A Method for Downscaling FengYun-3B Soil Moisture Based on Apparent Thermal Inertia

Chengyun Song,Li Jia

doi:10.3390/rs8090703

Abstract

FengYun-3B (FY-3B) soil moisture product, retrieved from passive microwave brightness temperature data based on the Qp model, has rarely been applied at the catchment and region scale. One of the reasons for this is its coarse spatial resolution (25-km). The study in this paper presented a new method to obtain a high spatial resolution soil moisture product by downscaling FY-3B soil moisture product from 25-km to 1-km spatial resolution using the theory of Apparent Thermal Inertia (ATI) under bare surface or sparse vegetation covered land surface. The relationship between soil moisture and ATI was first constructed, and the coefficients were obtained directly from 25-km FY-3B soil moisture product and ATI derived from MODIS data, which is different from previous studies often assuming the same set of coefficients applicable at different spatial resolutions. The method was applied to Naqu area on the Tibetan Plateau to obtain the downscaled 1-km resolution soil moisture product, the latter was validated using ground measurements collected from Soil Moisture/Temperature Monitoring Network on the central Tibetan Plateau (TP-STMNS) in 2012. The downscaled soil moisture showed promising results with a coefficient of determination R2 higher than 0.45 and a root mean-square error (RMSE) less than 0.11 m3/m3 when comparing with the ground measurements at 5 sites out of the 9 selected sites. It was found that the accuracy of downscaled soil moisture was largely influenced by the accuracy of the FY-3B soil moisture product. The proposed method could be applied for both bare soil surface and sparsely vegetated surface.

Highlights

Soil moisture is one of the important variables in hydrological and meteorological models, and has been used in numerous applications, such as weather forecasting, drought monitoring and flood area mapping, among others [1,2]
This study presented a method of downscaling the FY-3B 25-km soil moisture product based on the relationship between soil moisture and Apparent Thermal Inertia (ATI) by using satellite data from optical/thermal bands at high spatial resolution (1 km)
The proposed method assumes that the relationship between soil moisture and ATI is the same but the parameters of the model are different at different spatial scales

Summary

Introduction

Soil moisture is one of the important variables in hydrological and meteorological models, and has been used in numerous applications, such as weather forecasting, drought monitoring and flood area mapping, among others [1,2]. Passive microwave remote sensing is suitable for monitoring soil moisture for its high sensitivity to soil moisture, ability to penetrate to vegetation canopy and less influence by clouds [3,4]. The Microwave Radiation Imager (MWRI) onboard the Fengyun-3B (FY-3B) satellite is a passive microwave sensor, which provides brightness temperature observations at five frequencies from 10.7 GHz to 89 GHz. FY-3B soil moisture product is retrieved using 10.7 GHz brightness temperature based on the Qp model [5]. Onboard NASA’s Earth Observing System and Soil Moisture and Ocean Salinity (SMOS) [6,7], the spatial resolution of MWRI onboard FY-3B is from 18 km to 85 km and the resolution of its soil moisture product is about 25 km .

Methods

Discussion

Conclusion