Personal tools

Get notifications on new reports and products. Frequency: 3-4 emails / month.
Sign up to receive our reports (print and/or electronic) and quarterly e-newsletter.
Follow us
Twitter icon Twitter
Facebook icon Facebook
YouTube icon YouTube channel
RSS logo RSS Feeds

Write to us Write to us

For the public:

For media and journalists:

Contact EEA staff
Contact the web team

Call us Call us


Phone: (+45) 33 36 71 00
Fax: (+45) 33 36 71 99


Skip to content. | Skip to navigation

Sound and independent information
on the environment

You are here: Home / Publications / European Freshwater Monitoring Network Design / 4. Need for Different Types of Monitoring Station

4. Need for Different Types of Monitoring Station


The ETC/IW was asked to assess the need for different types of monitoring station to be included within the network. Such stations would provide different types of information for use by the Agency to meet the requirements described in Section 3. Such a structured or tiered monitoring network would also imply that there might be a need for different sample site densities, sampling frequencies and determinands for measurement.

The EEA’s need for different types of stations for fresh surface water and groundwater, quality and quantity monitoring is summarised in the following sections and is based on more detailed submissions by members of the ETC/IW The source documents are reproduced in the Project Record for 1994 (ETC 1995).

4.1 Surface water quality

There is a very wide range of terminology used to describe and define different types of monitoring stations, many related to the type of information provided. For example, in Europe and North America the following types of station are described:

  • Statutory stations, providing data to fulfil legal commitments, either national (for instance control of raw water for a public water supply) or international (agreement between two countries to control water quality in a transboundary water course, or obligations arising from a EU directive);
  • Benchmark (or reference) stations, aiming at characterising catchments undisturbed, as far as possible, by man;
  • Boundary stations, aiming at characterising fluxes, either between legal boundaries (between countries or regions), or between media (from a river to a lake or ocean, from a surface stream to groundwater, etc.);
  • Impact stations, aiming at controlling the effect of man's interference, namely well defined pollution sources;
  • Representative stations that can be used to provide summary information on a larger area, usually with long records;
  • Operational stations, located for day-to-day water quality management by local, regional or national agencies;
  • Research stations, installed and operated during scientific projects.

There are also examples of aggregating or summarising data from a number of stations to characterise relatively large areas or river catchments. These stations have been termed virtual stations (Santos and Costa 1991).

It would appear that in the context of the EEA network that three types of monitoring station are relevant:

  1. Reference stations to give reference points for analysis, regarding what the natural or pristine water quality is likely to be across Europe, and how this is changing with time.
  2. Flux stations to estimate fluxes between media, between Member States and between EEA and the rest of Europe;
  3. Representative stations to give an assessment of general quality of waters across Europe.

Ideally there would be reference stations for each eco-hydrological zone in a country, though in some countries reference conditions will probably not exist. For instance Portugal should have at least four, characterising the arid Mediterranean area south of the Tagus, the Atlantic basins of the north, the coastal basins and the interior forest areas. There should be flux stations on all major rivers crossing borders among Member States, and on each major river just before discharging into estuaries or coastal waters.

Possible ways of aggregating data from a number of representative stations to give broader overviews of quality on a catchment or regional basis will be investigated further during the implementation of the network in 1996 and subsequent years. It will be important to assess whether aggregating stations loses important information and to determine what would be the optimum scale of such aggregation.

4.2 Surface water quantity

Europe has a dense network of flow measurement stations: approximately 19,000 at an average density of 1 per 270 km2 (WMO, 1987). This is justifiable owing to Europe's wide physical diversity with respect to climate, morphometry and geology and the anthropogenic factors of population density, land and water use. This diversity is reflected in the varieties of river regimes which present a wide range of challenges for flow measurement and hydrometric data acquisition, so much so that estimates of runoff and thence resources may be significantly compromised.

