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Hydrology and Land

The earliest altimetry missions were dedicated to studying the open ocean and some ice measurements. However, every stretch of water (enclosed seas, lakes, rivers, flooding areas...) or even flat surfaces over lands can give valid data - as long as the satellite fly over them. Swot will change drastically the latter point, with a near-complete, high resolution monitoring of the continents (except Antarctica). 

Satellite altimetry has the advantage of being able to take global, homogeneous, repeated measurements, unhindered by clouds, night or even vegetation, thus enabling systematic monitoring to be carried out over several years. The measured surface heights are referenced to the same frame. However, classical radar altimetry technique has been mainly optimized for the ocean and takes measurements only at the nadir (i.e. just under below the satellite), with a rather narrow footprint -- and averaging everything in that footprint. Specific processing can be applied to retrieve all the possible data. Moreover, the satellite acquisition modes have been improved since 2017 so that the altimeter can be in a receiving mode over lands using a Digital Elevation Model. Over non-ocean surfaces (wet or dry), the accuracy of the altimetry measurements can be degraded by several centimetres or tens of centimetres, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged land surfaces), and because of reflections on the river banks. Another important source of error lies in the propagation of the signal through the atmosphere. It must also be noticed that the satellites repeat-orbits are rather long (10 to 35 days), which do not fit real-time monitoring of river or lake level variations (e.g. flood alerts), but agree well with seasonal or interannual monitoring.

Hydrological simulator
High-resolution Hydrology
Swot, by providing an image everywhere will enable to know where is the water, its slope and thus compute its discharge at least every 22 days. Comparisons between two successive measurements will also provide with variations of those quantities.
Lakes and enclosed seas
The level of lakes varies along the seasons. Enclosed seas level is a major indicator of their good (or bad) health. Altimetry able us to continually monitor these levels.
For certain major rivers and wetlands, hydrological information can often be difficult to obtain due to a region's inaccessibility, the sparse distribution of gauge stations, or the slow dissemination of data. Satellite radar altimeters can potentially monitor height variations of inland waters.
Wetlands are ubiquitous on Earth, and provide with a number of "services" for life on Earth - including human beings' ones. Their observation is thus an important stake for all.
Reception of return echoes on land is less reliable, since a field or a forest do not reflect radar pulses as well as water, but conclusions can still be drawn.
Hydrology from space CTOH
Our data base contains time series over water levels of large rivers, lakes and wetlands around the world. These time series are mainly based on altimetry data from Topex/Poseidon for rivers, but ERS-1 & 2, Envisat, Jason-1 and GFO data are also used for lakes. At present, water level time series of about 100 lakes (in Europe, Asia, Africa, North and South America) including Aral & Caspian seas are available. About 250 sites (called virtual stations) on large rivers are also available (see maps below). Users of the data base can visualize the water level time series as well as Landsat images showing the geographic location of the site. Users can download the numerical values of the time series as well as associated errors.
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