Specific Notes:
Regarding surface elevation products:
  1. It is important to consider the location of the satellite ground track AND the section of track used in complex or drought-prone regions where height variability may be location dependent. In such cases check the specific geographical location in the text product header.
  2. High latitude lakes and rivers will freeze, and the ice-on period is not known accurately. Microwave energy can penetrate into ice and reflect back from a layer at an unknown depth. Further, for small or narrow water bodies, or for any water body where the satellite overpass is close to the coastline or islands, the radar altimeters may monitor the build-up of snow and ice on the surface or be affected by the build-up on the surrounding terrain. Winter elevations remain in the radar-based time series but they could be potentially erroneous due to the presence of snow and ice. High latitude observations from the laser altimeters should not suffer the same limitations.
  3. The more recent radar altimeters are using an on-board DEM to more rapidly acquire the water surface. There are cases where this DEM setting is incorrect resulting in either a) the seasonality not being captured or b) the observed seasonality being dampened. End users must check the product Advisories or the product summary Tables for notes on potential DEM issues for that particular lake or reservoir.
  4. The smoothed height variation (lower graphs) products are not available. These can vary according to the smoothing function applied and so are provided as a visualization aid only.
  5. The products are routinely upgraded. End users should note the product version number in the graph legend and table header, and avoid utilizing products of mixed heritage.
10-day River/Wetland Water Level
Water Bodies Date
Monthly River/Wetland Water Level
Water Bodies Date
Satellite Mission Legend:
Sentinel-3A
Sentinel-3B
ENVISAT
SWOT
Project BackgroundTop

This web site provides satellite-based information relating to the current and historical status of the global oceans, lakes & reservoirs, and rivers & wetlands. Here, “status” is primarily centered on variations in surface elevation as provided by a suite of satellite-based radar altimeters. These instruments are onboard various space-agency (NASA/CNES/ESA/ISRO) missions with data set and auxiliary information sourced from NOAA, NASA/GSFC, and RADS. Variations in surface elevation enables changing sea level to be monitored and reveals the seasonal and inter-annual fluctuations in continental waters. The products integrate responses to regional and global climate variations as well as anthropogenic influences. Over land, short- and long-term drought, as well as approaching potential flood conditions, can be highlighted. The data sets span multiple decades and are available in near real time, they thus serve both science and applied science programs. Applications include water resources management, estimating irrigation potential for crop analysis, hazard monitoring, hydro-electric power stability, and regional security.

Satellite AltimetryTop

A satellite radar altimeter is a nadir-pointing instrument continuously recording average surface heights directly below the satellite, as it transverses over the Earth's surface. Operating at Ku or Ka band frequencies, each altimeter emits a series of microwave pulses towards the surface. By noting the two-way time delay between pulse emission and echo reception, the surface height can be deduced. Each returned height value is an average of all surface heights found within the footprint of the altimeter. The diameter of the footprint depends on the surface roughness, but can typically range between 200m (for open pools of water in calm conditions) to a few kilometers (open water with surface waves). Each satellite is placed in a specific repeat orbit, so after a certain number of days the same point (to within 1km), on the Earth's surface is revisited. In this way, time series of surface height changes can be constructed for a particular location along the satellite ground track during the lifetime of the mission. Elevation measurements can be retrieved over ocean surfaces, ice sheets, sea ice, and continental waters such as lakes, reservoirs, river reaches, and wetland regions. Data interpretation techniques are mature, well-validated, and published, and mission continuity is assured to at least to 2030. Ongoing research continually strives to make improvements in both the quality and quantity of available elevation measurements.

Satellite laser altimeters are also nadir-pointing instruments though they have the capability to off-point to specific targets of interest. Operating at green or infrared wavelengths these missions are primarily focused on mapping ice-dominated regions, mapping canopy or vegetation cover, and studying the atmosphere. While GEDI (laser waveform data, mid to low latitudes) resides on the International Space Station, ICESat-1 (laser waveform data) and ICESat-2 (laser photon data) operate from their own orbits and record surface elevations to high geographical latitudes.

There have been a number of altimetric satellite missions to date and follow-on missions can be expected (see Table below).

Instrument Summary
Satellite Operation Repeat Period
Topex/Poseidon 1992-2002 10 days
Jason-1 2002-2008 10 days
Jason-2/OSTM 2008-2016 10 days
Jason-3 2016-2022 10 days
Sentinel-6A MF 2020-current 10 days
HY-2A 2011-2023 14 days
HY-2B 2018-current 14 days
HY-2C 2020-current 14 days
HY-2D 2021-current 14 days
Seasat 1978 17 days
Geosat 1985-1989 17 days
GFO 2000-2008 17 days
SWOT-nadir altimeter 2022-current 21 days
Sentinel-3A 2016-current 27 days
Sentinel-3B 2018-current 27 days
ERS-1 1992-1993, 1995-1996 35 days
ERS-2 1996-2003 35 days
ENVISAT 2002-2010 35 days
SARAL 2013-2016 35 days
ICESat-1 (laser) 2003-2009 90 days
ICESat-2 (laser) 2018-current 91 days at high latitudes
Cryosat-2 2010-current 365 days
SWOT-KaRIn swath altimeter 2022-current variable
GEDI (laser) 2018-2023, 2024-current variable
Contact UsTop

For general information relating to the satellite products, contact
Dr. Charon Birkett
Charon.M.Birkett@nasa.gov
Tel: 301-614-6464

For technical issue relating to the web site and product access, contact
Dr. Martina Ricko
Martina.Ricko@nasa.gov

Funding SourcesTop

The following programs and funding sources are acknowledged,

  1. NASA Applied Sciences/Water Resources
  2. NASA Making Earth Science Data Records
  3. NASA Ocea Surface Topography
  4. NASA ICESat-2 Mission
  5. NASA EIS
  6. The US Department of Agriculture/Foreign Agricultural Service

Lake/Reservoir elevation products can also be found at the USDA/FAS CropExplorer and the NASA PO.DAAC portals . Global mean sea level and gridded sea level products can also be found at the NASA PO.DAAC portals.

Data AcknowledgementTop

The project team acknowledges NASA/PODAAC, NASA/GSFC, NOAA, AVISO, ESA, and the TU-Delft/NOAA-NESDID/STAR “RADS” database, for the provision of altimetric datasets and associated physical parameters. The team also gratefully acknowledges Google Earth.

DisclaimerTop

Many elements of this program reside in the research domain. The project management, support staff, and associated science investigators accept no responsibility for the accuracy and application of the surface elevation products