Abstract
This study uses the brightness temperature at the given microwave frequency (18.7 GHz) from the Microwave Radiation Imager (MWRI) on-board the Fengyun-3B (FY-3B) satellite to improve the τ-ω model by considering the radiative contribution from waterbody in the pixels over the wetland of the Yellow River source region, China. In order to retrieve vegetation optical depth (VOD), a dual-polarization slope parameter is defined to express the surface emissivity in the τ-ω model as the sum of soil emissivity and waterbody emissivity. In the regions with no waterbody, the original τ-ω model without considering waterbody impact is used to derive VOD. With use of the field observed vegetation water content (VWC) in the source region of the Yellow River during the summer of 2012, a regression relationship between VOD and VWC is established and then the vegetation parameter b is estimated. The relationship is employed to derive the spatial VWC during the entire vegetation growing period. The VOD retrieved is invalid and failed in some part of the study area by using the previous τ-ω model, while the results from the improved τ-ω model indicate that the VOD is in the range of 0.20 to 1.20 and the VWC is in the range of 0.20kg/m2 to 1.40kg/m2 in the entire source region of the Yellow River in 2012. Both VOD and VWC exhibit a pattern of low values in the west part and high values in the east part. The largest regional variations appear along the Yellow River. The comparison between the remote-sensing-estimated VWC and the ground-measured VWC gives the root mean square error of 0.12kg/m2. These assessments reveal that with considering the fractional seasonal wetlands in the source region of the Yellow River, the microwave remote sensing measurements from the FY-3B MWRI can be successfully used to retrieve the VWC in the source region of the Yellow River.
Highlights
Vegetation water content, vegetation canopy biomass, and soil moisture strongly affect fluxes of the surface water vapor and energy, control the proportion of surface sensible heat, and latent heat allocated to the usable surface energy, as well as the proportion of rainfall among the surface runoff, infiltration, and evapotranspiration, and can have significant impacts on numerical weather, climate and hydrologic predictions [1,2]
The primary objectives of this paper are to (a) obtain more accurate vegetation optical depth (VOD) information on seasonal vegetation growth in the source region of the Yellow River based on theτ-ω model and open water fraction by using the brightness temperatures (Tb) at 18.7 GHz (K band) of the Fengyun-3B (FY-3B) Microwave Radiation Imager (MWRI) and (b) use canopy vegetation water content (VWC) from the field measurements and the correspondence between VOD record to obtain spatiotemporal distribution of VWC in the whole source region of the Yellow River
To examine if the FY-3B MWRI has a capacity to catch the vegetation signals and obtain more accurate information on seasonal vegetation growth in the source region of the Yellow River, we use FY-3B MWRI 18.7 GHz (K band) brightness temperatures provided by the National Satellite Meteorological Center and other ancillary data inputs to derive the VOD and VWC
Summary
Vegetation water content, vegetation canopy biomass, and soil moisture strongly affect fluxes of the surface water vapor and energy, control the proportion of surface sensible heat, and latent heat allocated to the usable surface energy, as well as the proportion of rainfall among the surface runoff, infiltration, and evapotranspiration, and can have significant impacts on numerical weather, climate and hydrologic predictions [1,2]. Measurements using passive microwave radiometers are more sensitive to variations in the characteristics of vegetation canopy, and can be directly used to retrieve canopy VWC without affected by weather conditions These observations have the potential to be used for all-weather monitoring of regional vegetation canopy. The primary objectives of this paper are to (a) obtain more accurate VOD information on seasonal vegetation growth in the source region of the Yellow River based on theτ-ω model and open water fraction (fw) by using the brightness temperatures (Tb) at 18.7 GHz (K band) of the Fengyun-3B (FY-3B) Microwave Radiation Imager (MWRI) and (b) use canopy VWC from the field measurements and the correspondence between VOD record to obtain spatiotemporal distribution of VWC in the whole source region of the Yellow River.
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