Last modified on Jun 07, 2024 14:09

Contributors: B. Calmettes (CLS), G. Calassou (CLS),  N. Taburet (CLS)

Issued by: B. Calmettes

Date: 06/03/2024

Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1

Document reference: C3S2_312a_Lot4.WP2-FDDP-LK-v2_202312_LWL_PUGS-v5_i1.0 

History of modifications

List of datasets covered by this document

Related documents

Acronyms

General definitions

'Orbit' is one revolution around the Earth by the satellite.

A satellite 'Pass' is half a revolution of the Earth by the satellite from one extreme latitude to the opposite extreme latitude.

'Repeat Cycle' is the time period that elapses until the satellite flies over the same location again.

'Range' is the satellite-to-surface pseudo distance given from the 2-way travel time of the radar pulse from the satellite to the reflecting water body.

'Retracking' is the algorithm which computes the altimetric parameters (range, backscatter coefficient...) from the radar echoes.

'Altitude' is considered as the height over the reference ellipsoid (orthometric height).

'Mean Profile' is an average over several years of the surface heights with respect to geoid, computed following the satellite's repetitive tracks.

'Lake' is a body of water that is surrounded by land. In this document, the term "lake" refers to lake or reservoir

Scope of the document

The Product User Guide and Specification (PUGS) is the primary document that users need to read before handling the Lake Water Level (LWL) and the Lake Water Level-Single Track Lakes (LWL-S) products distributed through the Copernicus Climate Change Service (C3S). It gives an overview of the product characteristics and background, in terms of algorithms used to generate it (see the Algorithm Theoretical Basis Document [D2] for more detailed information), key technical characteristics, and a summary of key limitations identified from the validation exercise (see the Product Quality Assessment Report [D4] for more detailed information). Meanwhile, Appendix A contains the specifications for LWL/ LWL-S products towards which this C3S LWL product is aligned towards, based on guidance from the Global Climate Observing System (GCOS).

Executive summary

This Product User Guide and Specification (PUGS) is a self-contained document which gathers all the necessary information to use the Water Level products in an efficient and reliable way.

The Copernicus Climate Change Service (C3S) Lake production system (C3S ECV LK) provides an operational service, generating climate datasets for lake surface water temperature and lake water level for a wide variety of users within the climate change community. This document covers the lake water level system. The water level products distributed through the C3S aim to provide users with water level time series on lakes located beneath satellites tracks. Since satellite altimetry is not an imagery technique (with measurements only being possible at nadir), both the ground coverage and the temporal repeatability is strongly dependent on the orbital characteristics of the altimeters and their host satellite platform.

Chapter 1 contains a description of the data in the Climate Data Record (CDR) and a brief description of the water surface height estimation process. A detailed description of the algorithm is included in the Algorithm Theoretical Basis Document (ATBD) [D2]. Chapter 2 contains a description of the C3S lake water level product including the description of the data in the NetCDF files and the product characteristics in terms of temporal and spatial coverage and resolution. Chapter 3 recalls the requirements as defined by the GCOS and the results obtained in the C3S Climate Data Record V4.0. The last chapter  includes information on the use of the data, with examples of metadata and figures. 

Product Change Log

The 'Product change log' table provides an overview of the differences between different versions of the product up-to, and including, the current version (LWL-v5.0 and LWL-S-v1.0). 

Changes in product

1. Data description

Hydrological information is traditionally obtained via ground-based observation systems and networks that suffer from well-known inherent problems. These include issues such as high cost, sparse coverage (often limited to political local/national instead of geographical/hydrological boundaries), slow dissemination of data, heterogeneous temporal coverage, destruction of the stations during floods, absence of stations in remote areas, and the absence of management strategies, amongst others.

Although originally conceived to study open ocean processes, radar altimeter satellites have nevertheless acquired numerous useful measurements over lakes. This technique, that can complement ground-based observations systems, potentially provides a major improvement in the field of continental hydrology, due to the global coverage (despite being limited to Earth's portion at nadir of the orbital ground tracks of current missions), regular temporal sampling and short delivery delays.

