Space-borne Passive Microwave Remote Sensing of Soil Moisture: A Review

Soil moisture (SM) plays an important role in groundwater recharge, runoff, air moisture, atmospheric temperature, land degradation, geo-chemical process, surface and sub-surface biota including agriculture, etc. Several studies have reported comparable results of space-borne passive microwaves remote sensing of surface (2 to 5 cm) SM using relationship between volumetric SM and brightness temperature, dielectric constant, polarimetry, roughness parameter, mathematical inferences, etc. However, reported success studies are very site-, image -, overpass time-, model- and situation specific and insufficient to reach at global conclusions. Therefore, reported methods, techniques and algorithms should thoroughly be tested in different biophysical environments to assess applicability in SM retrievals using microwave data captured at different frequencies, polarization, angle, time, etc.