In general, data on water issues are sparse, particularly with respect to water quality and pollution; informative time series are often difficult to construct. Existing data typically come from national or regional studies undertaken by independent researchers, or data are the result of comprehensive surveys performed (infrequently) by international organizations. Since data have disparate sources, data collection and calculation methodologies may vary.
The majority of country-level water resource data provided on EarthTrends comes from the Food and Agricultural Organization's AQUASTAT database (follow the "database" link). The AQUASTAT database incorporates roughly 75 water-related variables; approximately one-quarter of these are included on EarthTrends. AQUASTAT variables pertain to geography and population, available water resources, water use and withdrawal, waste water produced and treated, and irrigation (both land area and water use). Data are available from 1958 to the present for most countries and regions. However, since data are not frequently measured/updated or do not change substantially over time, AQUASTAT reports values as annual averages for five-year time increments (e.g., 1998-2002). Values are often only available for the most recent survey year (which may date back several years to decades) or a single, unchanging value is applied to the entire time-span of the AQUASTAT database. As a result of the temporal scarcity of available water data, where appropriate, WRI has condensed the AQUASTAT datasets for presentation purposes on EarthTrends and reports only the most recent data, or provides only a single value for a country for a particular variable. Please note that existing (older) estimates in AQUASTAT are frequently revised and although EarthTrends attempts to incorporate these changes through regular updates of the relevant datasets, there is sometimes disagreement between databases.
All of the data related to water use and water resources provided on EarthTrends can be found in the searchable database of the "Water Resources and Freshwater Ecosystems" research topic. These data include water availability, actual and internal renewable water resources, water withdrawals, and groundwater data. EarthTrends also features a special collection entitled Watersheds of the World, full of data and maps pertaining specifically to the geography and hydrology of river basin watersheds around the world.
Data related directly to water quality and pollution are extremely limited. One gauge of pollution—the amount of biochemical oxygen demand (BOD)—is available on EarthTrends via the World Bank Indicators. Additionally, the United Nations Environment Programme-Global Environmental Monitoring System (UNEP-GEMS) produces the GEMSTAT database, an impressive collection of data from approximately 1400 lake and river monitoring sites set-up throughout the world. Station data include general water chemistry (e.g., pH, alkalinity), specific ion contents, and metal, nutrient, and organics abundances.
Other water data can be found on the websites of water divisions or resource management offices of certain governments. For example, the United States Environmental Protection Agency (EPA) and United States Geological Survey both provide sub-national water data for the U.S.
