Water on the continents is stored in different reservoirs: snow layers, glaciers, aquifers, root zones and surface waters (rivers, lakes, reservoirs, wetlands). These components are in permanent interaction with the oceans and the atmosphere, together forming the hydrological cycle, as illustrated in the figure below.

Thus, characterising all the water stocks and flows on land, and their variations, is a key element in understanding the water cycle and allowing fine-tuned control of the planet’s resources. These needs are particularly critical in the inter-tropical belt, a key region for global climate dynamics, or in the boreal regions, which are true sentinels of global change. However, our knowledge of the spatial and temporal variations of continental water flows and stocks, of the associated hydrodynamic phenomena, as well as of water mass exchanges between continent-atmosphere-ocean, is still very limited, both at the global level and at the scale of river basins, and many questions related to the water cycle on the continents remain open.

Scientific issues

ECHOS hydrologists are working on these issues by integrating and developing new observations and innovative analyses combining spatial data, in situ and numerical modelling.

The main scientific questions underlying our research are the following:

• How do continental water stocks and flows vary at different spatio-temporal scales (from daily to decadal and from regional to continental and even global scales)?
• How is the terrestrial water cycle and water resource impacted by the combined effect of climate variability and anthropogenic pressure?
• Can we better characterise recent changes in the hydrological cycle and their possible links to extreme events (droughts, riverine floods and coastal floods)?

Space-based remote sensing techniques provide valuable information on the characterisation of the water cycle and the spatio-temporal variations of many hydrological parameters. These new techniques represent a real revolution in the field of hydrology, in which the ECHOS team specialises. Moreover, they are associated with the development of tools combining several types of spatial sensors, as well as in situ measurements and hydrological/hydrodynamic models, in collaboration with several other national and international laboratories.

Main results

Important results achieved by the team in recent years include:

• the development of algorithms for the calculation of hydrological variables (water levels and surfaces, water volumes, satellite bathymetry);
• several studies on recent changes in high mountain lakes in Asia and the Caucasus;
• the study of ice rings and their formation on Lake Baikal;
• the dynamics of hydro-ecosystems in the thermokarst depressions of Central Siberia and the potential of grasslands for local agriculture;
• the estimation of water stocks (surface and groundwater) in the large tropical basins (Amazon, Congo, Mekong, Ganges-Brahmaputra), and their climatic variability, as well as the identification of the decline of aquifers in the Bengal Delta due to anthropogenic pumping;
• the first quantification of the spatio-temporal variations of groundwater in the Amazon;
• the estimation of the flows of the large tropical rivers (Mekong, Ganges, Amazon, Orinoco, Congo, Niger) and boreal rivers (Ob, Yenesey, Lena) by multi-altimeter methods and/or combined with models and their impact on the near ocean and the coast.

ECHOS also federates the national and international activities on cal/val of current and future altimeters and, as such, carries out numerous field missions on these different hydrological sites (Russia, Lake Baikal, Lake Issyk-Kul, Ob, Amazon, Bangladesh, Madagascar, Colombia, Niger, Central Asia, France).

The team members are also heavily involved in the development of the HydroWEB database of water levels and volume variations of lakes and rivers using satellite altimetry, as well as in current (AltiKa, Jason-3, Sentinel-3A/B) and future (SWOT) altimetry missions.