Hydrology is the study of water as it moves through the water cycle. Water levels in collections of groundwater and rivers are recorded and assessed to determine if variations are the result of man-made influences.

What is hydrology?

Hydrology is the study of the water cycle or the movement of water around its cycle. This goes from rainfall to water in soil, to groundwater flow, to river flow, to the oceans and evaporation. It also includes the movement of water vapour through the atmosphere to become rainfall again.

Water in rivers, lakes and groundwater, as well as being the source of drinking water, represents an important natural resource that supports ecological diversity. River flows rise and fall naturally as a result of different weather conditions throughout the year. They are also affected by human practices.

The main factors determining river flow at any time are:

  • size of area drained by the river, known as its watershed or catchment
  • amount of any rainfall within this area reaching the river
  • nature of the soil, any undersoil layers and solid rock beneath
  • removal or addition of water and any control of the rate of flow


River levels are monitored by the Rivers Agency with a network of over 110 permanent river flow gauging stations across the province. Lough Erne and Lough Neagh are also monitored for water levels.

Groundwater levels in Northern Ireland are monitored by Northern Ireland Environment Agency (NIEA) in a number of groundwater aquifers (underground zones that contain important water reserves).

River and groundwater level information for Northern Ireland and the rest of the United Kingdom may be viewed on the National River Flow Archive.

Water resource assessment

Assessment of water resources is needed to see the ecological impact of any man-made influences and changing climate conditions on rivers, lakes and other bodies of water.

A water resource assessment in the form of a river water balance will show if a proposed abstraction (removal of water from a water body) is likely to have an ecological impact. Licence conditions can then be set according to this.

Water balance

There is a key flow level within the range of a river’s flow variation, which is recognised as being ecologically important. This flow, known as the Q95, is the average flow for any one day expected to be greater for 95 days in any 100 days.

An example of the approach to water resource assessment due to abstractions and other influences is outlined below.

River natural flow (for example, Q95) = 1000 litres/second
Abstraction (removal of water) = - 110 litres/second
Discharges (addition of water) = + 10 litres/second
Influenced river flow = 900 litres/second
Effect on river flow = 1000 - 900 ÷ 1000 = 10% reduction
Apply Environmental Standard (allowable abstraction) = 15%

In this case the outcome is that the abstraction does not have a significant ecological impact on the river as the abstraction is less than the limits for that river type.

Rivers are assigned a type based on their physical character and observed ecology. A range of water resource environmental standards are applied at different times of the year. This is to ensure the effect of the abstractions and other influences do not impact on the river’s ecological status. 

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