Early studies have demonstrated the great potential of satellite altimetry to monitor large continental water bodies such as the Caspian Sea or the East African lakes (Birkett et al, , 1995; Birkett 1998; Mercier et al., 2002). However, these studies also raised many important issues related to the need for a dedicated treatment and interpretation of the measurements acquired over continental waters. More specifically, the radar echoes (waveforms) returned by inland waters can strongly differ from those returned by the ocean (because of the contamination of the signal by reflections over emerged lands), thus altering the accuracy of the height retrievals.

However, the recognition of these systems potential by the hydrological community is increasing. New altimeter sensors are currently being designed following the requirements expressed by both hydrologists for scientific use, and the water resources management community for emerging operational services.  Currently, scientific research dominates data use (Crétaux et al, 2011) and mainly concerns the global scale: monitoring of large climatically sensitive lakes or the study of the temporal water masses redistribution within a large basin.

1.1. The retrieval methodology

1.1.1. Background

1.1.1.1. Satellite Radar Altimetry

Radar altimetry from space consists of vertical range measurements between the satellite and water level. The difference between the satellite altitude above a reference surface (usually a conventional ellipsoid), determined through precise orbit computation, and satellite-water surface distance, provides measurements of water level above the reference. Placed onto a repeat orbit, the altimeter satellite observes a given region at regular time intervals (called the orbital cycle), during which a complete coverage of the Earth is performed.

Water level measurement by satellite altimetry has been developed and optimised for open oceans. However, techniques can also be applied to obtain water levels of inland seas, lakes, rivers, floodplains and wetlands. Several satellite altimetry missions have been launched since the early 1990s: European Remote Sensing (ERS)-1/Radar Altimeter (RA) (1991-1996), TOPography EXperiment (TOPEX)/Poseidon (1992-2006), ERS-2/RA (1995-2005), Geosat-Follow-On (GFO) (2000-2008), Jason-1 (2001-2012), ENVISAT/RA-2 (2002-2012), Jason-2 (2008-), CryoSat-2 (2010-), Haiyang-2A (HY-2A) (2011-), satellite with Argos and Altika (SARAL)/AltiKa (2013-), Sentinel-3A (2016-), Sentinel-3B (2018) and Sentinel-6A (also called Jason Continuity of Service (CS) or Michael Freilich, 2020-). ERS-1, ERS-2, ENVISAT and SARAL have a 35-day temporal resolution (duration of the orbital cycle) and 80 km inter-track spacing at the equator. TOPEX/Poseidon, Jason-1, Jason-2, Jason-3 and Sentinel-6A have a 10-day orbital cycle and 350 km equatorial inter-track spacing. GFO has a 17-day orbital cycle and 170 km equatorial intertrack spacing. Sentinel-3A and Sentinel-3B orbits have a revisit time of 27 days, and its inter-tracking separation is 104 km. It is reduced to 52 km in the two-satellite configuration (Sentinel-3A and B).

The Sentinel-6A tracks tracks over some Russian lakes are shown in Figure 1 below. The combined global altimetry data set has 30 year-long history and is intended to be continuously updated in the coming decade. Combining altimetry data from several in-orbit altimetry missions increases the space-time resolution of the sensed hydrological variables. 

 Figure 1. TOPEX/Poseidon (Jason-1, Jason-2, Jason-3, Sentinel-6A) tracks over the lakes Balkash, Yssik-Koul, Alakol and Saysan. Note that in the areas between those tracks, no measurements are acquired by the altimeters on board these satellite missions.

1.1.1.2. Computing surface height

Figure 2 shows the technique used to estimate water level or water height. Satellite altimetry measures the time taken by a radar pulse to travel from the satellite antenna to the surface and back giving a distance: range. The water surface heigh is estimated thanks to the knowledge of the precise satellite location obtained with the Doppler Orbitography and Radiopositioning Integrated by Satellite System (DORIS) and considering the impact of  the passage of signal through the troposphere and ionosphere as well as changes in the earth surface (tides). 

