Sahelian dry and wet seasons seen by altimetry

Image of the Month - December 2014

Rainfall (a & b), time-variations of altimetry backscattering coefficients (c&d) for Envisat (2003-2010) in S and Ku band, Saral (since 2013) in Ka band, Jason-2 in C and Ku-band, and soil moisture (e&f). Backscatter levels are inversely proportional to frequency during dry seasons, but the backscatter dynamics linked to rains is stronger for higher frequencies (Credits Midi-Pyrénées Observatory)

Initially designed to monitor the dynamic topography of the ocean, nadir-pointing radar altimeters have proven their capacities for the study of continental surfaces and for land hydrology, allowing long-term observations of water level variations for land hydrology. The magnitude of radar altimeter backscattering coefficients is also related to the dynamics of surface properties. Spatial and temporal variations of radar altimeter backscattering coefficients were found to be related to soil roughness and surface soil moisture changes in semi-arid areas, such as Sahel.

In Sahel, soil moisture drives many surface processes including soil organic matter mineralization, vegetation productivity, land surface fluxes and land surface–atmosphere interactions. Radar altimetry offers a new opportunity for the monitoring of this key variable in regions where in-situ observations are scarce.

See also:


  • C. Fatras, F. Frappart, E. Mougin, M. Grippa, P. Hiernaux, 2012: Estimating surface soil moisture  over Sahel using Envisat radar altimetry, Remote Sensing of Environment, 123 (2012), 496-507.
  • Result shown hereF. Frappart, E. Mougin, P. Borderie,  C. Fatras, personal communication