Impact of Moisture Distribution Within the Sensing Depth on L- and C-Band Emission in Sandy Soils

The performances of the soil moisture retrieval and assimilation algorithms using microwave observations rely on realistic estimates of brightness temperatures (T_B) from microwave emission models. This study identifies circumstances when current models fail to reliably relate near-surface soil moisture to an observed T_B at L-band; offers a plausible explanation of the physical cause of these failures; and recommends improvements needed so that L-band observations can provide reliable estimates of soil moisture, more universally. Physically consistent soil parameters and moisture at the surface were estimated by using dual-polarized C-band observations during an intensive field experiment, for an irrigation event and subsequent drydown. These derived parameters were used in conjunction with the in situ moisture in deeper layers and different moisture profiles within the moisture sensing depth to obtain estimates of H-pol T_B at L-band, that provided best matches with the observed T_B. The general assumptions of linear moisture distribution, with uniform or exponentially decaying weighting functions provided realistic T_B during the later stages of the drydown. However, the RMSDs of the T_Bs were up to 10.37 K during the wet period. In addition, the use of one value of moisture representing the entire moisture sensing depth during this highly dynamic stage of the drydown provides unrealistic estimates of emissivity, and hence, T_B at L-band. This study recommends use of a hydrological model to provide dynamic, realistic soil moisture profiles within the sensing depth and also an improved emissivity model that utilizes these detailed profiles for estimating T_B.