Definitions

Annual averages, five-year average and season averages

Annual and seasonal water extent in square kilometers is available for download. Additionally, five-year rolling averages are available. The annual and seasonal data show high variability between years due to periods of drought and intense rainfall. The five-year rolling averages are useful for tracking long term changes in water extent. These include the current year and four previous years, thus the five-year rolling average for 2004 in the chart would be based on 2000-2004.

The five-year averages are used for the Sustainable Development Goal (SDG) indicator 6.6.1 as the purpose of the SDG is related to tracking longer term change. Note that in the context of the SDGs, the 2000-2004 data is considered as a baseline.

Annual permanent water and seasonal 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 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.

The surface area measurements reported here have been extracted from the Global Surface Water Explorer (GSWE) developed by the European Commission Joint Research Centre. The current data product includes inter and intra-annual changes in the coastline, which may be quite dramatic. Further analysis a ‘reference’ coastline could be established for some defined time period in the future to reduce coastal variations . The coastal variations will add an offset, but shouldn’t influence the trends too much.

The analysis applied a consistent algorithm to all 35 years of the Landsat observations. Current efforts are being made to add Sentinel data to the analysis.

The water transitions from the Global Surface Water Explorer is shown as the map underlying this site. The map shows changes in water state between the first year and the last year of observation. It documents:

• Permanent water surfaces
• New permanent water surfaces (i.e. conversion of a no water place into a permanent water place.)
• Unchanging permanent water surfaces
• Lost permanent water surfaces (i.e. conversion of a permanent water place into a no water place)
• New seasonal water surfaces (i.e. conversion of a no water place into a seasonal water place)
• Unchanging seasonal water surfaces
• Lost seasonal water surfaces (i.e. conversion of a seasonal water place into a no water place)
• Conversion of permanent water into seasonal water
• Conversion of seasonal water into permanent water
• Ephemeral permanent water (i.e. no water places replaced by permanent water that subsequently disappeared within the observation period)
• Ephemeral seasonal water (i.e. no water places replaced by seasonal water that subsequently disappeared within the observation period)

References:

Pekel, J. F., Cottam, A., Gorelick, N., and Belward, A. S. (2016). High-resolution mapping of global surface water and its long-term changes. Nature, 540(7633), 418-422.

Surface water aggregations have been precomputed by political boundaries and hydrobasins. Each of these are available at two levels for a total of four sets of aggregations.

FAO Global Administrative Unit Layers (GAUL)

GAUL has been developed by FAO to represent information on administrative units and coastlines for all countries in the world. GAUL 0 depicts national level boundaries while the first level of administrative boundaries within countries are depicted by GAUL 1. Note that the GAUL layers maybe quite large for countries with long coastlines. More information can be found on the GAUL site.

The use of GAUL does not represent an endorsement of the boundaries in GAUL, but the GAUL has been approved by the UN Inter-Agency and Expert Group on the Sustainable Development Goals for analytical purposes in the context of monitoring sustainable development.

WWF HydroBASINS

HydroBASINS is a series of polygon layers that depict watershed boundaries and sub-basin delineations at a global scale. The goal of this product is to provide a seamless global coverage of consistently sized and hierarchically nested sub-basins at different scales (from tens to millions of square kilometers), supported by a coding scheme that allows for analysis of watershed topology such as up- and downstream connectivity. The current data product provide information for Hydrobasin levels 3, 4, 5 and 6. More information can be found on the HydroBASINS site.