Figure 2. Altimetry principle of estimating water level from altimetry measurements (Credit CNES – Mira production / Aviso)

For all satellites, the following operation is done:

Surface Height = altitude – corrected_range        [Eq.1]

With:

          Corrected_range = range + Wet_tropo + Dry_ tropo + iono + polar tide + solid earth tide        [Eq.2]

where:

• "Wet_tropo" is a correction to account for the delay of the radar signal due the wet part of the troposphere.
• "Dry_tropo" is a correction to account for the delay of the radar signal due the dry part of the troposphere.
• "Iono" is a correction to account for the delay of the radar signal due the electronic content of the ionosphere.
• "Polar Tide" and "Solid Earth Tide" are two corrections that account for the temporal deformation at the Earth' surface.

Note that for SARAL/AltiKa, ionospheric correction is not taken into account due to the frequency used for this mission (Ka-band), which is not sensitive to the ionosphere electron content.

Finally, the water surface height is expressed with respect to the geoid:

  Water Surface Height = Surface Height – geoid        [Eq.3]

The geoid value considered for LWL products is extracted from a mean profile file over large lakes, while for lakes observed by a single mission (LWL-S products) the geoid value from EGM2008 geoid grid is used.

1.1.2. Processing

Altimetry missions used are repetitive, meaning that the satellite overflies the same point at a given time interval (10, 27 or 35 days for current missions, depending on the satellite). The satellite usually does not deviate from more than +/- 1 km across its track.

A given lake in LWL products can be overpassed by several satellites, with potentially several passes, depending on its surface area.

In the current version of the LWL dataset, 251 lakes are monitored. Among them, 165 lakes are monitored with data from a single mission (Jason family data, Sentinel-3A or Sentinel-3B), 83 lakes with data from two missions and 3 lakes  with the data from all three missions. Several tracks can be used to derive the lakes' Lake Water Level (LWL) timeseries according to their surface areas.

In the first version of the LWL-S dataset, 10037 lakes are monitored: 4367 are monitored with measurements from Sentinel-3A mission, 4711 with Sentinel-3 measurements and 959 with Jason-3/ Sentinel-6 measurements. 

Water level series starting before 2016 are based on data from TOPEX/Poseidon (T/P), Jason-1, Jason-2, Envisat, SARAL, IceSat and GFO data provided by European Space Agency (ESA), National Aeronautics and Space Administration (NASA) and Centre National d'Etude Spatiale (CNES) data centres. 

Inputs

Inputs to the processing are:

Altimetry data from missions Sentinel-3A /Sentinel-3B, provided by Copernicus Data Space Ecosystem, and data from missions Jason-3/Sentinel-6A,  provided by CNES.  Altimetry data consist of Short Time Critical (STC) and Non-Time Critical (NTC) data, and they are available within 2-3 days and 1 month, respectively, after measurement acquisition. We advise the user that NTC and STC nomenclature are used for Sentinel data (since these data are provided by Eumetsat), these denominations correspond respectively to the GDR (Geophysical Data Record) and the IGDR (Interim Geophysical Data Record) for Jason-3 (data provided by CNES, hence CNES nomenclature is used). These two types of files include the altimetric measurements as well as the necessary corrections for their exploitation (such as orbit, and atmospheric corrections, amongst others). GDR data were used for time series initialisation, IGDR data are now used for operational processing. GDR are still used for the research products. The main differences between GDR and IGDR data reside in the orbit and radiometer derived values (the GDR benefits from more precise values as well as more recent GDR calibrations). Differences between these corrections are in the order of a few cm.

• Information data on the monitored lakes, including the location of the lake, the mission and the associated number of the tracks that cross each processed lake.
• For LWL dataset, the mean geoid profile along the track for each one of the tracks crossing the  processed lake: this information is needed for the correct estimation of the water level. For LWL-S dataset, the geoid grid EGM2008 is used. 

Method

The algorithm and process to derive Lake Water Level estimates from altimetry data are described in detail in this product's associated Algorithm Theoretical Basis Document [D2]. However, a summary of the approach is as follows. The altimeter range measurements used for lakes consist of 20 Hz IGDR/GDR for Sentinel-6 data and NTC/STC for Sentinel-3A/B data. All classical corrections (orbit, ionospheric and tropospheric corrections, polar and solid Earth tides and sea state bias) are applied. Depending on the size of the lake, the satellite data may be averaged over very long distances. It is, thus, necessary to correct for the slope of the geoid (or equivalently, the mean lake level). Because the reference geoid provided with the altimetry measurements (e.g., Earth Gravitational Model (EGM)96 for T/P data or EGM2008 for Sentinel-3A) may has significant variations for large lakes, we have computed for LWL products a mean lake level, averaging over time the altimetry measurements themselves. Such mean lake level surface along each satellite track across the lake provides a better estimate of the model geoid. Mean profiles are therefore used for lake water level computations which are then referenced with respect to this estimate of the geoid. If different satellites cover the same lake, the lake level is computed in a 3-step process:

