1. Hydrological data exchange and the formation of global data bases is a difficult, time-consuming and sensitive process. Unlike meteorological data, hydrological data are usually only held and distributed at national level. Limited international exchange of hydrological data has taken place, but usually only when required for operational purposes, such as flood forecasting on international rivers. Some exchange for research purposes has existed, though. Hydrological data are the responsibility of a wide variety of organizations and ministries within individual countries, which makes the problem of data exchange difficult at the national and even more so at the international level.
2. In spite of the above, a number of global hydrological data bases do exist, such as:
(i) The Global Runoff Data Centre (GRDC) of WMO in the Federal Institute of Hydrology, Koblenz, Germany;
(ii) The Global Precipitation Climatology Centre (GPCC) of WMO in the German Weather Service, Offenbach, Germany;
(iii) The Global Environment Monitoring System (GEMS-WATER) of UNEP with assistance of WHO, water quality data being held in the Quality Monitoring Centre managed by the Canadian Centre for Inland Waters in Burlington, Canada;
(iv) The Flow Regimes from International Experimental and Network Data (FRIEND) data base (flow data, originally from research basins in Europe but broadening) co-ordinated from the Institute of Hydrology, Wallingford, UK within the context of UNESCO's IHP-V;
(v) The National Snow and Ice Data Centre (WDC-A Glaciology) held by CIRES, University of Colorado, Boulder, USA;
(vi) The World Glacier Monitoring Service acting under the umbrella of ICSI of IAHS with the support of IGSU/FAGS, UNESCO/IHP and UNEP/GEMS, with its Secretariat in ETH Zurich, Switzerland;
(vii) The Global Resources Inventory Database (GRID) of UNEP, which has a wide range of different types of data, some of it water-related, held in several locations;
(viii) The remote sensing programme at the Mullard Space Science Laboratory of the University College London aims to monitor and interpret short and medium term lake volume changes. The MSSL Global Lakes Database contains location and lake type information along with lake levels and areas for over 1400 inland water bodies including, as far as possible, all lakes and reservoirs but not lagoons, with surface areas in excess of 100 km2.
(ix) A data base of the state of the world lakes has been created by the International Lake Environment Committee (ILEC) and the Lake Biwa Research Institute (Japan).
(x) The Global Network “Isotopes in Precipitation” (GNIP) of IAEA is operated with the assistance of WMO and collects data worldwide on the contents of stable environmental isotopes in precipitation. These data, kept in IAEA, Vienna, are useful as a benchmark for testing the relations between isotopic composition of precipitation and climate (temperature, volume of precipitation, source of moisture).
3. WMO, as the specialized agency of the United Nations in charge of meteorology and operational hydrology, is promoting, in association with the World Bank and other UN agencies concerned, the establishment of the World Hydrological Cycle Observing System (WHYCOS) which is intended to provide real-time information on a number of hydrological and meteorological variables. WHYCOS should act as a tool for the improvement of collection, dissemination and use of high quality, standardized and consistent hydrological and related information at a national, river basin, regional and international levels. The system is to be based on a world-wide network of key stations linked by satellite with an associated quality-controlled data base for variables characterizing the quantity and quality of water, plus basic hydrologically-relevant meteorological variables. WHYCOS is potentially of great importance to water resources assessment on the global, regional and national scales. Initially, WHYCOS will consist of about 1000 stations world-wide, sited on major rivers, including one site near to the river mouth, yet beyond the extent of saline intrusion. Each station will monitor up to 15 variables such as river water parameters (stage, temperature, pH, conductivity, turbidity, dissolved oxygen) and atmospheric parameters (precipitation, air temperature, relative humidity, wind speed and net radiation). The data collected will be transmitted via satellite to regional and global centres. WHYCOS stations will generally be selected as sub-sets of the existing station networks agreed with the national agencies, usually those with long time series of records. In the developed world, where many good quality stations already operate, WHYCOS will capitalize on them. The effort will essentially be a matter of choosing benchmark, or reference stations, up-grading and networking. However, in the developing world, substantial investment will have to be allocated for establishing the stations themselves and the capacity building programme for the National Hydrological Services participating in WHYCOS. The implementation of the first component of the WHYCOS has recently been initiated for countries of the Mediterranean and the Black Sea Basin (MED-HYCOS) under a World Bank grant. The concept is being developed also for other regions, such as SADC Africa (SADC-HYCOS), Latin America and the Caribbean (CARIB-HYCOS) and for the Aral Sea Basin (ARAL-HYCOS).
4. There are a wealth of other important hydrological data bases being formed in a number of international programmes. One of the main objectives of the Global Energy and Water Cycle Experiment (executed under the World Climate Research Programme jointly sponsored by WMO, ICSU, and the IOC of UNESCO) is to determine the hydrological cycle and energy fluxes by means of global measurements of observable atmospheric and surface properties. The scientific plan for GEWEX calls for a major data collection effort to complement the existing river discharge data. Such GEWEX projects as GCIP (GEWEX Continental-scale International Programme), GEWEX Asian Monsoon Experiment (GAME), MAGS in the Mackenzie drainage basin, BALTEX in the Baltic Sea Basin, LBA, that is Large-scale Biosphere-atmosphere Experiment in Amazonia and others collect hydrological data, needed for research into energy and water transfer. These projects may substantially complement the available hydrological data by variables which are not observed routinely (e.g. remotely sensed monitoring of soil moisture). Because most of the GEWEX hydrological activities are organised on a regional level, it may be expected that a number of regional hydrological data bases will contribute to the overall goals of GCOS.
5. The Biospheric Aspects of the Hydrological Cycle (BAHC) Project was established within the International Geosphere - Biosphere Programme (IGBP) of ICSU. BAHC has developed a framework for interdisciplinary research activities to address the question - how does vegetation interact with physical processes of the hydrological cycle. BAHC will use different data sources, and one of the Project's task is to gather and summarize worldwide data on vegetation and soils. Observations from space are expected to support data obtained from land-surface experiments.
