FAO has been collecting and analyzing data on water resources and their use through its AQUASTAT country surveys since 1992. The periodicity of country surveys is about 10 years due to budgetary constraints, but it could be reduced to 5 years relatively easily, if more resources would be available.
Data on water resources, expressed in long-term averages of annual values, can be considered stable (they vary annually around a long term average which is constant, except on a long term basis where climate change can induce variations). They have been estimated by FAO on the basis of country information, complemented with available global datasets, and are available for all countries at
Water use data are collected from information available at country level through the AQUASTAT country surveys. Estimates of water use, based on a standardized methodology, have been made by FAO for most countries for the year 2000 and 2005 and are available at http://www.fao.org/nr/water/aquastat/dbase/index.stm. They are based on country information, complemented, when needed, with expert estimates based on unit water use figures by sector.
AQUASTAT data on water resources and use are also published every 3 years through the United Nations World Water Development Report.
A comparative analysis of available country water resources data is carried out at regular intervals. On that basis, AQUASTAT compiles and updates its best estimates of the main elements of the water balance for each country.
AQUASTAT collects statistics on water resources and data on water resources obtained from national sources are systematically reviewed to ensure consistency in definitions and between countries located in the same river basin. A methodology has been developed and rules established to compute the different elements of national water balances. A brief overview of the rules is given in the article “Key water resources statistics in AQUASTAT”, available at ftp://ftp.fao.org/agl/aglw/docs/PaperVienna2005.pdf.
In the case of conflicting sources of information, the difficulty lies in selecting the most reliable one. In some cases, water resources figures vary considerably from one source to another.
There may be various reasons for such differences:
- Differences in computation methods or definitions used in computing water resources.
- Differences in the reference period used to calculate the long-term average annual water resources.
- Overestimation of resources where there is double counting of surface water and groundwater.
- Transboundary rivers: Methods used by countries to compute transboundary rivers flows are not always transparent and countries sharing the same river, might each count the same water resources.
- Misuse of the concept of renewable water resources. Some sources may or may not include extraction of fossil water as part of water resources. Others include non-conventional sources of water such as desalinated water and wastewater, or return flows from agriculture.
- Reasons for changes in estimates on long-term average annual values can also be attributed to the availability of better data due to improvements in knowledge, methods or measurement networks.
It is hoped that through the comparative analysis of available country statistics on water resources the most reliable and complete dataset of water resources by countries is obtained and that the results can assist in harmonizing existing water resources databases.