| As a link of the surface water and groundwater,soil moisture plays a significant role in the earth surface energy balance and material exchange,which is the main source of water deprivation of crops and forest and grassland and has a close connection with water circulation,plants growth and groundwater environment.It is also an important parameter in the scientific research field of hydrometeorology,ecological environment and agriculture forest and husbandry.In the arid and semi-arid areas of the northern part in China,water resources shortage becomes the most obvious environment characteristic of this region due to little rainfall and strong evapotranspiration,thus,achievement of soil moisture inversion in the regional scale and analysis of its spatial characteristics have a great guiding significance for the water resource rational allocation and environment protection of this region.Microwave remote sensing technologies compensate the shortcomings(influenced by clouds and sun illumination conditions)of optical remote sensing,offers the ability to retrieve surface parameters with the characteristics of all time,all weather and penetrability,rendering it promising for soil moisture monitoring in large scale over bare soil surfaces and vegetation covered areas.In the present,the difficulty in the soil moisture inversion via microwave remote sensing is how to efficiently eliminate the effects of covered vegetation and surface roughness on the radar backscattering coefficients.The objective of this paper is to develop a suitable soil moisture inversion for the over sparsely vegetation covered areas in Wushen banner of Ordos city of Inner Mongolia through combing Radarsat-2 SAR and GF-1 data.For this purpose,based on the advanced Water-Cloud model,the impacts of vegetation on the radar backscattering coefficient was removed,and then,a coupling model for the soil moisture inversion of this study area was in turn developed through ATEM model and combined roughness parameter,and the distribution map of soil moisture content(SMC)levels(0-6cm)for sparsely vegetation covered surfaces was generated.Based on the soil moisture map,the responding relationships between soil moisture and environmental factors(geomorphic type,vegetation and landform)were further analyzed.The main study results are drawn as follows:(1)Based on different size of filter window and methods,the filtering effect for RADARSAT-2 SAR data was estimated,with an overall consideration of evaluation results,it indicates that the Gamma filter is the optimal method with a filter window of 5*5 for the study area.(2)The developed soil moisture inversion model has a good inversion precision based on the advanced Water-Cloud model,AIEM model and combined roughness parameter.The root mean square error(RMSE)and mean absolute error(MAE)were 5.38% and 3.83%,respectively.In addition,the model proposed in this paper has a higher precision with a comparison to other models in the same study area or the similar regions.(3)The soil moisture inversion results indicated that most soil moisture content in the study area is below 35%,the distribution of soil moisture is characterized by stripes directed northwest to southeast in accordance with the directions of sand dunes in the area,which is consistent with field observations.(4)Based on the distribution characteristic of soil moisture in the study area,the relationship between soil moisture and geomorphic type was further studied.The relationship of each geomorphic type and the corresponding averaged soil moisture is: bottomland(averaged soil moisture 17.31%)> sand dunes(averaged soil moisture 3.76%).(5)The distribution characteristics of soil moisture under different vegetation types were herbaceous vegetation(averaged soil moisture 22.8%)> salix mongolica(averaged soil moisture 8.03%)> nutans(averaged soil moisture 6.12%).The soil moisture content in the study area has a good positive linear relationship with NDVI(R2=0.8735).(6)The relationships between soil moisture with landform factors show that soil moisture is mainly affected by the elevation and slope.The results show that humid level is in a preponderant distribution when the elevation is lower than 1350 m,while in the range of 1350 ~ 1430 m,the arid level is in the dominant,and the humid level is in the dominant as the elevation is higher than 1430 m.In the lower slope range(< 3°),the humid level is in a preponderant,while the arid level is in the dominant with the slope of over 3°.The above conclusions are in accordance with the practical monitoring situation,which further verifies that the inversion model in this paper has a good performance in reflecting the real soil moisture situation of the study area and has a good reliability. |
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