Status of microwave soil moisture measurements with remote sensing

Active and passive zaicrowave remote sensing techniques have demonstrated their potential for measurements of soil moisture. However, the soil moisture response from them is coupled to vegetation and surface roughness effects, and therefore the interaction among all three needs to be understood. This paper reviews the progress made in the measurement of soil moisture and the factors such as vegetation and surface roughness that affect these measurements. The active techniques, particularly those employing synthetic aperture radar (SAR), provide opportunities for soil moisture studies over a large area, and varbus aircraft and space missions have been carried out to achieve them. Still, there are unresolved questions about deriving soil moisture from these missions, and research is underway to develop algorithms so that soil moisture information can be obtained on a local as well as a global basis.