There is growing recognition of the need to measure and monitor river mass fluxes systematically to assist with the rational management of the environment at all spatial scales, from individual river reaches and basins (e.g. UK National Rivers Authority Catchment Management Plans) and semi-enclosed seas bordered by several countries (e.g. the North Sea Conference), through to the world's coastal zones and oceans. This recognition has resulted in significant research initiatives on flux assessment in a number of countries.

There is also a need to monitor in respect of eu legislation and international obligations and conventions. There is only one piece of EU legislation requiring measurement of water quantity relative to surface freshwater, the Exchange of Information Decision (77/795/EEC) and there are nine European international conventions which require flow measurement.

For inland water quantity monitoring, two broad categories of station may be recognised in philosophical terms; in practice, there may be considerable overlap between the categories at individual measurement sites:

  • Statutory and operational monitoring arising from national or international obligations or to provide information for the business and operational needs of the regulators, suppliers, users and reclaimers of water.
  • Surveillance monitoring to characterise and allow appraisal to be carried out of the state of water resources and, in conjunction with water quality measures and biodiversity, the state of the water environment.

The separation is between that which has to be done to ensure legal compliance or efficient utilisation of the water resource and that which is beneficial in allowing the longer or broader view to inform policy making, assist planning decisions and increase the knowledge base.

Surveillance monitoring may be further subdivided into:

  • Reference stations that characterise regimes in catchments undisturbed as far as possible by man.
  • Baseline stations which, in total, characterise the generality of runoff behaviour of the region or country and whose data are appropriate for the transfer of hydrological characteristics to ungauged sites.
  • Representative (or Index) stations that are a subset of the network to provide summary estimates of the regional or national picture. Typically, these sites will have long records to provide a good historical perspective.
  • Impact stations that record and characterise the effects of man's interference with the natural regime.

The EEA requires access to information from a sound surveillance network that would capitalise on existing networks as far as is prudent and establish new sites where justified. The network design should be driven by the need for information to address the legitimate interests of the EEA. These interests extend beyond the monitoring and characterising of the state of the environment (the classical surveillance justification), investigating pressures and assessing remedies, into reviewing selected scenarios related to development strategies. In turn, this requires that the network is capable of quantifying effects and providing insights to processes sufficient to frame scientifically based management or mitigation procedures. This may not coincide with the classical methodology of the theoretical sampling of representative subsets of the geographical, climatic and aquatic environments (WMO 1976, 1982) but would be a pragmatic response to the difficulties in maintaining networks with too narrow a user base. The networks related to water quantity measurement, notwithstanding their historical development, are probably closer to a representative ideal than those related to water quality and ecology measures. Attempts to harmonise the location of water quantity and quality sampling sites are likely to demand an extension or relocation of the flow-gauging network (as flow measurement sites are likely to be less critical in their siting).

Example topics for which information on water quantity and mass loads would be obligatory or desirable include:

  • sustainable development - extension of models of sustainability from those related to water volumes and temporal distribution to concerns related to resource quality and maintenance and preservation of aquatic environments;
  • defining environmentally acceptable flows and quality standards;
  • climate change impacts appraisal - examination of the implications for water and environmental management (e.g. development of programmes to monitor extremes of river flow behaviour and to predict possible changes in extremes frequency, review irrigation and agricultural practises) (Kuusisto et al., 1994);
  • monitoring and prediction of fluxes of chemical species and suspended solids with possible changes in flow regimes;
  • the impact of groundwater pumping in modifying base flow behaviour;
  • disaggregation of the effects of man's interference in fluvial regimes.
  • providing the knowledge/data base that will allow the complex interactions between flows and concentrations to be more fully understood.
  • assessing the inputs to sensitive water bodies and the oceans; partitioning inputs between river systems/countries.

It is concluded from this review that:

  1. A hierarchy of gauging stations, including reference, baseline, representative and impact stations will be necessary to match the perceived information needs of the EEA.
  2. The Agency must capitalise on the capacity of the present networks to provide it with surveillance information.
  3. The Agency should assess the information delivery capacity of existing networks in the light of its requirements for the range of information and accuracy it desires
  4. There is a need for a class of monitoring station that addresses the measurement of mass loads as its primary purpose.