1. Extraction of input data: including an editing to remove erroneous measurements
2. Lake level computation and filtering
3. NetCDF files for each lake, and populating the files with the derived lake water level estimates.

For LWL-S products, geoid model variations along tracks are negligible and the utilization of a geoid model, such as EGM2008, is enough to compute Lake Water Level.  

Each satellite-derived altimetry track of data is processed independently. Potential radar instrument biases between different satellites are removed using TOPEX/Poseidon data as reference. We generally observe an increased precision of lake levels when multi-satellite processing is applied.

1.2. Limitations of the product

The lake processing chain, initially developed at LEGOS (Laboratoire d'Etudes en Géophysique et Océanographie Spatiales), is based on the use of altimetry measurements from the CTOH database (Center for Topography of the Oceans of LEGOS), and includes Envisat GDRs. This database includes, in addition, some enhanced corrections that were not in the first data sets of institutional suppliers (Archiving, Validation and Interpretation of Satellite Oceanographic data (Aviso), ESA), such as for the wet and dry tropospheric as well as for the ionospheric corrections. As part of the online version of HYSOPE (HYdrométrie Spatiale OPErationelle) which underpins the C3S Lakes Service, these tailored corrections were replaced by their corresponding standard corrections included in the operational input products. In recent mission products, the geophysical corrections have been regularly updated by agencies which improves the quality of the estimates and makes it unnecessary to replace them. The main source of uncertainty comes from the wet tropospheric correction, which, depending on the region, varies from 2-3 cm. In addition, a module for calculating the height of the geoid has been developed to improve assessment of water levels. Moreover, the overall uncertainty on water surface height depends mainly on the length of the track over the lake and comes mainly from retracking. It varies from 8-10 cm for large lakes (with longer transects) to 1m for small and narrow lakes.

2. Product description

The products provide estimation of water level relative to a reference at the times that a given satellite overpasses the lake. It also contains the standard deviation associated with this measurement and the time of the overpass. Each dataset in the product collection contains the full time of data available for a single named lake, and also contains the metadata necessary for the user to quickly gain a good understanding of the dataset itself.

2.1. Product content

The products contain all the descriptive metadata in the global attributes of the netCDF file (see Table 1).

Table 1. Global attributes included in the product files. Values that are duplicated across datasets are shown in italics

The data is provided as a variable in the netCDF file as indicated in the Table 2.

Table 2. Variables in the Lake Water Level product

2.2. Product characteristics

2.2.1. Projection and grid information

Longitude and Latitude values indicating the lake barycentre are expressed with respect to the coordinate system WGS84. Concerning the reference surface (Datum), for the timeseries using past missions (Topex/Poseidon, Jason1, ERS1, ERS2, Envisat), the water level was estimated with respect to the high-resolution global gravitational model GGM02C (Gravity Recovery And Climate Experiment - GRACE Gravity Model 02), released in 2004. For stability reasons, this reference is maintained when data for current missions (Jason 3, Sentinel-6A, Sentinel-3A, Sentinel -3B) is used to extend these timeseries. However, for the timeseries starting after the Jason2 mission in 2008, including Jason3, Sentinel-6A  and Sentinel-3A/3B, the reference surface used is the Earth Gravitational Model 2008 (EGM2008). Data from this more up-to-date model is included in the agency's products.

It's very important to note here, that for the lake water level, the C3S product contains the mean value of the water height over the lake and the variation of this level over the time has priority over an absolute value.

