SMAP Active And Passive Microwave Observations Synergy For Soil Moisture Retrieval

Soil moisture is the critical factors for hydrological,biological and biogeochemical processes.It affects the climate change through affecting the incident energy on the distribution of sensible heat and latent heat flux,which is of great significance to the research of hydrometeorology,disaster monitoring,crop yield estimation,water resource management and other related disciplines.Therefore,the rapid acquisition of soil moisture with different spatial scales and precision is extremely important to improve the simulation quality of water,energy and carbon models between the surface and the atmosphere,and to better understand the global water,energy and carbon cycle.Since microwave remote sensing has the ability to observe the earth’s surface throughout all-time and all-weather conditions,and has a certain ability to penetrate the vegetation,which has become one of the main means of estimating soil moisture.The active microwave is characterized by high spatial resolution,low time resolution and sensitive to vegetation and surface roughness;the passive microwave is characterized by high temporal resolution,low spatial resolution and sensitive to soil moisture.If the advantages of active and passive microwave could be combined,soil moisture products with high spatial resolution,high temporal resolution and high accuracy would be obtained.SMAP is an earth observation satellite equipped with L band active and passive microwave.This research is based on the active and passive observation data of SMAP satellite.Based on the active and passive observations of SMAP satellite,the study was carried out.1.The dynamic relationship between the microwave polarization difference index(MPDI),the soil moisture and the vegetation was analyzed with the active and passive observations of SMAP at the global scale.By calculating the explained variance(EV)of soil moisture and vegetation,it was found that the explained variance of soil moisture could largely explain the dynamic change of microwave polarization difference index in low vegetation coverage areas(vegetation water content was less than 3kg/m3).For densely vegetated areas,the explained variance of soil moisture was smaller because the microwave polarization difference index was less affected by soil moisture change.In addition to the transition between dry and wet seasons in farmland and subtropical regions,extensive vegetation growth had been found during the study period,so that the explained variance of vegetation was particularly small and almost zero.While the Russian and the northeastern United States were higher,probably due to the seasonal crop growth in the northern hemisphere in spring and summer.2.In this study,the cost function between the forward model simulation value and the satellite observations was constructed by using the active backscatter and passive brightness temperature observations of SMAP.The Levenberg-Maquardt algorithm was used to calculate the optimal solution of the cost function.When the optimal solution of the cost function was calculated,the soil moisture simulated by the forward model was regarded as the retrieval soil moisture.In this study,the inversion results of all possible channel combinations were compared and analyzed.The best inversion channel combination was screened out to retrieve the soil moisture with the spatial resolution of 36 km.The soil temperature and moisture observation network located in Qinghai Tibet Plateau was selected to verify the accuracy of the inversion results.The results showed that the accuracy of soil moisture retrieved by the latest inversion algorithm of this study is higher than that of SMAP 36 km soil moisture products.At the same time,the effect of SMAP satellite observation underestimating the surface effective temperature on the inversion result was also improved.3.The soil moisture products of 9km spatial resolution were retrieved by the descending scale with the low spatial resolution brightness temperature(36km)and the high spatial resolution backscatter(9km)of SMAP satellite.In the study,because the brightness temperature of 36 km didn’t match the backscatter data of 9km,the brightness temperature of the 36 km was used to substitute for the brightness temperature of the 9km.Finally,the soil moisture of 9km can be retrieved synergisticallly by the cost function of construction.For the verification of soil moisture retrieval results,the soil temperature and moisture observation network of Naqu watershed was also selected for verification.The results showed that the inversion accuracy with the brightness temperature of 36 km instead of the brightness temperature of 9km was higher than that of the soil moisture products of 9km with SMAP active and passive microwave observations.