Soil Moisture Retrieval Techniques Using Satellite Remote Sensing

Abstract

Soil moisture is required to understand the land surface processes, land-atmosphere interaction, drought forecasting, crop growth patterns, etc. It is a dynamic variable that changes significantly on different spatial and temporal scales even in a smaller area. Remote sensing (RS) techniques provide an alternative way to estimate the high spatial and temporal variability of soil moisture. This chapter includes the state of art and techniques used to retrieve soil moisture from satellite RS in different parts of the world. Several techniques have been developed to retrieve soil moisture either from optical/thermal/microwave sensors or fusion of these sensors, but microwave sensors either with a fine spatial resolution (and coarse temporal resolution) or with a coarse spatial resolution (and fine temporal resolution) seem promising than optical and thermal sensors depending on applications. However, microwave sensors have shown its high potential and capability for deriving global soil moisture information due to all weather capability and longer penetration depth. Many operational products of soil moisture have been developed using passive microwave sensors; however, coarse spatial resolution and penetration depth over vegetation-covered surfaces are the major factors that limit the utility of these soil moisture products for agricultural purposes. The major initiatives have been taken by various space agencies across the globe to develop the microwave sensors with L and/or S bands for its potential use in soil moisture besides other applications. Sentinel-1 synthetic aperture radar (SAR, active microwave sensor) data has opened up a new research area to develop high-spatial-resolution soil moisture products for agricultural applications. Several studies have shown the potentials of Sentinel-1 SAR data and high-resolution optical data along with operational products (like SMAP) to downscale the coarse soil moisture data to retrieve high spatial soil moisture at a regular interval over vegetation-covered surfaces. However, future sensors are required to estimate soil moisture from depth up to 0.7–1 m (microwave sensors with P band) over sparse vegetation areas and varying surface roughness using active and passive microwave sensors.