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.

Altimetry has the advantage of taking being able to take global, homogeneous, repeated measurements (thus enabling systematic monitoring to be carried out over several years), unhindered by clouds, night or even vegetation. The measured surface heights are referenced to the same frame. However, it this technique is mainly optimized for the ocean (but although specific land re-tracking can be applied) 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. Over non-ocean surfaces (wet or dry), the accuracy of the altimetry measurements can be degraded to by several centimetres or tens of centimetres, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged land surfaces). Another important source of error lies in the signal's propagation of the signal through the atmosphere. The satellites repeat-orbits are rather long (10 to 35 days), which do not fit with suit real-time monitoring of river or lake level variations (e.g. flood alerts), but agree do work well with seasonal or interannual monitoring.

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.
Land
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.
Rivers
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.
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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