Review on Soil Moisture Remote Sensing Techniques and Spatial Resolution Downscaling Methods

Soil moisture is a valuable parameter for water cycle over lands, controlling water fluxes between the atmosphere, land surface and subsurface, through evaporation and plant transpiration. Measurement and recording of soil moisture observations as in-situ data cannot meet human needs. The availability of global soil moisture maps which is possible using remote sensing sensors, will benefit many application, including precipitation forecasting, flood prediction, drought monitoring and agricultural related applications. These maps must be provided at suitable scale. This is done by spatially downscaling of the soil moisture observations. Spatial downscaling of soil moisture measured by satellites should provide two purposes in order to use this parameter in hydrological, meteorological and agricultural applications: 1- Achieving to medium resolution (approximately 10 km), 2- Sufficient retrieval accuracy. It is evident that fulfilling both purposes using a single sensor is difficult. Therefore, complementary downscaling using a range of observation types has been proposed as an approach to overcome these scale and accuracy issues, by combining the merits from different sensors. In this paper, the soil moisture remote sensing techniques and as well, two basic available downscaling approaches which have the potential to fulfil the stated requirements on resolution and accuracy are introduced and their pros and cons are investigated.