FAQ & Definitions

The following provides a list of frequently asked questions (FAQ) and definitions for commonly used terms and key concepts. If your question is not answered, please contact us.

Frequently Asked Questions

What is Sustainable Development Goal indicator 6.6.1?

Water bodies are crucial for sustaining life and ecosystem services. As part of the 2030 Agenda for Sustainable Development, UN Member States committed to achieving Sustainable Development Goal (SDG) 6, ensuring water and sanitation for all. Within SDG 6, target 6.6 specifically aims to protect and restore water-related ecosystems, including mountains, forests, wetlands, rivers, aquifers, and lakes. Monitoring progress on target 6.6 uses one global indicator, indicator 6.6.1, change in the extent of water-related ecosystems over time, which can provide the necessary data for countries to take action. The indicator is framed around the monitoring of different types of water-related ecosystems including lakes, rivers, wetlands, groundwater and artificial water bodies such as reservoirs.

What is the Water-Related Ecosystems site?

To make progress on indicator 6.6.1, UN Environment has led the process since 2015 to develop an indicator monitoring methodology. However, the pilot data collection for indicator 6.6.1 conducted by UN Environment in 2017 found that only 20 per cent of United Nations Member States were able to provide the information from a self-reporting questionnaire. To fill the data gap, UN Environment and Google have collaborated with the European Commission’s Joint Research Centre and launched this Water-Related Ecosystems platform, aiming at providing free and open access to national, sub-national, basin and sub-basin aggregated data on water extent. Satellite-based Earth observations on this site partially supplement countries’ in situ monitoring and nationally reported data. Currently the site covers only parts of indicator 6.6.1 components (i.e. change in spatial extent of open water bodies) but will include others (e.g. water quantity and quality, natural and artificial water bodies, etc.) in the future.

What are the next steps for the project?

Currently the site covers most parts of indicator 6.6.1 components (i.e. permanent and seasonal surface water, including a separation of natural and artificial, wetlands, and mangroves and water quality. In the future it will include water quantity (volume) dynamics of lakes and reservoirs as well as river flow discharge.

How can I use this site to report progress on SDG target 6.6?

The components of SDG indicator 6.6.1 that are currently covered by the site (i.e. change in spatial extent of open water bodies) will be shared with countries before being submitted to the UN SDGs database, maintained by the UN Statistics Division. This will need to be augmented by countries’ in situ monitoring and nationally reported data. In the future there are plans to include more information (e.g. water quantity and quality, natural and artificial water bodies, etc.) to help countries in their reporting of SDG target 6.6.

What kind of data can I access and what is the data source?

Various formats of data can be accessed including GeoTiff, tabular CSV, and shapefile layers. Data products such as annual averages, five-year average and season averages, annual permanent water and seasonal water have been extracted from the Global Surface Water Explorer (GSWE), developed by the European Commission’s Joint Research Center in partnership with Google Earth. Political boundaries such as FAO Global Administrative Unit Layers (GAUL) is extracted from the GAUL site. Watershed boundaries such as WWF HydroBASINS is extracted from HydroBASINS site.

Earth observation data (i.e., spatial extent of open water bodies) is acquired by Landsat 5, 7 and 8 satellites at a 30 m resolution from 2001-2015. From 2016 to 2030, higher spatial and temporal resolution satellites, including both optical and radar satellites, will be used. For example, 20 m Sentinel 1 (radar) and 10 m Sentinel 2 (optical) satellites, used in combination with Landsat satellites, will allow for a more precise delineation of water bodies both in spatial terms (due to the higher spatial resolution) and in temporal terms (due to the higher revisit time). Learn more about the monitoring methodology by reading the SDG Indicator 6.6.1 Monitoring Methodology document in the Resources page.

How often is the data updated?

Satellite data will be updated annually by the first quarter of each year.

How can I interpret the data from this website?

Based on the satellite images accompanied by downloadable statistical data and trend analysis, countries can understand changes occurring in the spatial extent of open water bodies; identify new or lost water bodies, and where changes are happening to seasonal (ephemeral) water bodies. This is valuable for informed decision making on water bodies, and conducive to cross-country collaboration and learning. However, arbitrary conclusions should be avoided such associating a gain of water extent as a positive trend and loss of water extent as a negative trend without further site-specific studies. Data on this site should be used solely as a way to flag potential areas of concern that need to be further researched. Considering unique conditions of each country and complex mechanisms of dynamics of water-related ecosystems, more supportive information are needed to interpret data properly and make decisions comprehensively.

