Hydrology

Water Cycle

Hydrology is the study of the water cycle or the movement of water around its cycle from rainfall, water in soil, groundwater flow, river flow to the oceans, evaporation, and 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, also represents an important natural resource that supports diversity of ecology. River flow rises and falls naturally due to different weather conditions throughout the year and is 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.

Monitoring

River levels are monitored by the Rivers Agency (links to website) with a network of over 110 permanent river flow gauging stations across the province. Lough Erne and Lough Neagh are also monitored for water level. Groundwater levels in NI are monitored by us in a number of groundwater aquifers. River and groundwater level information for Northern Ireland and the rest of the United Kingdom may be viewed on the National River Flow Archive (links to centre for Ecology and Hydrology website).

Water Resource Assessment

The assessment of water resources is critical to the evaluation of the ecological impact of any man-made influences and changing climate conditions on rivers, lakes and other bodies of water. An initial assessment of such man-made influences in Northern Ireland was carried out under the

Water Framework Directive

(2000/60/EC) and a Characterisation Report published March 2005. This report outlined the particular bodies of water at risk of not achieving their ecological objectives due to the presence of significant water resource impacts and other pressures. In Northern Ireland certain abstractions will require a licence under the Abstraction and Impoundment (Licensing) Regulations (Northern Ireland) 2006. A water resource assessment in the form of a river water balance will inform if a proposed abstraction is likely to have an ecological impact and license conditions can be set accordingly.

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.

Water Balance Resource Assessment

River natural flow (e.g. 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%

Outcome is that the abstraction does not have a significant ecological impact on the river as abstraction is less than allowable threshold for that river type.

Rivers will be assigned a type based on their physical character and observed ecology. An appropriate range of water resource environmental standards will then be 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.