GNIP — Frequently Asked Questions

Network and Coverage

Who operates GNIP stations and produces the isotope data?

GNIP stations are usually operated by hydrological services within the framework of national observation networks, or by water research institutions and individual researchers, and even private citizens. Similarly, these organisations and researchers often analyse their samples to produce the corresponding isotope data. GNIP stations are generally monitored for stable water isotopes (18O, 2H), and a subset for radioactive tritium (3H).

Why are there no GNIP stations in my part of the world?

GNIP relies on voluntary participation (collecting samples or analyses) by various scientific organizations, institutes, universities, and private citizens. IAEA has no mandate to initiate sampling. While we encourage member states to establish precipitation isotope sampling networks, temporal gaps may arise when stations cease to operate. When priorities shift, , funding terminates, and staff or volunteers are not available , a station may face closure. If data coverage is lacking in your area, please consider setting up your own GNIP station to help fill the gaps (see below).

What is a 'national GNIP network', and what is the difference from a single station?

National networks within GNIP are sampling efforts coordinated by a national authority in the field of isotope hydrology or meteorology. National networks usually manage all aspects of the work (i.e. manage the stations, conduct sampling and isotopic analyses). They are regular participants in laboratory intercomparison exercises organized by IAEA (WICO, TRIC) to maintain QA/QC with the 'core' GNIP network. National networks may have different data dissemination policies, or their data may be distributed via different portals.

Examples of some national networks are ANIP (Austrian Network of Isotopes in Precipitation), USNIP (USA), CNIP (Canada), CHNIP (China), IWIN (Indian Water Isotope Network), CWIN (Canadian Water Isotope Network), REVIP (Spain) or the German and Swiss Networks (NISOT). Smaller networks include SLONIP (Slovenia), DANIP (Denmark), Greece or Turkey and are supported by the IAEA.

What are long-term, medium-term, and short-term stations?

•   Long-term stations have been operated by collaborators over decades, some since the early 1960s. Among the long-term stations are Valentia (Ireland, 1959), and the Tritium reference stations in Vienna (Austria), Ottawa (Canada) and Kaitoke (New Zealand).

•    Medium-term stations operate over 10-15 years. Over their operational period, they often represent at least one major climatic cycle (ENSO etc.).

•    Short-term stations operate for 2-3 years, usually dictated by a project or grant cycle. Nevertheless, their operation constitutes a valuable contribution by filling in spatial gaps of isotopic information.

Which criteria are applied for optimizing GNIP network coverage?

GNIP coverage is cross-checked against the climatic clustering of IAEA's precipitation isotope prediction model RCWIP Within these climatic clusters, the spatial distribution of stations is considered (x, y, and z domain), but also realistic logistical aspects.


Can I establish a GNIP station? What are the time commitments, cost, etc.?

Yes, volunteers are always appreciated! The IAEA welcomes interested organizations, institutes, or individuals who can volunteer time to operate a new or ongoing GNIP station. Depending on the sampling methodology, the commitment may range from a few minutes per day for daily collections to just 15-30 minutes per month for the collection of monthly integrated samples. Costs mainly involve a rain collector (a one-time expenditure), sampling bottles and isotopic analyses. Some operational expenses like isotopic analyses may be sponsored by the IAEA. A background in hydrology with access to an isotope laboratory is helpful, but not a prerequisite.

Where are new GNIP stations most needed around the world?

Regions deficient in GNIP coverage include:

•   Europe: Russia, parts of France and British Isles, Belgium, Norway and Sweden, large parts of eastern and southern Europe.
•   Eastern Mediterranean and Middle East: Adjacent to the Mediterranean Sea, and the south and east of the Arabian Peninsula.
•   Asia: all areas (except Sri Lanka and Malaysia )
•   Africa: all areas
•   Australia / Pacific: all areas
•   North America: all areas of USA, Canada and Mexico, including Greenland
•   South America: all areas (except Chile)

Proposals for GNIP sampling in other regions are welcome, and will be handled on a case-by-case basis.

What rain/snow collection equipment do I need for a GNIP station?

Suitable equipment for GNIP sampling are standard rain gauges (provided daily collection of water is feasible), or monthly totalizing rain samplers. These can be hand-built , although excellent inexpensive samplers (around 150 EUR) are available commercially (e.g. here.) Snow sampling is conducted using traditional box/bucket samplers, or by using carefully heated rain gauges. Guidelines on the placement of a GNIP rainfall sampler are given in the GNIP operations manual .

Will the IAEA pay me or provide a grant to run a GNIP station?

No. Participation in the GNIP network is voluntary. IAEA may provide other types of in-kind support (see below).

Can the IAEA support my GNIP station operation in other ways?

Yes, in-kind IAEA support may include:

•   Providing sampling consumables (QA/QC sample bottles)
•   Providing analytical H/O isotope services (if labs are not available)
•   Provide assistance with calibration of laboratory standards
•   Provide annual shipment cost of water samples to be analysed in Vienna.

I don't own a stable isotope laboratory – who can measure my GNIP samples?

IAEA may be able to link you to a national GNIP participating isotope laboratory. In other cases, your samples may be analysed at the IAEA Isotope Hydrology Laboratory in Vienna.


For general information on rainwater sampling for isotopic assays, kindly refer to the GNIP station operations manual.

