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Satellite altimetry


Satellite altimetry over continental lakes



The hydrological balance of a lake depends water supply to the basin where it is located against water losses due to runoff, evaporation and infiltration. It can be affected by changes due to human activities (dams, irrigation) or climatic changes. It is therefore a sensitive indicator of both climate conditions and water resources within a country or group of countries that lie in the same basin.
The parameters used to establish the water balance (precipitation, evaporation, surface and underground water supplies and losses) are currently obtained using field measurements or modelling. In situ water level measurements, however, are not always sufficient or may not even exist; precipitation is measured by unevenly distributed rainfall stations; underground water supplies are often unknown; and evaporation, rarely obtained through direct measurements. Thus, the water balance is usually calculated using models that rely on a number of parameters that are not always well understood. Observations provided by existing altimetry missions are therefore of major importance, as they provide additional information useful in monitoring continental lake water levels on a global scale, even if in situ data cannot be completely replaced by satellite data.

Given the number of satellite altimetry missions currently in orbit, several hundred continental lakes can be accessed through space observation.

The purpose of studies currently conducted by the team is to quantify the spatio-temporal variations of surface water mass balances, and study the relationship between this component and climate variability (in particular the response of surface water to ENSO events, NAO fluctuations, etc). Another area of study involves large man-made reservoirs, which have multiplied in number in recent years and whose role in the regional hydrological balance is not fully understood.

Jean-François Crétaux is in charge of several international regional hydrology programmes that bring together specialists of space observation, hydrology, climatology and even biology. These programmes involve the following regions: Lake Issyk-Kul (Kyrgyzstan), the Aral Sea (Central Asia), the Caspian Sea and the great lakes of the Andes (South America).

Research conducted and main findings

Studies carried out in the last two or three years have placed a particular focus on a few lakes for which significant in situ data is available, making it possible to achieve a certain synergy between satellite altimetry observations and field data. The studies have examined interannual variations of Lake Issyk-Kul and the Aral Sea, which can be linked to the hydrological balance of these two continental waters (Aladin et al., 2005, Crétaux et al., 2005a, 2005b), and the African Great Lakes, studied in relation to climatic variability of the Indian Ocean (Mercier et al., 2002).

The Aral Sea

The Aral Sea study (north basin and south basin), which combined water level variations measured by altimetry with rainfall data, runoff data and evaporation model results, have allowed improved hydrological balance of this basin. The results have suggested that a significant underground water supply, totally concealed until now, may be present (Crétaux et al., 2005a). Figure 1 shows the variation in volume of the Aral Sea (both Small Aral and Big Aral) for the period 1993 to 2004, by combining Topex/Poseidon and Jason-1 altimetry measurements (for water levels) with satellite images (for surface area). The estimate based on in situ measurements has been superimposed on the image. A significant reduction in the volume of the Big Aral Sea is observed, dropping from 240 km³ at the beginning of 1993 to 90 km³ in 2004.

The Small Aral Sea study reveals significant interannual fluctuations related to construction of a dam. The level of the Small Aral has started rising since 2002.

Altimétrie spatiale d-fig1

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Figure 1: Variations in the volume of the Aral Sea based on data from Topex Poseidon and Jason
compared against variations in volume deduced from in situ hydrological measurements.
(a) : Small Aral ; (b) : Big Aral

Other lakes and reservoirs

In some cases, lack of knowledge of the geoid may bias the mean lake water level. Thus complicates comparisons with in situ data, especially when attempting to complete the historical time series for lake water levels established by tide gauges, by satellite altimetry data. In 2004 J.F. Crétaux and S. Calmant conducted several calibration campaigns using GPS receivers on Issyk-Kul lake and Aral sea to establish geoid profiles along the various satellite tracks. Other field campaigns were also conducted in 2005 (Caspian Sea, Lake Issyk-Kul, lakes in the Andes).

In another study conducted over the East African Great Lakes (Mercier et al., 2002), significant interannual variations in the lake water levels were revealed at the end of 1997 from Topex/Poseidon altimetry data (Figure 2). This rise was related to the 1997-1998 ENSO event through excess rainfall over East Africa and the Western Indian Ocean in response to warming of that ocean at this period.Figure 3 shows the water level variations of a man-made reservoir (Lake Buharyat) in Iraq.

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Figure 2 : Water level variations of Lake Mweru (East Africa) based on Topex/Poseidon.


Altimétrie spatiale d-fig4


Figure 3 : Water level variations of Lake Buharyat (Iraq). This lake is a reservoir, affected by human activities.


Contacts : J.F. Crétaux, M. Bergé-Nguyen, M.C. Gennero, A. Kouraev, S. Calmant, A. Cazenave

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