Assessment of Soil Moisture SMAP Retrievals and ELBARA-III Measurements in a Tibetan Meadow Ecosystem

This letter presents the results evaluating retrievals of liquid water content ( $\theta _{\mathrm {liq}}$ ) performed with a zero-order radiative transfer ( $\tau $ – $\omega$ ) model under frozen and thawed soil conditions from Soil Moisture Active Passive (SMAP) and ELBARA-III brightness temperature ( $T_{\mathrm {B}}^{p}$ ) measurements collected over a Tibetan meadow ecosystem. A good agreement is found between time series of the SMAP and ELBARA-III measured $T_{\mathrm {B}}^{p}$ resulting in a Pearson product-moment coefficient ( $R$ ) larger than 0.87. Differences noted between the two data sets can be associated with discrepancies in $\theta _{\mathrm {liq}}$ measured in the specific footprints, whereby the SMAP measurements are best explained by the in situ $\theta _{\mathrm {liq}}$ . Furthermore, the in situ $\theta _{\mathrm {liq}}$ has a better agreement with the horizontally polarized SMAP and ELBARA-III measurements ( $T_{\mathrm {B}}^{\mathrm {H}}$ ) in the cold season, whereas the vertically polarized measurements ( $T_{\mathrm {B}}^{\mathrm {V}}$ ) are better correlated with $\theta _{\mathrm {liq}}$ in the warm season. With the implementation of new vegetation and surface roughness parameterizations for the $\tau $ – $\omega $ model, the dynamics of in situ $\theta _{\mathrm {liq}}$ is better reproduced by corresponding retrievals for both frozen and thawed soil conditions, leading to the reduction in the unbiased root-mean-square error (ubRMSE) by more than 31% in comparison with these retrievals using SMAP default parameterizations. Notably, the single-channel algorithm configured with the new parameterizations using SMAP $T_{\mathrm {B}}^{\mathrm {V}}$ measured during the ascending overpass provides the best $\theta _{\mathrm {liq}}$ retrievals with a ubRMSE of 0.035 $\text{m}^{3}\cdot \text{m}^{-3}$ that is well within the SMAP mission requirements.