Information on vegetation water content (VWC) is important in retrieving soil moisture using microwave remote sensing. It can be also used for other applications, including drought detection, bushfire prediction, and agricultural productivity assessment. Through the Soil Moisture Active Passive (SMAP) mission of the National Aeronautics and Space Administration, radar data may potentially provide the VWC information needed for soil moisture retrieval from the radiometer data acquired by the same satellite. In this paper, VWC estimation is tested using radar vegetation index (RVI) data from the third SMAP airborne Experiment. Comparing with coincident ground measurements, prediction equations for wheat and pasture were developed. While a good relationship was found for wheat, with    $r=0.49, 0.62,\ \mbox{and}\ 0.65$   and root-mean-square error    $(\mbox{RMSE})=0.42, 0.37,\ \mbox{and}\ 0.36$   kg/m 2 , the relationship for pasture was poor, with    $r=-0.06,-0.14,\ \mbox{and}\ -0.002$   and    $\mbox{RMSE}=0.15,0.15,\ \mbox{and}\ 0.15$  , kg/m 2 , for 10-, 30-, and 90-m resolutions, respectively. These results suggested that RVI is better correlated with VWC for vegetation types having a greater dynamic range. However, the results were not as good as those from a previous tower-based study (   $r=0.98$   and    $\mbox{RMSE}=0.12\ \mbox{kg/m}^2$  ) over wheat. This is possibly due to spatial variation in vegetation structure and surface roughness not present in tower studies. Consequently, results from this study are expected to more closely represent those from satellite observations such as SMAP, where large variation in vegetation and environment conditions will be experienced.
['Yuancheng Huang', 'Jeffrey P. Walker', 'Ying Gao', 'Xiaoling Wu', 'Alessandra Monerris']
Estimation of Vegetation Water Content From the Radar Vegetation Index at L-Band