River Flow Data (CEH)

The Gauged Daily Flow is the mean river flow in cubic metres per second (abbreviated to m³ s^-1 and sometimes also referred to as 'cumecs') in a water-day, (09.00 to 09.00 G.M.T.) or, where indicated, a calendar day.

Further information: NRFA Gauged Daily Flows

The NRFA holds Naturalised Daily (NDF) and Monthly mean Flow (NMF) series for some UK gauging stations. The naturalised discharge is the gauged river flow adjusted to take account of net abstractions and discharges upstream of the gauging station.

Further information: NRFA Naturalised Flows

Similarly to the daily flow series, peak flow data are derived by relating the stage (water level) measured at a station to the flow (discharge) using a rating curve.

The Annual Maximum (AMAX) series contains the largest observed flow in each water year. Further information can be found here.

The Peaks Over Threshold (POT) series contains all peak flows that are greater than a given threshold flow, the threshold is generally set to include an average of 5 events per year. Where multiple peak flows occur in a single event, the largest is used for the POT series (See FEH Volume 3, section 23.5.1).

Further information: NRFA Peak Flow Data

The National River Flow Archive provides systematic storage for river flow data, either daily or monthly; the normal minimum requirement for inclusion in the national archive is several successive years of flow data. However, much valuable hydrological information is aperiodic, sporadic or event-specific in nature, and thus less amenable to storage in conventional archives designed to hold time series data; this is particularly true of data and information relating to the pre-instrumentation period.

The NRFA operates a programme of capturing and storing long hydrometric records as part of the national archive.

Further information: NRFA Long Hydrometric Records

Gauged Monthly Flows

The National River Flow Archive is able to calculate monthly mean gauged river flow series for analysis and retrievals. In most cases, the series are not held as a separate dataset but are calculated by taking the mean of all the Gauged Daily Flows in each calendar month. Only complete monthly records are used in the derivation of the monthly mean gauged river flow series.

Daily Catchment Runoff

The notional depth of water in millimetres over the catchment equivalent to the daily mean flow as measured at the gauging station. It is computed using the relationship:

Runoff in mm = Mean Flow (m³ s^-1) x 86.4 / Catchment Area (km²)

Runoff is computed on the basis of naturalised flows for the small minority of catchments where daily naturalised flows are available.

Monthly Catchment Runoff
The notional depth of water in millimetres over the catchment equivalent to the mean flow for the month as measured at the gauging station. It is computed using the relationship:

Runoff in mm = Mean Flow (m³ s^-1) x 86.4 x n / Catchment Area (km²)

where n is the number of days in the month.

Runoff is computed on the basis of naturalised flows for the small minority of catchments where daily, or monthly, naturalised flows are available.

Further information: NRFA Derived Data Series

The National River Flow Archive calculates flow statistics for individual flow records / gauging stations to aid selection and analysis of river flow data. Unless otherwise stated all flows statistics are calculated using the Gauged Daily Flow series. Users should note that statistics are not available for all flow records / gauging stations. While derived statistics are not commonly available as retrievals, many feature in the latest Hydrometric Register.

The following flow statistics are commonly calculated by the NRFA:

• Mean Flow
• Flow Percentiles
• Mean Annual Catchment Runoff
• Mean Annual Loss
• Base Flow Index
• Percentage Data Complete
• Period of Record

Further information: NRFA Derived Flow Statistics