Low Flow Statistics

The usual measures to assess low flow in Ireland are:

  • Dry weather flow
  • 95 percentile flow

Dry weather flow

The Dry Weather Flow (DWF) is defined as the annual minimum daily mean flow rate with a return period of 50 years. The DWF is a statistical measure of low flow and usually requires reliable long term low flow data or sufficient information that would allow the estimation of the DWF. It should normally be based on long term records of flow data. In the absence of processed flow data, it is not possible to use the records to calculate a dry weather flow and, in their absence, recourse has been made to best estimates based on measured flows.

Investigations of low flow data indicate that in large areas of the country, the return period of the 1976 drought had a return period of about 50 years.

95 percentile flow

Where pollution control is concerned, interest is more often centred on low flows of a more common occurrence.

The practice in relation to the effects of an effluent discharge is that computation is based on detailed hydrometric data, the critical flow being that which is equalled or exceeded at least 95% of the time (or, on average, on all but 18 days per annum).

The 95 percentile flow is a statistical measure of flowrate and ideally should be based on the processing of water level records for a considerable period of time. This measure of flow may be sensitive to the length of record. In the absence of processed flow data, it is not possible to use the records to calculate a 95 percentile flowrate and, in their absence, recourse has been made to best estimates based on measured flows.

In certain parts of the country, the low flow in the droughts in 1989, 1990, 1991 and 1995 approximated to the long term 95 percentile flowrate.

Learn more

Download Estimates of Dry Weather Flow and 95%tile Flow at selected hydrometric stations.

Read reports on Surface Water Level and Flows.