2.2.2. Spatial information

As described above, the water level heights can only be measured directly below the satellite's orbital ground tracks (nadir), if favourable measurement conditions are encountered. Unfortunately, it is not always possible to retrieve valid measurements at each crossing between hydrographic networks and satellite ground tracks. Figure 3 shows the location of the lakes that are currently monitored to generate LWL products and figure 4 illustrates the location of lakes monitored to generate LWL-S products.  The list of lakes for both products, including their location, is also provided in csv format. This allows the user to take a closer look at the spatial coverage. This very common format can be used for a wide range of applications, from simple text editors to geographic information software such as QGIS.

Spatial coverage depends aslo on the characteristics of the mission's orbit. For example, for the Jason family of missions, the inclination of the orbit is limited to +/- 66° latitude and, as a result, certain targets may not be monitored in the Arctic region. The ground track coverage of all altimetry missions can be found on the Aviso website¹. As an example, the offset between two ground tracks at the Equator is 315 km for Jason-3 tracks (10-days revisit time) and 80 km for Envisat (35-days revisit time).

Figure 3. Spatial Coverage - LWL V5.0 dataset collection. The lakes for which time series are provided are shown in red

Figure 4. Spatial Coverage - LWL-S dataset v1.0 collection. The lakes for which time series are provided are shown in red.

2.2.3. Temporal information

Lake Water Level series (LWL & LWL-S products) are updated each time a given satellite passes over the lake. This can range from daily for bigger lakes, as lake Baikal (Russia), to as much as once every 27 days for lakes monitored by a single track of Sentinel-3A or Sentinel-3B. Thus, if several satellites monitor the lake as is the case for LWL products, the update time does not depend on the repetitivity of one orbital cycle (10 days for Sentinel-6A, 35 days for Envisat, 27 days for Sentinel-3A/3B).  It may be more frequent depending on the satellites involved and the tracks over a given lake.

3. Target requirements

The target requirements and the gap with the current product characteristics are described in the Target Requirement and Gap Analysis Document [D5]. Table 3 shows target requirements based on GCOS. See the Product Quality Assessment Report (PQAR) [D4] for an assessment of product quality.

Table 3. User requirements for LWL estimates as defined by the Global Climate Observing System (GCOS)

4. Data usage information

4.1. File naming

Files are named:

C3S_<lake water level product>_<region_name>_<lake_name>_<lake_id>_<latitude>_<longitude>_altimetry_<version>_<time_coverage_start>_<time_coverage_end>_R<file_generation_date>.nc

Where

         <lake_water_level_product> is the name of the Lake Water Level product: LWL or LWL-S.

         <region_name> is the name of the reigion. Currently, five zones are defined: 

• •  N-AFRICA: for lakes in Africa north of 0 degrees
  •  S-AFRICA: for lakes in Africa south of 0 degrees
  • N-ASIA: for lakes in Asia north of 50 degrees
  • SE-ASIA : for lakes in Asia south of 50 degrees and east of 85 degrees 
  • SW-ASIA : for lakes in Asia south of 50 degrees and west of 85 degrees 
  • N-NORTH_AMERICA: for lakes in North America north of 50 degrees
  • S-NORTH_AMERICA: for lakes in North America south of 50 degrees
  • N-EUROPE: for lakes in Europe north of 50 degrees
  • S-EUROPE: for lakes in Europe south of 50 degrees
  • OCEANIA: for lakes in Oceania
  • SOUTH-AMERICA: for lakes in South America

<lake_name> is the name of the lake, in English or the Local name. If this one doesn't exist, it is set to 'NONE'.

<lake_id> is the identifier of the lake based on well known dataset. It is defined as followed:

• • 4 letters defining the dataset on which is based the identifier:
    • Hydrolakes: HYLA
    • Global Lakes and Wetlands Dataset: GLWD
    • Hydroweb: HYWB 
  • 7 digits (following the 4 letters) defining the identifier number.

          <latitude> is the latitude of the lake followed by N (North) or S (South).

          <longitude> is the longitude of the lake followed by E (East) or W (West).

          <version> is the product version number.

          <time_coverage_start> is the first date of the timeseries.

          <time_coverage_end> is the last date of the timeseries.

          <file_generation_date> is the date of the file generation.

4.2. File format

The file format is netCDF-4 Classic Model.

