Sixty years ago, the very first samples of rainwater were collected for the Global Network of Isotopes in Precipitation (GNIP), while the science of isotope hydrology was still taking its first steps. How has the GNIP evolved since and how is it used today? We have asked the Head of the IAEA Isotope Hydrology Laboratory, Leonard Wassenaar, to discuss GNIP’s past, present and future.
Initiated in 1960 by the IAEA in cooperation with the World Meteorological Organization (WMO), the GNIP has become the world’s largest and most comprehensive collection of isotope data in atmospheric waters, helping scientists worldwide to study the global water cycle and the origin, movement and history of water. With such information, policy makers can better plan strategic management of water resources.
Thanks to the GNIP, which houses monthly isotope records at over 1100 sites located in more than 90 countries, data is accessible online to researchers and practitioners to study water resources, environmental processes, interpret past climatic archives, and validate and further improve climatic prediction models. In the last 60 years, the GNIP has collected more than 130,000 monthly isotope records in collaboration with many contributors from around the world.
1. Why was the GNIP created?
Due to atmospheric tests of thermonuclear devices in the 1950s, levels of tritium in the atmosphere sharply increased in the early 1960s and then gradually declined to pre-bomb levels by the late 2000s. The primary aim for GNIP was to understand and make use of this ‘accidental global tritium spike’ for hydrological research and to increase scientific understanding of the water cycle, with stable isotopes added as an essential component. Anticipated applications of the GNIP were groundwater age dating, tracing of ocean mixing, estimations of residence times of surface waters and evaporation losses.
2. Why was it set up in cooperation with WMO?
WMO was a logical collaborative counterpart, as it oversaw climate and meteorological stations worldwide. In the late 1950s, the WMO was setting up a new global network of stations and they approached IAEA Member States’ national meteorological services inviting them to participate in advanced scientific endeavours, including the future GNIP. Many stations of the current GNIP were top class WMO stations with permanent staff needed for collecting rain samples. Sites included major cities, airports, research sites and military bases. In the past two decades, the GNIP partnerships have broadened through direct contacts with research centres, counterparts and meteorological services. Today, the GNIP is affiliated with WMO’s Global Terrestrial Networks for Hydrology (GTN-H) group of surface observational networks.
3. How did the GNIP function before personal computers and the internet?
Initially, GNIP data were stored on mainframe computers using card readers, only accessible by IT staff until 1987. Following that, GNIP data were available for researchers through IAEA printed publications (10 volumes) or by computer tapes accessible at research centres in a few countries until the early 1990s. The advent of personal computers in the 1990s allowed distribution of GNIP data on floppy disks and, in 1995, the first GNIP website was made public for downloading data using Lotus spreadsheets. In 2002, data, maps and animated maps were distributed online. Today, all information collected over the last 60 years is available here.
4. How have the GNIP’s operations evolved?
The basic principles of the GNIP sample collection and the isotope observation programme have remained unchanged since its inception, making use of monthly composite rainfall samples. Nowadays, smaller sample volumes are needed for stable isotope analyses and the equipment to conduct sample collection and analyses has become more affordable and easier to use. For tritium, however, owing to the decline back to natural levels, a more sensitive and costly analysis is required. More of the data in the GNIP is now produced by national collaborators instead of the IAEA, partly as result of IAEA efforts to train experts around the world. Communication and interaction with national participants via the Internet has replaced posted letters and faxes.
5. What is the GNIP’s plan for the next 10 years?
In the shorter term, the field of researchers using the GNIP is still growing rapidly with the use of stable isotope data in non-traditional hydrological disciplines such as ecology, agriculture, archaeology, food authenticity and atmospheric sciences. There was new technology development in stable isotope measurements in the past 10 years, which now allows for high-frequency, real-time and remote location measurements of liquid water and vapour isotopes using lasers. The use of the GNIP in Big Data is only beginning. Not many changes are expected for tritium, as its levels have declined to near natural pre-bomb levels, but the tritium/helium-3 dating method will be increasingly used to map vulnerable groundwater supplies.
6. How do you imagine the GNIP after another 60 years?
It remains to be seen if hydrogen fuels are going to eventually have an impact on the hydrogen isotopic signatures in rainfall. Will fusion energy produce huge releases of tritium, again masking the Earth’s natural tritium patterns at some point in the future?
As global climatic changes continue, this should become evident through changes in the isotope patterns and trends recorded in the GNIP stations around the globe.
It is possible that the GNIP could begin to include other isotopes of interest to water quality or for atmospheric studies as methods and instrumentation evolve to detect lower concentrations of isotopes, for example naturally-occurring isotopes of nitrogen and chloride or isotopes of airborne pollutants that are deposited through rainfall.
We expect the GNIP will be increasingly used to validate climate models as satellites begin to be able to remotely measure isotopes in the Earth’s water vapor or precipitation.
