Earth Observation

36 esa bulletin 117 - february 2004 www.esa.int

Monitoring Riverand Lake Levelsfrom Space

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Rivers and Lakes

The growing concern about our planet’s water resources,coupled with the economically driven decrease in ground-based gauge measurements, has focused attention on thepossibility of using space-based data sets for remotemeasurements of river and lake heights. The most directmeasurements of inland water heights are obtained fromsatellite altimeters, with long time series of such observationshaving been built up over the past decade.

ESA has been developing new global river and lake monitoringproducts, including an expert processing system, with the helpof De Montfort University (UK). A seven-year-long timeseries of samples was distributed to the hydrology communityat the CNES Hydrology from Space Workshop in Toulouse lastOctober in order to validate the user requirements. Theyresponded very favorably and are now impatient to get theirhands on a decade of global river- and lake-level products, aswell as the latest near-real-time products coming from ESA’sEnvisat mission.

The great majority of the World's populationlives alongside, and is often dependent upon,continental water bodies. Inland water bodies

– rivers, lakes, wetlands and floodplains – playimportant roles in a variety of interdisciplinaryapplications. They are a source of both water andprotein, often a means of navigation and for theproduction of hydroelectric power, and have beenshown to be good proxy indicators of local and regionalclimatic change. Many catchment areas are regions ofgreat biodiversity and are often focal points in terms ofenvironmental and conservation issues. Routinemonitoring of these basins has further importance forregional and continental-scale hydrological,biochemical and climatological studies concerning, forexample, the measurement of river discharges, theproduction of wetland methane, and the estimation ofevaporation losses for land/atmosphere interactions.

Changes in the stored volumes of surface water alsohave geodynamical implications for the Earth’s rotationand gravity, and for estimating the global water mass inrelation to sea-level changes. With synergisticinundation extent measured by remote-sensingimagers, radar or optical, the monitoring of bothsurface level and area has relevance for studies ofwater-related epidemics such as malaria, cholera, andtuberculosis and the financial losses and humansuffering therefrom.

Jérôme BenvenisteEarth Observation Science and Applications Department,ESA Directorate of Earth Observation, ESRIN, Frascati, Italy

Philippa BerryDe Montfort University, Leicester, United Kingdom

© Ilce Campos

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The Amazon River Basinobserved with the ERS-1Radar Altimeter

The Rationale

ESA’s ERS-1 and ERS-2 missions and theTopex/Poseidon satellite have beenacquiring radar-altimeter data for morethan ten years now. With the recentsuccessful launch of Jason-1 and ESA’sEnvisat, this time series will be prolongedinto the future, as well as being processedin near real time. It means that there is awealth of long-term hydrologicalinformation at hand, but extractingmeaningful products from the radar-altimeter signal echoes is a complexprocess due to the unpredictable nature ofthe echo’s shape over continental surfaces.This is precisely why ESA launched thedevelopment of a new ready-to-usehydrological product as part of its ‘Riverand Lake Project’.

The project’s main objective is toprovide the scientific community witheasy-to-use, effective and accurate riverand lake level measurements from both theERS and Envisat satellite altimeters. Thehydrologist’s requirements pose a veryinteresting challenge, because thetraditional satellite products have beenradically different from those based onground data, with both the verticalprecision and temporal sampling beingmore limited.

This new development will enable thevaluable hydrological data encoded insatellite altimeter echoes returned fromrivers and lakes to be translated intoaccurate height estimates, thus permittingtime series of more exact water heights tobe produced. Of the many potentialapplications of radar altimetry tohydrology, the simplest in concept is themonitoring of river and lake levels.Previous work on extracting suchinformation from satellite data wasconstrained by mission limitations (Seasat)and a combination of data limitations andinstrument and pointing difficulties overland surfaces (Geosat). This meant thatonly very limited applications of altimetryfor surface hydrology could bedemonstrated over land, with the primarytargets being wetlands (for direct mapping)and large lakes (for water-leveldetermination).

Considerable work has been done withTopex, which has also demonstrated theusefulness of such data, but only over asmall number of selected targets. One keyconstraint for hydrological applicationsfrom these missions is that their altimeters,originally designed for ocean measure-ments, can ‘keep track’ only over verylimited areas on land.