4.3. Quicklook

For Lake Albert, in Democratic Republic of Congo (DRC) and Uganda (North Africa), the time series has been computed since June 1996, starting with ERS-2 and it is currently being monitored by 5 tracks from Sentinel-3A/B. An example of the metadata for Lake Albert is shown it Table 4,  containing the netcdf file header. A time series plot can then be generated from the data shown in Figure 5. 

 Table 4. Header of the netCDF file for Lake Albert 

 netcdf C3S_LWL_N-AFRICA_ALBERT_HYLA0000159_01.67N_030.92E_altimetry_5.0_19950605_20230627_R20230803 {
dimensions:
	time = UNLIMITED ; // (437 currently)
	lat = 1 ;
	lon = 1 ;
variables:
	double time(time) ;
		time:standard_name = "time" ;
		time:long_name = "time" ;
		time:units = "days since 1950-01-01" ;
		time:calendar = "gregorian" ;
	double lat(lat) ;
		lat:standard_name = "latitude" ;
		lat:units = "degrees_north" ;
		lat:long_name = "latitude" ;
		lat:valid_min = -90. ;
		lat:valid_max = 90. ;
		lat:axis = "Y" ;
		lat:reference_datum = "WGS84 datum" ;
	double lon(lon) ;
		lon:standard_name = "longitude" ;
		lon:units = "degrees_east" ;
		lon:long_name = "longitude" ;
		lon:valid_min = -180. ;
		lon:valid_max = 180. ;
		lon:axis = "X" ;
		lon:reference_datum = "WGS84 datum" ;
	double water_surface_height_above_reference_datum(time, lat, lon) ;
		water_surface_height_above_reference_datum:standard_name = "water_surface_height_above_reference_datum" ;
		water_surface_height_above_reference_datum:long_name = "water surface height above geoid" ;
		water_surface_height_above_reference_datum:units = "m" ;
		water_surface_height_above_reference_datum:unit_long = "Meters" ;
		water_surface_height_above_reference_datum:comment = "The reference surface (Datum) for the  current data set is  GGMO2C (high-resolution global gravitational model)." ;
	double water_surface_height_uncertainty(time, lat, lon) ;
		water_surface_height_uncertainty:standard_name = "water_surface_height_uncertainty" ;
		water_surface_height_uncertainty:long_name = "water surface height uncertainty" ;
		water_surface_height_uncertainty:units = "m" ;
		water_surface_height_uncertainty:unit_long = "Meters" ;

// global attributes:
		:title = "Lake Water Level from satellite altimetry" ;
		:institution = "LEGOS, CLS, CNES" ;
		:source = "Copernicus Climate Change Service" ;
		:references = "https://cds.climate.copernicus.eu/" ;
		:conventions = "CF-1.8" ;
		:product_version = "5.0" ;
		:summary = "Copernicus Lake Water Level database contains time series over water levels of large lakes around the world" ;
		:keywords_vocabulary = "GCMD, GEMET and is19115 keywords" ;
		:gcmd_keywords = "TERRESTRIAL HYDROSPHERE, SURFACE WATER" ;
		:gemet_keywords = "water level, water management, hydrometry, hydrology, climate, seasonal variation, environmental data, environmental monitoring, monitoring, remote sensing" ;
		:iso19115_topic_categories = "elevation,inlandWaters" ;
		:cdm_data_type = "vector" ;
		:comment = "" ;
		:creator_name = "LEGOS, CLS, CNES" ;
		:creator_url = "http://www.legos.obs-mip.fr, https://www.cls.fr/, https://cnes.fr/" ;
		:creator_email = "inlandwaterlevel@groupcls.com" ;
		:project = "Copernicus Climate Change Service - C3S" ;
		:geospatial_lat_min = -90. ;
		:geospatial_lat_max = 90. ;
		:geospatial_lon_min = -180. ;
		:geospatial_lon_max = 180. ;
		:standard_name_vocabulary = "NetCDF Climate and Forecast (CF)" ;
		:license = "Free after registration" ;
		:platform = "Topex/Poseidon, Jason-1, Jason-2, Jason-3, Sentinel-6A, Envisat,  SARAL, Sentinel-3A, Sentinel-3B" ;
		:sensor = "Poseidon-1, Poseidon-2, Poseidon-3, Poseidon-3B, Poseidon-4, RA-2, Altika, SAR-Altimeter, SAR-Altimeter" ;
		:key_variables = "water_surface_height_above_reference_datum, water_surface_height_uncertainty" ;
		:processing_mode = "Delayed Time" ;
		:processing_level = "LEVEL3B" ;
		:copyright = "Copernicus Climate Change Service" ;
		:inspire_theme = "Hydrography" ;
		:credit = "Lake Water Level products are generated by the Copernicus Climate Change Services, the Earth Observation Programme of the European Commission and the Theia-land program supported by CNES. The research leading to the current version of the product has received funding from CNES, LEGOS, IRD, CLS and ESA." ;
		:lake = "Albert" ;
		:basin = "Nile" ;
		:country = "Congo&Uganda" ;
		:lake_barycentre_latitude = "1.67 N" ;
		:lake_barycentre_longitude = "30.92 E" ;
		:surface_reference = "GGM02C (high-resolution Global Gravitational Model)" ;
		:history = "File generated on 2023-08-03" ;
		:id = "C3S_LWL_ALBERT_altimetry_5.0_19950605_20230627_R20230803.nc" ;
		:time_coverage_start = "1995-06-05" ;
		:time_coverage_end = "2023-06-27" ;
		:time_coverage_duration = "P28Y5M12D" ;
}

