Global Assessment of Environmental Flows and Scarcity

Freshwater ecosystems need a certain amount of water (Environmental Flows- EF) to sustain their ecological processes and biodiversity. As more water is withdrawn for human use (such as agriculture), the pressure on freshwater ecosystems grows, and the need to estimate and maintain EF becomes urgent. Sustainable Development Goal 6, target 4 suggests to explicitly use EF in the estimation of the indicator 6.4.2- "Water stress".

This site is set to support the input to the above target and indicator, and suggests the numerical values of EF per country and a map of EF distribution globally. These EF values shall be seen as a starting point for more detailed EF estimates that countries can subsequently undertake. These estimates are based on the first global study of EF summarized in Smakhtin, V.; Revenga, C.; Doll, P. (2004a) and Smakhtin, V.; Revenga, C.; Doll, P. (2004b).

The study considers hydrological variability to be the primary driver of ecological processes in freshwater ecosystems. As river flow regimes change worldwide, so too will their EF. It is assumed that ecosystems in river basins with highly variable hydrological regimes may need a lower proportion of natural runoff for their functioning since aquatic life is used to the extended periods of limited or no flow. Estimates of total EF in such basins are dominated by a proportion of high flows in the wet season. This applies to most monsoon-driven river systems (e.g., Indus, Ganges) and rivers in arid areas (e.g., Nile, Limpopo). The aquatic life in river basins with low hydrological variability, however, is more sensitive to water scarcity and may need a larger proportion of natural runoff. Estimates of total EF for such basins are dominated by a proportion of stable low flows throughout the year. This applies to rivers in the equatorial belt (Amazon, Congo) and basins with a high proportion of lakes (in Canada, Finland etc). EF consists of ecologically relevant low-flow and high-flow components, and depend upon the objective of environmental water management. Both components are related to river flow variability and are estimated by conceptual rules from discharge time series simulated by the global hydrology model (WATER GAP).

Under the above assumptions, EF in river basins range from 20 to 50% of the total renewable water resources (TRWR). On average, at least 30 % of the world's water resources have to be allocated for the environment to maintain a fair condition of freshwater ecosystems worldwide. Fair condition is understood as a “moderate modifications from natural freshwater ecosystem conditions”, and may be seen as equivalent to environmental water management class C (Class A being “pristine”, Class B – “slight modification”, and Class D – “high modification). Class C (“fair condition) is the most feasible to start with, given the overall condition of freshwater ecosystems globally. Suggestions on quick estimation of EF for other classes can be found in Smakhtin et. al (2004b).

Higher confidence EF estimates may be obtained using more detailed EF assessment methods. For details on existing EF methods see Acreman and Dunbar (2004).

The water stress indicator, similar to that of the SDG indicator 6.4.2, shows what proportion of the utilizable water in world river basins is currently withdrawn for direct human use and where this use is in conflict with EF. This indicator will need to be revisited by Countries with more recent data on water withdrawals and more detailed estimates of total water resources.

The above EF and WSI data are aggregated from 30’ by 30’ spatial resolution that was available at the time of the study. A more detailed data on EF for various management classes, with on-line data extraction and manipulation functionality, and with a finer resolution of 6’ by 6’ is now at the final stages of development and will be release soon to support indicator 6.4.2 monitoring at national and sub-national levels.