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4 Terrestrial Essential Climate Variables 
4.1 Global Terrestrial Network - Hydrology (GTN-H) 
The GTN-H is a joint effort of the World Meteorological Organization / Climate and Water 
Department (WMO/CLW), the GCOS, and the Global Terrestrial Observing System 
(GTOS 16 ), co-sponsored by WMO, UNESCO, ICSU, UNEP and FAO. 
GTN-H represents the observational arm of the Group on Earth Observations / Integrated 
Global Water Cycle Observations Theme (GEO/IGWCO). 
The following hydrological variables have been identified as essential for the GTN-H 17 net 
work: precipitation, river discharge, groundwater, water vapour, lake level/ area, isotopic 
composition, soil moisture, water use, snow cover, glaciers and ice caps, évapotranspiration, 
water quality/ biogeochemical fluxes. For most of the variables a global network is defined 
and a contact established. The GPCC and the GRDC, both operational in Germany, are part 
of the GTN-H Panel and represent their respective networks on precipitation and river dis 
charge. 
4.2 Global Terrestrial Network for River Discharge (GTN-R) 
The GRDC has been tasked with the implementation of the GTN-R to address the growing 
need for a global runoff network monitoring freshwater surface water fluxes. Due to resource 
constraints no progress was achieved with this GCOS Baseline River Discharge Network 
during the reporting period. Currently near real-time hydrological data are received from 12 
stations in 3 countries. 10 countries provided historical discharge data for 103 stations. The 
original objective was to include 380 stations from 81 countries, but only a quarter of the ap 
proached countries responded to the request to participate. A project proposal to boost the 
funding of the implementation of the GTN-R has been submitted to the European Union and 
currently the outcome is awaited before further action is taken. 
4.3 Global Terrestrial Network - Lakes (GTN-L) 
Lake level data are being observed by different agencies at Laender or community level. 
Though some of these observations are available in environmental information systems of 
the Laender on the Internet, there is no nation wide database established and it is therefore 
not known to which degree such data is made available to international data centres. 
4.4 Global Terrestrial Network- Glacier (GTN-G) 
The Commission for Glaciology of the Bavarian Academy of Sciences and Humanities runs a 
substantial recording programme in the catchment of the Vernagtferner, Oetztal, Austria. 
Maps of the glacier are available since 1889 and were repeated in 1912, 1938, 1954, 1969, 
1979, 1982, 1990, 1999, and 2006. Mass balance is determined annually since 1964/65, 
including the separation into winter and summer series. Runoff, precipitation, all radiation 
components, temperature, humidity, air pressure, and wind are recorded since 1974 on an 
hourly basis. The permanent observations at Vernagtferner are complemented/ extended by 
16 http://www.fao.org/gtos/ 
17 http://gtn-h.net