Dynamic b p in the L Band and Its Role in Improving the Accuracy of Soil Moisture Retrieval

The parameter b p in the tuo-omega ( τ – ω ) model is important for retrieving soil moisture data from passive microwave brightness temperatures. Theoretically, b p depends on the observation mode (polarization, frequency, and incidence angle) and vegetation properties and varies with vegetation growth. For simplicity, previous studies have taken b p to be a constant. However, to reduce the uncertainty of soil moisture retrieval further, the present study is of the dynamics of b p based on the SMAPVEX12 experimental dataset by combining a genetic algorithm and the L-MEB microwave radiative transfer model of vegetated soil. The results show the following. First, b p decreases nonlinearly with vegetation water content ( VWC ), decreasing critically when VWC becomes less than 2 kg/m 2 . Second, there is a power law between b p and VWC for both horizontal and vertical polarizations ( R 2 = 0.919 and 0.872, respectively). Third, the effectiveness of this relationship is verified by comparing its soil-moisture inversion accuracy with the previous constant- b p method based on the HiWATER dataset. Doing so reveals that the dynamic b p method reduces the root-mean-square error of the retrieved soil moisture by approximately 0.06 cm 3 /cm 3 , and similar improvement is obtained for the calibrated SMAPVEX12 dataset. Our results indicate that the dynamic b p method is reasonable for different vegetation growth stages and could improve the accuracy of soil moisture retrieval.