant distribution of soil average moisture content under the conditions of short-term precipitation and long-term non-precipitation.The overall recovery of moderate and heavily burned areas was poor,and the average soil moisture content under the conditions of short-term precipitation and long-term non-precipitation showed obvious dominant distribution,as well as the surface of moderate and heavily burned areas recovered well for the surface water holding capacity improved.In this study,a method of soil moisture content inversion based on SAR data has been established for fire scars in Genhe Forest Farm,and the overall inversion accuracy of the method was acceptable(R~2=0.87).Then,the characteristics of soil water content in different severity of burned areas under different environmental factors have been analyzed.Finally,the vegetation restoration in fire scars was indirectly analyzed.These inversion methods and analysis methods may play a certain technical guidance role for remote sensing monitoring of vegetation restoration in burning and cutting areas in Daxing'An Mountains and provide feasible ideas for soil moisture monitoring in other fire scars.