Abstract
Core Ideas In the proposed model, the wilting point and porosity are a function of organic matter. In organic‐rich soil, the model improves accuracy of the microwave radiative transfer model. The model is applicable for both portable and satellite soil moisture sensors. Most dielectric mixing models have been developed for mineral soils without extensive consideration of organic matter (OM). In addition, when used for in situ measurement, most of these models focus only on the real part of the effective dielectric constant without the corresponding imaginary part. Organic matter fractions in soils are found globally (57%), with an especially significant amount in the boreal region (17%). Without proper consideration of OM in dielectric mixing models and subsequent microwave radiative transfer modeling, brightness temperature (TB) calculations may be erroneous. This would lead to uncertainties in the estimation of higher level products, such as soil moisture retrievals from portable soil moisture sensors (e.g., time‐domain reflectometers) or passive microwave sensors onboard the Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), and Advanced Microwave Scanning Radiometer (AMSR2) satellites. We incorporated OM into a dielectric mixing model by adjusting the wilting point and porosity according to the OM content, i.e., the effective soil dielectric constant decreases with higher OM due to a decrease in the fraction of free water and an increase in bound water. With the proposed soil parameters in the dielectric mixing model, high levels of OM increase the TB for a specific soil moisture by decreasing the microwave effective dielectric constant. The simulated TB better reproduced SMAP‐observed TB (11% in RMSE) through the improvement of the effective dielectric constant (40% reduction in RMSE). We anticipate that the application of our approach can improve microwave‐based surface soil moisture retrievals in areas with high OM.
Highlights
Moisture and Ocean Salinity (SMOS)2 missions currently provide information about global soil moisture
In order to assess the effect of organic matter (OM) on the accuracy of soil moisture estimation, our proposed dielectric mixing models (DMMs)
We developed a dielectric mixing model to account for the effect of soil organic matter on microwave signals explicitly
Summary
Moisture and Ocean Salinity (SMOS) missions currently provide information about global soil moisture. In the process of such soil moisture estimation from microwave radiation observations, several factors may introduce uncertainty. The misspecification of the vegetation, soil surface roughness, or physical properties of the soil in a radiative transfer model (RTM) can lead to erroneous soil moisture estimates. In the RTM, dielectric mixing models (DMMs) are important in linking soil moisture to emissivity. DMMs mix the dielectric properties of water and soil, while accounting for physical soil properties such as soil temperature and texture. The efforts to build more sophisticated and accurate DMM are important for soil moisture estimation from satellite data, and for ground-based soil moisture instruments such as time-domain reflectometry (TDR) and groundpenetrating radar (GPR) sensors
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