Remote sensing applications to hydrology: Imaging radar

Synthetic Aperture Radar (SAR) data are very useful for hydrological studies because the dielectric properties of materials are generally dependent on the amount of liquid water in the material, and this affects the strength of the radar backscatter, sigma degrees. Additionally, the ability of microwaves to penetrate the surface of many materials often permits analysis of subsurface properties. Further, SAR sensors can image the Earth through both clouds and darkness which enables dynamic hydrological events, e.g. floods, to be captured. Both radar system parameters and surface characteristics affect sigma degrees. In the case of snow, sigma degrees is the sum of the surface scattering at the air/snow interface, volume scattering within the snowpack, scattering at the snow/soil interface and volumetric scattering from the underlying surface (if applicable). For glaciers, sigma degrees is dominated by scattering from the surface and is dependent upon the roughness of the glacier surface in relationship to the radar wavelength. Imaging radar is proving to be an important tool for measuring flooding beneath a variety of herbaceous and woody vegetation. Use of multi-frequency and multi-polarization sensors is necessary to improve the accuracy of mapping flooded vs non-flooded land in floodplains. SAR data have been shown to have potential for measuring stream discharge in braided rivers and for studying key characteristics of frozen lakes. Work with polarimetric SAR data is becoming increasingly significant in hydrological studies.