Methodology
WATER SCARCITY LAYER METHODOLOGY
The model for Water Scarcity is based on water data from grids of the size of 0.5x0.5 degrees. Each grid contains the population and overall water availability for that area. Based on this data, we calculate the average water availability per capita per year for each grid. The grids are summed up in order to obtain the country number. The sum of all countries is the headline number. Based on these grids, we are able to express what percentage of the population lives in areas with an average water availability of less than 500m³, 500m³ to 1,000m³, 1,000m³ to 1,700m³ and more than 1,700m³. The headline number shown is the sum of people living in areas with less than 1000m³ per capita per year, accross all countries. In order to account for small natural climate variations between years, we use 20-year rolling averages of water availability. Using these averages allows us to provide a more accurate estimate of water scarcity.
For example, a region may experience an unseasonably mild winter or perhaps endure particularly heavy snowfall. These years may be outliers and thus misleading when it comes to analysing and forecasting longer-term trends. For the population numbers, we use SSP2. The water data is based on 3 Global hydrological and 5 General Circulation models. By combining those, we get 15 different model combinations and we use the average of all of these. As a result, the figures shown will be independent from the choice of a certain model combination (implicitly assuming a uniform distribution across model combinations).
BIODIVERSITY LAYER METHODOLOGY
The distributional range of all comprehensively assessed freshwater taxa from the IUCN Red List (IUCN, 2021) were obtained. Freshwater taxa are defined as those with any inland wetland (IUCN Habitat Class 5) or coastal freshwater lake (IUCN Habitat Class 13.5) habitat listed as suitable or marginal for the species. Only comprehensively assessed taxa were considered to avoid biases which would be induced by counting the richness of taxonomic groups assessed for some regions and not others. These ranges were then subset to areas where the species is known to be extant, probably extant, or possibly extant, and to areas where it is native or reintroduced. For species with separate ranges corresponding to different seasons (e.g. breeding/non-breeding), these were combined into a single range to capture impacts at any point during the year. Species were then subset based on their threat status (threatened/non-threatened) and the type of pressures affecting them. Species listed as Vulnerable, Endangered, or Critically Endangered are considered threatened by IUCN.
In terms of pressures, both direct and indirect pressures relating to water quality were considered. Direct impacts were classed as any form of liquid, solid, or energy pollution (IUCN Threat Classes 9.1-9.4, 9.6), while indirect water quality pressures include disturbance due to freshwater aquaculture, shipping lanes, fishing and harvesting aquatic resources, recreational activities, ecosystem management, or invasive species (IUCN Threat Classes 2.4, 4.3, 5.4, 6.1, 7.3, 8.1). Pressures relating to water extraction and water management were considered to be primarily those relating to dams and water management use, as well as utility and service lines including aqueducts (IUCN Threat Classes 4.2, 7.2). Each species’ distributional range was then overlaid with each water basin to count the number of species within the basin that are threatened and known to be affected by one of the categories of water extraction or quality pressures. Each species was weighted by the inverse proportion of its global range intersecting with the basin (following Harfoot et al. 2021). This aims to take into account the fact that species with a greater proportion of their range within the basin are more likely to be representative of the basin’s impact on species’ threat status and pressures.
With respect to freshwater species and what defines threatened (from the above) is: Freshwater species: Data was used for all species available through IUCN and that made use of freshwater habitats, based on the legend used by IUCN (https://www.iucnredlist.org/resources/habitat-classification-scheme). Freshwater habitats were defined as freshwater + brackish freshwater, i.e. those in: level 1 == 5 (wetlands inland), level 2 == 13.5 (coastal freshwater lakes). In reality these will be both species which are exclusively aquatic (fish, crayfish) as well as amphibians and birds which utilise freshwater habitats and thus are dependent on them. Threatened species: IUCN conventions were followed and were categorised as species as threatened which have either a Vulnerable, Endangered or Critical endangered status according to the IUCN redlist (https://www.iucnredlist.org/).
WATER AREA SURFACE LAYER METHODOLOGY
Climate change and agricultural use are the leading factors in driving changes in freshwater habitats such as rivers, lakes and wetlands. We made use of time series of global surface water changes which were intersected with global delineations of key freshwater habitats, and other datasets, and summarise for each catchment area. Changes for the year 2030 were estimated based on the rate of change across time up to 2019, based on a Business as Usual assumption that changes in water surface area will continue.