4.3 Groundwater quality

4.3.1 Background

The purposes and objectives of groundwater quality monitoring include:

  1. Collection of basic data for general surveillance purposes for establishment of national groundwater quality databanks, which can be used as planning information for future groundwater abstraction.
  2. To gain information for the EC Directives on Groundwater. These data are also important for the assessment of compliance with national legislation. They are important for the future international and transboundary obligations on groundwater.
  3. For monitoring the quality of abstracted groundwater and the impact of pollution from both diffuse, air-borne and point sources. To forecast the impact of possible pollution from known sources and the changes of the quality caused by these sources. Identification of the groundwater quality trends.
  4. Pollution impacts and its consequences to new abstraction projects by, for example, lowering of the groundwater table. Identification of areas where specific programmes may be necessary to reduce pollution and the assessment of progress made in their implementation.
  5. Data support for groundwater quality and quantity modelling: modelling of the transport and decomposition of compounds in solution as a tool for understanding and forecasting of pollution components in the groundwater and as a tracer of the groundwater flow.
  6. Collection of groundwater quality data for mapping and observation of groundwater resources, (especially known and used resources, but also potential, but not yet exploited resources). Early warning system in recharge areas of the impact of diffuse pollution. Data can also be used in research and scientific work.
  7. Observation of the consequences of groundwater contact with natural minerals and chemical compounds, which can affect the quality of the groundwater: Mineralisation in crystalline rocks and sediments, special compositions of the rocks and sediments and their alteration products, composition of water in areas of volcanic activity, contact with sea water and high salinity deposits (salt water intrusion), occurrence of deposits rich in organic matter as in, e.g. moors.
  8. Observation of the effects on groundwater from large-scale processes like global climate changes or man-induced changes in reaction-rates of natural processes like acidification caused from acid rain.

4.3.2 Types of monitoring network

The following types of monitoring network can be distinguished:

  1. Basic networks;
  2. Specific networks;
  3. Temporary networks.

Basic networks

The basic network should deliver general information about the quality of the groundwater. The network should cover the entire country, and the monitoring programme should have a permanent character over long time. Stations yielding background information of the natural quality of the groundwater can be a part of the basic network. To be consistent with the surface water quality stations these could be termed ‘reference’ stations. The information from this network forms the basis of the evaluation of the quality trends in the future and is the basis for both countrywide and local hydrogeological scientific and practical investigations. The design of the basic network can follow different concepts of which the following should be considered.

  • Representative stations could be placed in a square net or other geometrical pattern with a fixed distance between the stations;
  • They should be placed in the main aquifers; and, also
  • In other important aquifers of the area selected on the basis of representativeness.

Reference stations providing background information should be established outside areas affected by direct human activities such as groundwater pumping and other anthropogenic changes. In some areas within the Eea (small countries or in densely populated areas) this will not be possible.

Specific networks

Specific networks are constructed for monitoring selected areas or for specific kinds of pollution, for example, point sources. Therefore, they act as impact stations. The stations can form a separate network, or they can be an extension of the basic network, and thereby fulfil the need for data in areas between points on the larger basic network. The specific network can have a permanent character, or will be in operation as long as there are needs for information at that specific place. Around landfills, this could be during the period of activity and for a period after the landfill has been closed. These kinds of networks are regional or local and are often the most important.

Temporary networks

The temporary network stations are established to collect data in connection with particular groundwater projects, and will normally be impact stations. The network will be operational during the project period after which it is closed. Eventually, some stations may be transferred to the basic or specific network. The network will often be very dense and the quality data are included into transport and process studies of an area, often contributing to the verifications of the project findings.

4.3.3 Conclusions

It is concluded that the EEA will need information from reference, representative and possibly impact monitoring stations initially selected form existing national monitoring networks. Information will also have to be obtained from important flux points (stations), for example, between media (surface and groundwater, groundwater and sea) and between countries.

