The Soil Moisture Active Passive (SMAP) "Soil Moisture 9 km (L2, Passive, Night, Single Channel Algorithm, V Polarization)" layer displays surface soil moisture in cm³/cm³ derived from the Single Channel Algorithm V-Pol (SCA-V), the baseline soil moisture algorithm, for the 6:00 p.m. ascending (Night) half-orbit passes of the SMAP radiometer. The SMAP radiometer measures natural thermal emission emanating from the soil surface. To enhance the grid resolution, Backus-Gilbert optimal interpolation techniques are used to extract maximum information from SMAP antenna temperatures and convert them to brightness temperatures, which are posted to the 9 km EASE-Grid 2.0. In the SCA-V, the vertically polarized brightness temperature observations are converted to emissivity using a surrogate for the physical temperature of the emitting layer. The derived emissivity is corrected for vegetation and surface roughness to obtain the soil emissivity. The Fresnel equation is then used to determine the dielectric constant from the soil emissivity. Finally, a dielectric mixing model is used to solve for the soil moisture given knowledge of the soil texture.

The SMAP spacecraft carries two instruments, a radar (active) and a radiometer (passive), that together make global measurements of land surface soil moisture and freeze/thaw state. It is useful for monitoring and predicting natural hazards such as floods and droughts, understanding the linkages between Earth’s water, energy and carbon cycles, and reducing uncertainties in predicting weather and climate.

References: SMAP Enhanced L2 Radiometer Half-Orbit 9 km EASE-Grid Soil Moisture

Data field: soil_moisture_option2