Figure 5. Water level series for the lake Albert spanning the DRC and Ugandan border

Appendix A - Specifications: GCOS User Requierements for Lake Water Level

References

Birkett, C.M., and  I.M. Mason, (1995). A new Global Lakes Database for a remote sensing programme studying climatically sensitive lakes, J. Great Lakes Res., 21 (3), 307-318.

Birkett, C. (1995). The global remote sensing of lakes, wetlands and rivers fro hydrological and climate research. International Geoscience and Remote Sensing Symposium, IGARSS'95. Quantitative Remote Sensing for Science and Applications.

Birkett, C. (1998). Contribution of the TOPEX NASA Radar Altimeter to the global monitoring of large rivers and wetlands. AGU1000. Doi: https://doi.org/10.1029/98WR00124 (URL resource validated 12th December 2023)

Crétaux J-F and C. Birkett, (2006). Lake studies from satellite altimetry, C R Geoscience, Vol 338, 14-15, doi: 10.1016/j.crte.2006.08.002 (URL resource validated 12th December 2023)

Crétaux J-F, W. Jelinski, S. Calmant , A. Kouraev , V. Vuglinski , M. Bergé Nguyen , M-C. Gennero, F. Nino, R. Abarca Del Rio, A. Cazenave, and P. Maisongrande. (2011) SOLS: A Lake database to monitor in Near Real Time water level and storage variations from remote sensing data, J. Adv. Space Res. Vol 47, 9, 1497-1507, doi: 10.1016/j.asr.2011.01.004 (URL resource validated 12th December 2023)

Crétaux, J. F., Abarca-del-Río, R., Berge-Nguyen, M., Arsen, A., Drolon, V., Clos, G., and Maisongrande, P. (2016) Lake volume monitoring from space. Surveys in Geophysics, 37(2), 269-305. 

Michał Halicki, Tomasz Niedzielski, The accuracy of the Sentinel-3A altimetry over Polish rivers, Journal of Hydrology, Volume 606, 2022, 127355, ISSN 0022-1694, Doi: https://doi.org/10.1016/j.jhydrol.2021.127355 (URL resource validated 12th December 2023)

Mercier, F. et Al. (2002) Interannual lake level fluctuations (1993-1999) in Africa from Topex/Poseidon: connections with ocean-atmosphere interactions over the Indian Ocean. Global and Planetary Change, 32(2-3) 141-163.

Related articles

• Page:
  LWL v5.0 and LWL-S v1.0: Product Quality Assessment Report (PQAR)
• Page:
  LWL v5.0 and LWL-S v1.0: Product User Guide and Specification (PUGS)
• Page:
  Lakes Service version 2023: Target Requirements and Gap Analysis Document (TRGAD)
• Page:
  LWL v5.0 and LWL-S v1.0: Algorithm Theoretical Basis Document (ATBD)
• Page:
  LWL v5.0 and LWL-S v1.0: System Quality Assurance Document (SQAD)