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3. How much freshwater is available in different countries?

    The source document for this Digest states:

    Efforts to characterize the volume of water available to a given nation have been ongoing for several decades. The primary input for many of these estimates is an information database (AQUASTAT) that has historically been developed and maintained by FAO. It is based on data related to the quantity of water resources and uses a water-balance approach for each nation (FAO, 2003a). This database has become a common reference tool used to estimate each nation’s renewable water resources. FAO has compiled an Index of Total Actual Renewable Water Resources (TARWR). The details of how the TARWR Index and its national Per Capita Equivalent of ‘Availability’ (PCA) are determined and some of the considerations that should be taken into account when using the database index are explained in Box 4.1. The TARWR and PCA results for most nations from the latest 2005 update of the FAO AQUASTAT database are found in Table 4.3.


    Total Actual Renewable Water Resources

    (TARWR) is an index that reflects the water resources theoretically available for development from all sources within a country. It is a calculated volume expressed in km3/year. Divided by the nation’s population and adjusted to m3/yr, it is expressed as a per capita volume more readily allowing a relative evaluation of the resource available to its inhabitants. It estimates the total available water resources per person in each nation taking into account a number of individual component indicators by:

    • adding all internally generated surface water annual runoff and groundwater recharge derived from precipitation falling within the nation’s boundaries,
    • adding external flow entering from other nations which contributes to both surface water and groundwater,
    • subtracting any potential resource volumes shared by the same water which comes from surface and groundwater system interactions, and
    • subtracting, where one or more treaty exists, any flow volume required by that treaty to leave the country.

    It gives the maximum theoretical amount of water actually available for the country on a per capita basis. Beginning in about 1989, TARWR has been used to make evaluations of water scarcity and water stress.

    Considerations related to availability in the TARWR index

    It is important to note that the FAO estimates are maximum theoretical volumes of water renewed annually as surface water runoff and groundwater recharge, taking into consideration what is shared in both the surface and groundwater settings. These volumes, however, do not factor in the socio-economic criteria that are potentially and differentially applied by societies, nations or regions to develop those resources. Costs can vary considerably when developing different water sources. Therefore, whatever the reported ‘actual’ renewable volume of water, it is a theoretical maximum, and the extent to which it can be developed will be less for a variety of economic and technical reasons. For example, Falkenmark and Rockstrom (2004) point out that, globally, approximately 27 percent of the world’s surface water runoff occurs as floods. That is not considered a usable water resource even though it would be counted as part of the annual renewable surface water runoff component of TARWR. Therefore, the usable volumes available as resources to meet societal demands will be considerably less than the maximum number given as a nation’s TARWR.

    Four additional limitations are inherent in the TARWR information. First, seasonal variability in precipitation, runoff and recharge, which is important to regional and basin-level decision making and water storage strategies, is not well reflected in annualized quantities. Second, many large countries have several climatic settings as well as highly disparate population concentrations and the TARWR does not reflect the ranges that can occur within nations. The recently developed small-scale Relative Stress Index Map (Vörösmarty) could assist in overcoming this oversight. Third, there is no data in TARWR that identifies the volumes of ‘green’ water that sustain ecosystems – the volumes that provide water resources for direct rain-fed agriculture, grazing, grasslands and forests – nor does it account for the volumes of water that are potentially available from non-conventional sources (reuse, desalination, non-renewable groundwater). Finally, while the accounting-based method for a nation’s TARWR adds all water that enters from upstream countries, it does not subtract any part of the water that leaves the nation in the TARWR number although estimates of those volumes are available for each country from the database. Source: FAO, 2003a; FAO-AQUASTAT, 2005

    Table 4.3: Water availability information by country (AQUASTAT, FAO 2005)

    Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
    Section 2: Changing Natural Systems,
    Chapter 4 (UNESCO & WMO, with IAEA),
    Part 2. Nature, Variability and Availability, 2g. Water availability, p.130

    Relative Water Stress Index (from intro to Section 2 p.116)

    Water stress is commonly evaluated by comparing the volume of renewable water resources per capita at a national level. New mapping capabilities allow the geography of water stress to be better defined. High resolution water stress indices can be computed based on the ratio of total water use (sum of domestic, industrial and agricultural demand or DIA) to renewable water supply (Q), which is the available local runoff (precipitation less evaporation) as delivered through streams, rivers and shallow groundwater. Developed from actual statistics, the Relative Water Stress Index (RWSI), also known as Relative Water Demand, is useful because it is a dimensionless quantity, which can be applied at different scales. The map below shows populations living in water stressed (RWSI ≥ 0.4) and relatively unstressed (RWSI < 0.4) conditions highlighting substantial within-country differences that national estimates often obscure. The map shown below (at approximately 50 km resolution globally) nearly tripled earlier nation-wide estimates of those people living under severe water stress, with obvious impacts on the degree to which water problems can be appropriately identified and managed.

    Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
    Section 2: Changing Natural Systems,
    Relative Water Stress Index, p.116