Impact of temporal variations in vegetation optical depth and vegetation temperature on L-band passive soil moisture retrievals over a tropical forest using in-situ information

The Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) missions provide estimates of soil moisture (SM) at similar spatial resolutions using L-band brightness temperatures (T_B). These missions meet the requirement of SM retrievals with an unbiased root-mean-square difference (ubRMSD) < 0.04 (m³ m⁻³) compared to in-situ measurements over most of the ecosystems; however, their SM estimates over forests present an ubRMSD > 0.10 (m³ m⁻³). In this paper, we compared the SM retrievals from the SMOS and SMAP SM products with in-situ SM over a tropical forest in Southern Mexico. The L-band passive SM retrievals were evaluated in terms of four statistical metrics: root-mean-square difference (RMSD), bias, ubRMSD, and correlation coefficient (r). In-situ measurements of SM, soil and vegetation temperatures, precipitation, soil surface roughness, tree heights, diameters at the breast height of trunks, and forest cover fraction were collected during a field campaign from 6 January to 14 June 2015 in the Biosphere Reserve of Calakmul, Mexico, covering two areas of about 40 km × 40 km each. The comparison between SM retrievals from SMOS and SMAP and in-situ SM showed an RMSD ranging from 0.107 to 0.322 (m³ m⁻³) and an ubRMSD from 0.049 to 0.128 (m³ m⁻³). Overall, the SMAP SM estimates showed higher values of r and were closer to in-situ SM. Because the SMAP and SMOS radiometers performed very similar, these differences are due to the values assigned to the vegetation optical depth (τ), the scattering albedo (ω), and the representation of the dynamics in vegetation and soil temperatures in the SMOS and SMAP retrieval algorithm. Based on an optimization process, we estimated simultaneously optimal ω and τ values for the tropical forest by using T_B observations from SMAP and SMOS radiometers. The optimal value of ω was 0.0655 for the tropical forest, and constant over the study period. In contrast, the optimal values of τ showed to be variant on time and ranging between 1.0 and 1.7, with an averaged value of 1.4 and a standard deviation of 0.24. When applying the optimal values of ω and τ and in-situ soil and vegetation temperatures, the SM retrievals showed an ubRMSD of 0.035–0.070 (m³ m⁻³), improving the SM retrievals about 45%. A sensitivity analysis was conducted to evaluate the effect of the uncertainties in τ,ω, and soil and vegetation temperatures on the estimates of T_B. It was found that vegetation temperature (T_veg) was the most sensitive parameter, with r higher than 0.70 for both polarizations in T_B. When comparing in-situ T_veg and surface temperature values used in the SMAP and SMOS SM retrieval algorithms, differences up to 10 K were observed, affecting the SM estimates. The results presented in this paper could be useful in the preparation of the SMAP Calibration/Validation Experiment 2019, aiming at improving SM retrievals over forests.