Freshwater provision to human populations
This indicator reflects the importance of providing clean freshwater from natural habitats to downstream human populations per unit area.
By 2020, ecosystems that provide essential services, including services related to water, and contribute to health, livelihoods and well-being, are restored and safeguarded, taking into account the needs of women, indigenous and local communities, and the poor and vulnerable.
Higher value of the Freshwater Flow Index indicates a greater amount of clean freshwater ecosystem service per square kilometer that an area provides to the downstream human populations for consumption.
This indicator is available at the basin, country, and regional scales for the Tropical Andes, African Great Lakes, and Greater Mekong regions.
This indicator is available for the year 2010 as a baseline.
Freshwater provision data were developed by Larsen et al. (Larsen et al., 2011 and 2012) using spatially explicit maps of runoff from the global hydrological water model WaterGAP (Alcamo et al., 2003), hydrological drainage directions (Lehner et al., 2008; USGS, 2000), downstream human population density (Oak Ridge National Laboratory, 2006), and global land cover data (used to weight flow estimates by a quality coefficient, based on information from previous studies (Balmford et al., 2008; Brauman et al., 2007; Bruijnzeel, 2004; Dudley and Stolton, 2004). Estimated quality-weighted freshwater provision, reported as a freshwater flow index, was calculated for 2,592 km2 hexagonal grid cells. Using this grid, we calculated a mean value for each analysis unit of basin, country and region.
Specifically, first, we modelled the flows of water from upstream source cells to human beneficiaries in downstream cells. Although the freshwater provided by habitats often acts as a supporting service for downstream ecosystems, we here focus on water most immediately available to people. Thus, a key step weights freshwater services according to the presence of human populations downstream. We began with global maps of runoff fi among cells i (available for use within i or in downstream cells); and demand Di (computed as total global water consumption (Alcamo et al., 2003) allocated among cells in proportion to human population of cell i), and applied the following equations:
Equation (1) computes the total demand TDi across all cells downstream of site i. Sets of cells upstream of i (UPi) or downstream (DOWNi) are computed from a global 30-arc-second drainage direction map (Lehner 2008; USGS 2000). Equation (2) computes the total scaled flow Tsfj from all upstream cells into cell j. Tsf allocates flow from upstream cells to downstream cells j in proportion to demand in j, thus accounting for the fact that source cells generally supply water to more than one downstream cell. Equation (3) computes the water provision Iij of upstream cell i to downstream cell j. In so doing, it credits i's contribution to j only to the point where j's demand is met; no credit is given for contributions in excess of downstream demand. Finally, equation (4) sums up the total contribution Ii of cell i to all downstream demand.
This is necessarily a coarse model but it captures much of the relevant spatial variation in elevation, precipitation, and nearby population (which we expect to vary less within cells) and habitat (which varies within cells, but we account for that variation). We calculated the estimated water provision from a priority site as the mean water provision value among hexagon cells that the site covered (weighted by area of overlap).
Second, this global model for freshwater flow to downstream populations was then modified by using land cover distributions to derive values for freshwater flow based on the estimated influence of different land cover types on freshwater services (e.g., forests are more important for water quality than grassland). Because the water flow and quality implications of finely differentiated habitat types are poorly understood, we thus created a map of coarse land cover types (GlobCover) having pixel size of 1 km2 or less (30 arc-second). We derived quality-weighted water flow coefficients for the broad land cover types, based on existing literature (See Table 1 below; Balmford 2008; Brauman et al., 2007; Bruijnzeel 2004; Dudley & Stolton 2003); e.g., forests effectively reduce surface erosion and increasing water infiltration; wetlands effectively remove suspended solids, phosphorus, and nitrogen etc. By mapping these coarse land cover types to GlobCover land covers, each category thus had flow and quality coefficients.
Table 1. Flow coefficients of different land cover types
|Ecosystem||Stable water flow|
|Land converted to grassland||0.25|
|Snow and ice||1|
|Urban and bare soil||0.17|
The given freshwater values for flow are in principle an index, as the model on water provision (million m3 freshwater provided by that cell/polygon to downstream human population per year) has been adjusted by multiplying with the water coefficient.
Learn more about the methodology from the publications below:
Larsen FW, Londono-Murcia MC, Turner WR (2011) Global priorities for conservation of threatened species, carbon storage, and freshwater services: scope for synergy? Conservation Letters 4: 355-363.
Larsen FW, Turner WR, Brooks TM (2012) Conserving Critical Sites for Biodiversity Provides Disproportionate Benefits to People. Plos One 7.
Index values are relative, not absolute.
Because the freshwater provision data are currently only available for a single time step (2010), we cannot yet calculate trends.
Spatial resolution of the source data is too coarse (2,592 km2 pixels) to estimate freshwater provision in small areas.
Learn more about the method, result and discussion of this indicator from the publication:
Han X, Smyth RL, Young BE, Brooks TM, Sanchez de Lozada A, et al. (2014) A Biodiversity Indicators Dashboard: Addressing Challenges to Monitoring Progress towards the Aichi Biodiversity Targets Using Disaggregated Global Data. PLOS ONE 9(11):e112046
World WaterGAP 2 model runoff map is available through Water world; and the model is described in the article below:
Alcamo J, Doll P, Henrichs T, Kaspar F, Lehner B, et al. (2003) Development and testing of the WaterGAP 2 global model of water use and availability. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques 48: 317-337.
The hydrological drainage direction map is available through U.S.Geological Survey HYDRO1k Elevation Derivative Database (2000); and the methodology is described in the article below:
Lehner B, Verdin K, Jarvis A (2008) New Global Hydrography Derived From Spaceborne Elevation Data. Eos, Transactions American Geophysical Union 89: 93-94.
The Landscan Global Population Database was developed by and is available through Oak Ridge National Laboratory Landscan Global Population Database (2006).
The GlobCover land cover map is available through European Space Agency GlobCover Portal.
To learn more about the discussion on the impacts of broad land cover types on water flow and quality from the literatures, see:
Balmford A, Rodrigues ASL, Walpole M, ten Brink P, Kettunen M, et al. (2008) Review on the economics of biodiversity loss: scoping the science. European Commission.
Brauman KA, Daily GC, Duarte TK, Mooney HA (2007) The nature and value of ecosystem services: An overview highlighting hydrologic services. Annual Review of Environment and Resources. pp. 67-98.
Bruijnzeel LA (2004) Hydrological functions of tropical forests: not seeing the soil for the trees? Agriculture Ecosystems & Environment 104: 185-228.
Dudley N, Stolton S (2003) Running Pure: The importance of forest protected areas to drinking water. World Bank/WWF Alliance for Forest Conservation and Sustainable Use.