The major advance in land coverageprovided by the inclusion of an ‘ice mode’in the ERS-1 and ERS-2 Radar Altimetershas vastly increased the capabilities formonitoring the Earth’s river and lakesystems, compared with altimetersequipped only with an ‘ocean mode’, e.g.Topex/Poseidon and Geosat Follow-On.Envisat will both continue

this valuable data stream and, potentially,extend it, as an additional tracking modeallows successful radar-echo capture overeven rougher terrain.

The system developed by De MontfortUniversity (UK) for ESA uses both ERSand Envisat data to produce two types ofproducts: a River-Lake Hydrology (RLH)product and a River-Lake Altimetry (RLA)product. The first goal is to obtain 7 yearsof processed data for specific targets, thento propose the world-wide coverage oflarge rivers and lakes over 7 years, andfinally to make all RLH and RLA productsavailable to hydrologists in near-real-time,i.e. less than 3 hours after the satellite’soverflight.

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The Role of Satellite Radar Altimetry

Water bodies represent important economic and cultural resources, but also much economicactivity and development takes place close to the shorelines of lakes and can be adverselyaffected by flooding. Moreover, lake volumes respond to changes in precipitation integratedover their catchment basins and so can act as important, though indirect, indicators of climatechange on both regional and global scales. Major river systems are important targets forresearch covering a wide range of applications, such as transport, flooding hazards, water andfood resource management, as well as studies of the hydrological cycle and the impact of landuse and climate change.

For certain major rivers and wetlands, hydrological information can often be difficult to obtain,due to the inaccessibility of the region, the sparse distribution of gauge stations, or the slowdissemination of data. Satellite radar altimeters have the potential to provide accurate heightmeasurements not only for lakes, but also for large rivers such as the Amazon, which has beena primary target of environmental studies over the last 10 years.

Research into the application of altimetry for monitoring river and lake levels has been carriedout since 1982. It has highlighted the advantages of using data derived from satellites due totheir global coverage and regular temporal sampling of the processed data, but has alsoidentified the difficulties in interpreting radar-altimeter measurements made over inlandwaters. In general, the great improvement in altimeter measurement accuracy that hasoccurred over the past decade has been due to the progress in altimeter instrumentation,coupled with the substantial improvement in the precision of satellite orbit calculations.

Satellite altimetry coverage over land surfaces has also been greatly improved due to theinclusion by ESA of additional tracking modes in the ERS and Envisat altimeters, which

enable these instruments to cope with rapidly changing surfaces, leading tosubstantial advances in the study of ice, land and inland waters.

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Sifting the Data

To optimise the recovery of inland waterdata from spaceborne altimeters, and gainaccess to the unique time series of inlandwater heights contained in the ERS andEnvisat radar signals, it is necessary toreprocess the individual echoes to obtainan accurate ‘range to surface’. The task iscomplicated by the wide variety of echoshapes returned, especially in the presenceof land ‘contamination’. In fact, severalfactors affect the accurate recovery ofheight data from inland water echoes. Thefirst and in many ways the most seriouslimitation is the presence of very brightcomponents within the echo resulting fromstill pools. Further complications includethe presence of islands and sandbars withinthe water body, the surrounding still waterfrom, for example, irrigation and ricepaddies, and the effect of surroundingterrain. All of these factors affect the echoshape and complicate retrieval of the rangefrom the satellite to the water surface.

The accompanying figure shows foursequences of echo shapes to illustrate thedifferences between the echoes from largelakes and oceans, and the rapidly varyingradar returns from rivers and smaller lakesystems: they are typical ocean returnsfrom Topex, returns from Envisat over alarge lake (Lake Titicaca), and two series

Rivers and Lakes

Envisat Radar Altimeter return-echo shapes over rivers

Topex Ku Ocean 00N 150W

Envisat Ku Lake Titicata Topex Ku Amazon 15S 075W

ERS Ice-Mode Water Echoes Amazon

A C

B D

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of returns from inland water/landcomposites over the Amazon Basin fromERS-2 and Topex. This diversity in echoshapes means that it has not been possibleto design just one retracking algorithm toreprocess all waveforms optimally. Rather,a suite of such algorithms is required, andthis is the approach that has beenimplemented for ERS-1/2, Envisat andTopex data using the expert systemdeveloped at De Montfort University.

Over rivers, ERS-2 shows significantlybetter performance than Topex, thanks tothe Ice mode. The accompanying figureshows a typical 35-day cycle of ERS-2crossings (green) superimposed on theAmazon Basin bright-targets map developedfrom the ERS-1 geodetic mission (red).Four sample time series of river-heightvariations derived from the ERS-2retracked dataset are also shown. The largeannual variation is clear.

