Measuring water level in lakes and rivers
The national network of water level monitoring consists of some 310 stations, of which about 240 are operated by the Finnish Environment Institute or by a regional centre for economic development, transport and the environment. Hydropower companies, water supply organizations and the navigation administration are also important producers of water level data. Water level is observed daily, but a major part of the stations have now some kind of continuous automatic registration. The total number of water level stations in the hydrological database is around 850.
The oldest records began in 1847. Water levels are observed as distances in centimeters from the gauge zero, and in this specific level of the gauge the water level values are also given in the database. When a gauge is established, its gauge datum, i.e. its elevation above the sea level, is determined by levelling from a bench-mark of the national network. There are four level standards: NN, N43, LN and N60. If needed, the water levels can be given according to one of these standard levels. In such documents as licenses given by a water rights court, the prescribed water levels are usually mentioned referring to one of the standard levels. If there is reason to believe that a gauge or a bottom pole may have moved, its elevation can be controlled by levelling from the bench-mark.
At the water level stations operated by the Finnish Environment Institute and the regional centres for economic development, transport and the environment, the following devices and techniques are used:
- about 40 stations: a traditional water level gauge. About 20 ot these stations: a traditional level gauge, while data are transferred daily by telephone
- about 20 stations: mechanical recording (a limnigraph)
- about 15 stations: digital recording devices (logger, no realtime data transfer)
- 165 stations: automatic data transfer daily or more frequent
Manual water level observations are usually made daily 8 a.m., either by reading the gauge or by measuring the level by a funnel-top measuring stick placed on the bottom pole. For water levels registered by the limnigraphs, the database value used to be the 8 a.m. value until 1985, except for a few stations. Thereafter the database values for self-recording stations have been daily mean values.