Transferability of L-band SAR soil moisture retrieval parameters across surface conditions and view angles over wheat fields

Abstract

Synthetic Aperture Radar (SAR) offers high-resolution surface soil moisture in all-weather conditions, allowing continuous Earth observation. However, a significant challenge in soil moisture retrieval using SAR data exists in the calibration of retrieval parameters applicable to a broader range of environments and canopy densities. The site-specific calibration methods used in existing SAR soil moisture retrieval algorithms limit their operational applicability. In this study, we evaluated the transferability of soil moisture retrieval parameters across various sensing configurations and canopy densities to reduce the need for site-specific calibration and dependence on extensive ground data. We utilised quad-pol L-band imagery collected by UAVSAR during the SMAP Validation Experiment 2012 (SMAPVEX12), focusing on wheat fields. The backscatter from the vegetation-soil system is modelled using the Water Cloud Model (WCM). This approach resulted in accuracy (RMSE = 0.07 $m^3{m}^{-3}$, correlation coefficient = 0.7) similar to retrievals using ground-based vegetation parameters. The potential transferability of WCM parameters across fields, canopy densities, polarisations and incidence angles was examined using the posterior distribution of WCM parameters obtained from Markov Chain Monte Carlo (MCMC) calibration and extensive cross-validation. Both the posterior parameter distributions and cross-validation results suggest that WCM parameters are not transferable across different incidence angles nor between co-pol and cross-pol images. However, they are transferable across a wide range of canopy densities, between VV & HH polarisations, and across different locations when UAVSAR data with similar incidence angles are used as input.