Remote sensing applications to hydrology: Soil moisture

Passive and active microwave remote sensing instruments are capable of measuring the surface soil moisture (0-5 cm) and can be implemented on high altitude platforms, e.g. spacecraft, for repetitive large area observations. The amount of water present in a soil affects its dielectric properties. The dielectric properties, along with several other physical characteristics, determine the microwave measurement. In addition, the significance of the dielectric properties depends upon the sensor design, especially the wavelength. Instruments operating at longer wavelengths (>5 cm) have fewer problems with the atmosphere and vegetation, sense a deeper soil layer and maximize soil moisture sensitivity. Another instrument concern is whether to use an active or passive microwave approach. Active approaches, especially synthetic aperture radar, can provide extremely good ground resolution from space (<100 m). Passive methods currently provide much coarser resolution data (>10 km). The existing data interpretation algorithms for passive data are well tested for bare soil and vegetation and can be applied to a wide range of conditions. At the present time, the active microwave algorithms have not been widely verified. There has been a significant amount of recent research using both active and passive methods as a result of the availability of new sensor systems. With these new instruments have also come greater efforts to integrate the observations in large scale multidisciplinary investigations. A greater emphasis on the spatial distribution and temporal behaviour of soil moisture has produced some very interesting and valuable data sets that demonstrate the potential of a dedicated observing system for scientific and operational studies.