4.4 Groundwater quantity

There was close liaison and co-operation between the two groups undertaking the groundwater quality and quantity assessments, and as a result the main conclusions arising were similar which is not surprising as quite often both would be monitored at the same time for the same purposes.

Groundwater quantity measurement has proven to be indispensable to monitor the anthropogenic induced and/or natural changes in water levels in order to:

  • detect early signs of over-exploitation and/or other consequences of human impacts on groundwater levels (e.g. impact of hydraulic engineering, abandoned mines);
  • provide the necessary information allowing for ‘tailor-made’ use and need oriented groundwater quantity management; and,
  • provide information for the interpretation of groundwater quality data.

A feasible procedure to be followed when planning a network is to consider the multiple purposes and needs the network has to serve. In this manner the following types of networks can be distinguished:

  • basic networks;
  • bench-mark (or baseline) stations;
  • specific networks (or special hydrogeological networks); and,
  • temporary networks.

Descriptions of basic, specific and temporary networks have been given in the previous section. An additional type of network and station has been identified for groundwater quantity monitoring, the hydrological benchmark or baseline station. These provide a continuing series of consistent observations on hydrological and related climatological variables. They should reflect local, regional and geographic differences.

The type of the observed variables also varies with the purpose of the network, the necessary information and the particular characteristics of the groundwater and its regime in the area. As to spatial and temporal densities of the observations, these usually increase with the transition from the national or regional level to the project-specific sites and/or to the level of local warning requirements. The type of field record (e.g. autographic, telemetered, manual) is highly dependent on the available technology of data transmission and processing. Finally, the length of record depends on the duration or the purpose of the network.

Both the specific network and temporary hydrogeological network may be considered as "impact stations" since they monitor the influences of projects and water management systems on groundwater more on a local scale. They should be established in areas that are relatively uninfluenced by past or future anthropogenic changes. Since long records are the essence of a benchmark station, consideration should be given to existing stations if they meet the other requirements. Climatological benchmark stations are known as reference stations.

Monitoring and assessment of groundwater quantity is generally indispensable, but of particular importance in areas with quantity and/or quality problems. Detailed information about the situation and trend in water tables on a regional and local level are vital for a special tailored use and need oriented groundwater management. Lacking the necessary detailed information, a system of authorisations - as proposed by the groundwater action programme (EC 1995) - depending on permits and general rules may not be effective and may not meet the expectations put into such a system.

The different types and names of stations (e.g. benchmark stations, impact stations etc.) for monitoring groundwater quantity are mainly a result of the specific objectives the network has to serve. The type of stations with respect to the type of network (e.g. base-line etc.) has usually no influence on the design and the construction of the observation station as long as the observation of groundwater tables is concerned.

No current EU directive has specific requirements for groundwater quantity monitoring. Nevertheless the need for, and the importance of, monitoring groundwater quantity has been recognised at an European level especially when facing water shortages and quantity problems in large parts of the European Union over the last years. The need for such groundwater monitoring is stressed by the European Commission in its Groundwater Action Programme (EC 1995).

For the reasons mentioned the choice of the appropriate type(s) of groundwater quantity monitoring networks as well as the appropriate level of monitoring effort (density of stations, frequency of observations) are closely linked to the very needs the network has to serve. The economic and environmental benefits of appropriate and sustainable groundwater quantity management in regions with an excessive over-exploitation of limited groundwater resources may justify the costs of a dense network of stations including impact stations on a national, regional and local level. The same network and number of stations in regions with abundant groundwater resources and no quantity problems may be considered as pure luxury, as a much more limited and less dense network might serve the special purposes of water management in those regions.

A Europe wide comparison of results of groundwater quantity monitoring should mainly be based on the aggregated results (e.g. area, number of monitoring stations, monthly and annual changes in groundwater tables) of the basic or principal network, as all other networks (with "impact stations") take into account very specific local effects, which may be not fully comparable throughout Europe.


Geographical coverage

[+] Show Map

Document Actions


European Environment Agency (EEA)
Kongens Nytorv 6
1050 Copenhagen K
Phone: +45 3336 7100