How can I prevent sample evaporation ?

For event sampling, the best strategy is to collect rainwater immediately after each event, and pour it into an appropriately sized, tightly sealed, storage bottle for monthly accumulation. For monthly totalizer sampling, the use of a paraffin oil layer floating on the collected water seals it hermitically against evaporation with surrounding atmospheric moisture (but can cause problems for laser-based isotope analysers). Finally, a commercial evaporation-free rain totalizer can be purchased (above) which eliminates evaporation and the need for paraffin oil to prevent evaporation .

What is 'monthly integrated sampling', the pros/cons?

Monthly integrated sampling means the accumulation of all precipitation water over one whole month, obtained by using a rainfall totalizer or by transferring rain or snow into a single storage bottle after each precipitation event.

•   Pros: compliant with 50+ years of the GNIP database, lower analytical cost (only 12 annual samples per station ), and straightforward association with monthly climatic data. Sufficient temporal resolution for groundwater hydrology. Recommended sampling interval for Tritium is one month (analytical effort, water amount).
•   Cons: Can blur significant rainfall events (which may have distinctive isotope values), risk of sample evaporation during totalized collection or storage.

What is 'event-based sampling', and what are the pros/cons?

Event-based sampling means that an individual sample for isotopic analysis is prepared after each rain event. In the GNIP database, all rainfall isotope data with a temporal resolution less than monthly is stored in the 'GNIP-Events' section.

•   Pros: high-resolution isotopic data, especially for applications in hydrometeorology and catchment hydrology. Event-based sampling is also recommended in hyper-arid areas (where the risk of evaporation is too high for attempting a totalizing sampling strategy and the number of rainfall events is low).
•   Cons: high analytical effort (up to several hundred annual samples per station). Intensive sampling effort required (no events can be missed, i.e. a daily observer or sophisticated automated equipment).

Data Availability and Attribution

How do I cite GNIP data or a specific GNIP station in my research paper?

Please cite the GNIP database itself as given on the GNIP web page. Many of the GNIP stations include a specific literature reference, which is available when downloading the data. Please assure proper credit to the researchers who have devoted significant time, resources and funds for their GNIP contributions.

Who owns the GNIP data on the IAEA or WISER website?

Data uploaded to GNIP servers is public domain by permission of the contributor. Cite the original data source wherever possible.

How long does it take for GNIP to become available to the public?

IAEA strives to make GNIP data public as early as possible after analysis and evaluation. However, an embargo period pending publication of the contributed data may apply. Also, national networks may have different policies on reporting data publicly to GNIP.


Should I collect tritium samples?

The scientific value of Tritium is disputed since the thermonuclear "bomb peak" is barely present in modern precipitation. Tritium contents in precipitation have generally decreased to near background levels. Isotopic assays for current low levels of Tritium are complex and costly. Under these circumstances, the IAEA applies a conservative policy concerning the acceptance of new stations in the GNIP tritium network.

Why were there so many tritium stations in previous decades?

Previously, the presence of "bomb Tritium" in waters was used by hydrologists as a powerful artificial tracer for distinguishing modern from older pre-bomb water groundwaters. Consequently, the GNIP network was focused on tritium. Past high tritium levels in precipitation were easy to detect, since no electrolytic concentration was required.

Data Quality

What steps are undertaken to ensure isotopic data are OK before it enters into the public GNIP database?

Before new GNIP data enter the database, they are screened for obvious errors or outliers, and flagging of wrongly labelled samples, reporting errors, and removal of results which are clearly evaporated (flagged having a deuterium excess of less than zero).

Ancillary data

Where do the GNIP rainfall amount numbers come from?

The GNIP sampling procedures recommends that rainfall amount be recorded at the same location of the isotope sampler. Rainfall is highly stochastic, and can have large variance within short distances. Hence, it is paramount that rainfall amount is recorded as close to the GNIP rainfall sampler as possible. If the collector is located within a weather station's compound, the amount numbers of the corresponding rain gauge may be used. Otherwise, the rainfall amount may be determined volumetrically using the sampler, taking into account the funnel diameter.

Where does the GNIP temperature and vapour pressure data come from?

GNIP temperature and vapour pressure data originate from the corresponding weather station (stations at WMO locations) but may also originate from privately run equipment. In the extreme case, temperature and vapour pressure numbers come from a nearby 'official' WMO station. Vapour pressure is calculated using the WMO equation: Vp = (6.112 * Exp(17.62 * Temp / (243.12 + Temp))) * RH / 100 (cf. WMO: Guide to Meteorological Instruments and Methods of Observation, 7th ed. 2008)

Different accuracies are reported in for GNIP station LAT/LONG coordinates?

In the early days , GNIP station coordinates were reported in degree-minute format. In 2013, incorrect or inaccurate coordinates were identified and corrected where possible with modern digital mapping tools. Recently established stations usually report their stations as GPS coordinates in decimal degrees, with a spatial accuracy of a few metres .

Legal Stuff

What is the legal basis of GNIP?

GNIP began as the 'IAEA-WMO Network of Isotopes in Precipitation', formalized in a memorandum of understanding between the two UN Organizations. This MOU describes the basic structure and need for a network and database, as well as the roles of IAEA and WMO within the framework of GNIP.

Responsible/Contact: Isotope Hydrology Section | Last update: 07 May 2015