Space harnessed for water resource management
Satellite missions have enabled major breakthroughs in our knowledge of the environment. Among others, oceanographic altimetry measurements from space have revolutionized our knowledge of the oceans and the way they affect the climate (the most striking example is that we now understand the El Nino phenomena in a global context). This satellite technique for measuring sea surface height has been successfully developed for about 30 years through French-US cooperation.
R&D altimetry activities over this period have revealed improvements and new capabilities for future generations. Not only will they be able to scan larger areas of the ocean, but they will open up new measurement opportunities by recording the height of inland surface water such as lakes and rivers.
Hydrology depends mostly on limnigraphs (water level indicators) and depth sensors installed on the edges of rivers, streams or lakes. However, these instruments are unevenly distributed on the surface of the Earth and it is often difficult to maintain and calibrate them (see Figure below). Satellite observation offers regular, homogeneous measurements of all major lakes and rivers.
Only 30% of African catchment basins are equipped with in situ measurement devices.
The programmatic context
CNES and NASA, in collaboration with the Canadian Space Agency (CSA) and the United Kingdom Space Agency (UKSA) have proposed a satellite mission using a new technical concept: interferometric altimetry. The Surface Water & Ocean Topography (SWOT) mission will significantly improve both offshore and coastal ocean observation, while enabling global measurement of the water levels (and their variations over time and space) of rivers, lakes and flood zones. The partnership is based on 25 years of cooperation between CNES and NASA in the field of oceanographic altimetry through the TOPEX-Poseidon and Jason satellites (1, 2 and 3).
In comparison to conventional altimetry, which provides point data along profiles at intervals of several tens or hundreds of kilometres, interferometric altimetry provides a two-dimensional image with a horizontal resolution of the order of 50 to 100 m.
Respective coverage of a nadir altimeter (on the left) and a swath altimeter (on the right).
The SWOT mission concept is a major leap forward in satellite altimetry; it has created a new field of scientific investigation with the prospect of operational applications that can be met by French industry.
The SWOT mission should be seen as the precursor of a series of operational satellites.
SWOT science objectives
SWOT will provide altimetry data for a 2 x 50 km wide swath with a spatial observation resolution of 15 km, which is a considerable improvement over previous altimeters, which were only capable of along-track nadir measurements. Its main contribution to oceanography consists in precisely characterizing mesoscale and sub-mesoscale circulation patterns (see figure below) which play a major role in ocean energy transport. SWOT will also enable us to determine the effects of coastal circulation on marine life, ecosystems, water quality and energy transport and to improve ocean/atmosphere coupling models. It represents a major contribution to operational oceanography (GMES, Meteorology).
90% of the ocean’s kinetic energy is transported by eddies.
Time and space scale for ocean phenomena.
When investigating the hydrology of continental surface waters, the SWOT mission will provide large-scale measurements of changes in the water stocks of the main wetlands, lakes and reservoirs (it has been estimated that there are more than 30 million lakes in the world with a surface area of more than one hectare; see following figure), and offer a more accurate evaluation of discharge variations in major rivers:
- SWOT will map and monitor water level elevations for all bodies of water greater than 250 m x 250 m irrespective of the weather (limited only by very heavy rainfall), since radar measurements are unaffected by cloud cover.
- WOT will be able to measure the heights and discharge of rivers over 100 m wide (the ultimate aim being 50 m).
These hydrological observations are extremely important for understanding the global dynamics of terrestrial surface waters and their interaction at estuaries with the coastal area of oceans. This new capability offers the opportunity of monitoring the evolution of freshwater reserves in the context of global climate change, particularly in regions for which there are very few observations. In situ (limnigraph) or airborne measurements only provide a partial picture and at the current time, no other satellite instrument is capable of regularly and globally measuring water bodies worldwide.
Furthermore, SWOT data may be combined with other satellite observation data (such as radar, SAR, InSAR or optical data), weather forecasts and hydrological and hydraulic river models to significantly improve flood forecasting systems. SWOT will also be used to improve the mapping of flood basins after floods.
Modelling the number of lakes of between 1 and 10 km² by millions of km².
In addition to its unique contributions to high-resolution hydrology and oceanography, the SWOT mission will be used to observe and analyze the dynamic processes in estuaries, the marine geoid, ocean bathymetry and ice over part of the polar icecaps including pack ice.