To illustrate the relative performance ofa higher sampling frequency with an ice-mode altimeter, the next figure shows asimilar map for ERS-2 three-day crossings(green) over Amazon Basin bright targets.Again, time-series data have been includedto illustrate the satellite altimetry’spotential for identifying rapid temporalvariations in river and stream height whenusing a satellite with a short orbit-repeatpattern.

In this illustration, lakes seen by both ERS-2 andTopex are shown in orange, lakes seen only byERS-2 are in green, lakes not seen by eithersatellite are in blue, and three small lakesoverflown only by Topex are shown in cyan. Thisdramatic difference in coverage is a consequenceof the orbit patterns and the additional ice-tracking mode of the ERS Radar Altimeters

ERS-2 35-day crossings over the Amazon and samples of timeseries of river levels

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The Two New Products

As mentioned above, two novel types ofproduct have been designed and arecurrently available as samples. The River-Lake Hydrology (RLH) product, intendedfor hydrologists with no special knowledgeof radar altimetry, is grouped by river/lakecrossing point (one product per crossingpoint). The RLH product is distributed inXML format, which makes access andvisualisation very simple. The River-LakeAltimetry (RLA) product, designed forradar-altimetry experts, is grouped bysatellite orbital revolution. A detailedproduct description can be found in theProduct Handbook, downloadable from the River and Lake Project web site(http://earth.esa.int/riverandlake).

To familiarise the hydrology communitywith these novel products and to gatherfeedback concerning their suitability anduse, a set of 22 representative seven-year-long time-series samples were distributedat the Hydrology from Space Workshop inToulouse (F) last October. The 22 productsfocused on two regions:– Africa: Lake Nasser, the River Nile

including the Aswan Dam, Lake Tana,Lake Kyoga, Lake Victoria, Lake Mai-Ndombe and the River Congo.

– South America: Amazon Basin.

The accompanying locationmaps for both regions have therelated product crossing pointshighlighted and the corres-ponding product samples can bedownloaded at http:// earth.esa.int/riverandlake.

Rivers and Lakes

ERS-1 3-day crossings over the Amazon Basin and samplesof time-series river-level data

Maps of the African and South American sample-product locations

The River Congo as seen by Envisat’s AdvancedSynthetic Aperture Radar (ASAR)

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The accompanying graph is aquantitative representation of the productsover the River Amazon, showing thevariation from the mean water level in theriver at two crossing points. The annualcycle is clearly visible in this seven-yeartime series of data. The year-to-yearvariation in the annual cycle, which is ofmajor importance for climatologicalstudies, is also clearly detected.

Three New Processing Tools

To help the hydrology community toaccess and process the two new products,software tools have also been developedand made available. A software library,written in the C programming language,helps users to incorporate the reading/writing of RLA products into their ownprograms. A new Graphical User Interfacetool realised in IDL (Interactive DataLanguage) facilitates the plotting of boththe RLA and RLH products. An RLHDemonstration Viewer, written in Java andtherefore platform-independent, allowsscatter, line and area plots to be made ofmore than one RLH product at a time. TheRiver-Lake Viewer also includes a zoomfacility and the possibility to export plotsin Portable Network Graphics format.

Future Plans

Whilst the decade-long archive of satellitedata is being processed, more samples forother geographical regions, focusing onspecific user needs, will be prepared anddistributed next April. The Envisat groundsegment will be enhanced with the near-real-time river and lake level processor toprovide products to the hydrology com-munity in less than three hours aftermeasurement. A second Hydrology fromSpace Workshop will be organised nextyear, as was strongly recommended by theparticipants in the first workshop, alongwith the establishment of a Europeanscientific working group on HydrologicalObservations from Space. The WorkingGroup’s objective is to promote theapplication of existing space observationsto problems in hydrology and formulatethe requirements for future spacebornehydrology missions, as well as to organisethe necessary ground-based observationsto support the validation of the space-dataproducts. The Working Group will alsopromote communication between ESA andthe scientific communities representedwithin its membership. s

Sample products over theRiver Amazon, which clearly

show not only the annual cyclein water level, but also the

year-to-year variation, whichhave both ecological and

climatological consequences

The shrinking Aral Sea, as seen by Envisat's MERIS instrument. Its water level has dropped 13 metres since the 1960's

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