Fig. 4 presents a comparison between the L-, C-, and X-band RVIs and VWC over the soybean growth cycle. The correlation between the L-band RVI and VWC is very good (r = 0.98).
Prediction equations were developed using the RVI based on the observed linear relationships between RVI and VWC.
Root-mean-square error (rmse) was used as the primary metric for assessing retrieval performance. As a first step, prediction equations for VWC using the LAI and NDVI (rice only) were developed to establish a baseline (how well we might estimate VWC using visible–near-infrared observations) for intercomparison. Table II includes the results of this analysis and shows that both LAI and NDVI had good linear correlation with VWC
(r ≥ 0.89) and as a result the retrieval equations performed well.