How do I calculate percent change?

Using a baseline period, percentage change of spatial extent is calculated using the following formula:

Percentage change in spatial extent = (β - γ )/β*100

Where β = the average national spatial extent from 2001-2005

Where γ = the average national spatial extent of any other 5 year period (stats are present using the most recent 5 year period)

Where can I get further help or training?

If you cannot find the answer to your questions on this FAQ page, please contact us with further questions and inquiries.


Water-related Ecosystems

Water-related ecosystems are a subset of all ecosystems (MEA, 2005). They contain the world’s freshwater resources, natural and artificial, and include lakes and reservoirs; rivers, streams, canals and estuaries, groundwater; and several types of wetlands such as swamps, bogs, peat, paddies and mangroves. Water-related ecosystems can be defined as “a dynamic complex of plant, animal, and micro-organism communities and the non-living environment dominated by the presence of flowing or still water, interacting as a functional unit.” (MEA, 2005; Dickens et al, 2019).

Wetlands (full terminology: Inland vegetated wetlands)

Inland vegetated wetlands are mapped according to the following definition: “Inland vegetated wetlands include areas of marshes, peatlands, swamps, bogs and fens, the vegetated parts of floodplains as well as rice paddies and flood recession agriculture”. The method only measures inland vegetated wetlands and not coastal mangroves (see section 3.5 of this methodology on mangroves). This SDG indicator methodology is used for official reporting of SDG indicator 6.6.1 statistics. It does not apply the definition of wetlands defined by the Ramsar Convention on Wetlands, which is: “areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters”. The Ramsar definition of wetlands may be interpreted to mean all water within a country including the marine environment. The SDG indicator 6.6.1 definition refers to only a specific group of inland vegetated wetlands typologies.

Annual permanent and seasonal water (naturally occurring surface water)

A permanent water surface is underwater throughout the year (whilst a seasonal water surface is underwater for less than 12 months of the year). Some locations don’t have observations for all 12 months of the year (for reasons such as polar night). In these cases, water is considered as seasonal if the number of months where water is present is less than the number of months where valid observations were acquired.

A second consideration is lakes and rivers that freeze for part of the year. During the frozen period water is still present under the ice (true both for rivers/lakes and the sea). If water is present throughout the observation period (i.e. unfrozen period), the water body is considered as a permanent water surface. If the area of the water body contracts during the unfrozen period, then the pixels along the borders of the lake or river are no longer water, and those pixels will be considered as a seasonal water surface.

Gaps in the observation record are a source of uncertainty that needs to be recognised in the trends-analysis. Part of the water surface for any country may not be observed for specific years (the analysis occurs by pixel), which would underestimate the reported area. To document this uncertainty, the measured values of surface water area are presented with an estimate of the area of unobserved, but potentially surface water.

The actual permanent surface water area might be greater than the measured value due to the lack of valid observation. As the number of observations increases, then uncertainty decreases. The uncertainty is usually largest in the early years of the archive, and falls dramatically after 1999 (when Landsats 5 and 7 were both in operation) and to close to zero after April 2013 (with the new Landsat 8 data acquisition strategy) (see Pekel et al. (2016)). Note that these limitations affect all applications based on the Landsat archive. The GSWE analysis was specifically tuned to account for and mitigate against the spatial and temporal variability of the archive.

Reservoirs (artificially occurring surface water)

A permanent water surface is underwater throughout the year (whilst a seasonal water surface is underwater for less than 12 months of the year). Some locations don’t have observations for all 12 months of the year (for reasons such as polar night). In these cases, water is considered as seasonal if the number of months where water is present is less than the number of months where valid observations were acquired.

Reservoirs are artificial (or human-made) bodies of freshwater, as opposed to lakes which are naturally occurring. The reservoirs dataset represents surface area data on artificial water bodies including reservoirs formed by dams, flooded areas such as opencast mines and quarries, flood irrigation areas, and water bodies created by hydro-engineering projects such as waterway and harbour construction.


Turbidity is an indicator of water clarity, quantifying the haziness of the water and acting as an indicator of underwater light availability.

Trophic State

Trophic State Index refers to the degree at which organic matter accumulates in the water body and is most commonly used in relation to monitoring eutrophication.

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