A Field Verification of an Algorithm for Retrieving Vegetation Water Content From Passive Microwave Observations

Abstract

We present a field-verified algorithm for retrieving vegetation water content (VWC), which is the mass of water in vegetation tissue per ground area, using observed microwave brightness temperatures (TBs). We can use 6.925- and 10.65-GHz microwave observations to minimize the species dependence of the relationship between vegetation optical depth (VOD) and VWC. Then, we can easily estimate the VWC after obtaining the VOD. Although the VOD retrieved at these frequencies is highly affected by uncertainties in the surface roughness, we found that the effects of the bias of the roughness parameters in a radiative transfer model can effectively be eliminated by introducing leaf area index (LAI) data. The TBs observed using ground-based microwave radiometer and field-observed LAI were used to successfully reproduce the field-observed VWC below 2.0 kg/m2 ( $R^2=0.712$ and $\text{RMSE}=0.393 \text{kg/m}^2$ ). This strategy of surface roughness correction also positively impacts the performance of soil moisture retrieval. This field-verified algorithm can contribute to global-scale VWC retrieval using satellite microwave radiometers such as the Advanced Microwave Scanning Radiometer on Earth Observation Systems (AMSR-E) and its successor (AMSR2).