Annual tables of daily flow data for around 200 gauging stations throughout the UK are available for downloading. A map is provided to assist in locating the gauging stations for which data can be accessed; clicking on the appropriate region will bring up a list of stations for the selected measuring authority. (Note, with some browsers it may be necessary to scroll down to the gauging station of interest.) Several alternative selection criteria are available; stations may be selected according to:
The featured gauging stations have been selected to give a broad geographical coverage and to typify a wide range of catchment types. For each gauging station, the daily flow data are presented together with basic reference information and comparative average and extreme river flow and rainfall figures based upon the archived record. Also provided for each station is a succinct guide to the characteristics of the station and the catchment it commands.
Details of how to obtain data for other gauging stations in the national network, or pre-1996 data for the stations featured here, are given in the National River Flow Archive Retrieval Service.
The following explanatory notes are provided to assist in the interpretation of particular items in the flow data tabulations.
The gauging station number is a unique six-digit reference number which serves as the primary identifier of the station record on the National River Flow Archive. The first digit is a regional identifier being 0 for mainland Britain, 1 for the islands around Britain and 2 for Ireland. This is followed by the hydrometric area number given in the second and third digits. Hydrometric areas are either integral river catchments having one or more outlets to the sea or tidal estuary or, for convenience, they may include several contiguous river catchments having topographical similarity with separate tidal outlets. In Britain they are numbered from 1 to 97 in clockwise order around the coastline commencing in north-east Scotland. Ireland has a unified numbering system from 1 to 40, commencing with the River Foyle catchment and circulating clockwise; not all Irish hydrometric areas, however, have an outlet directly on the coast. The numbers and boundaries of the United Kingdom hydrometric areas are shown below. The fourth, fifth and sixth digits in the station number identify individual gauging stations within the hydrometric area - these numbers are usually allocated chronologically. Where the leading digit, or digits, are zero they may be omitted giving rise to apparent four or five-digit reference numbers.
An abbreviation referencing the organisation responsible for the provision of river flow data to the National River Flow Archive. A list of measuring authority codes together with the corresponding names and addresses for organisations currently contributing data to the National River Flow Archive is given in the Directory of Measuring Authorities.
The initial two-letter and two-figure codes each designate the relevant 100 kilometre National Grid square or Irish Grid square; the standard six-figure map reference follows.
Note: Irish Grid references - which are italicised - have only one prefix letter but it is common practice to precede it with the letter I to make the identification clear.
The surface catchment area, in the horizontal plane, draining to the gauging station in square kilometres. There are a few gauging stations where, because of geological considerations, or as a result of water transfers - for instance, the use of catchwaters to increase reservoir yields - the actual contributing area may differ appreciably from that defined by the topographical boundary. In consequence, the river flows, whether augmented or diminished, may cause the runoff values to appear anomalous.
The year in which the station started producing daily mean flow data, usually the first year for which data are held on the National River Flow Archive. Earlier data, often of a sporadic nature or of poorer quality, may occasionally be available from the measuring authorities or other sources.
The level of the station is, generally, the level of the gauge zero in metres above Ordnance Datum, or above Belfast Datum for stations in Northern Ireland. Although gauge zero is usually closely related to zero discharge, it is the practice in a few areas for an arbitrary height, typically one metre, to be added to the level of the lowest crest of a measuring structure to avoid the possibility of false recording of negative values by some digital recorders. Station levels are stored to the nearest 0.1 metre on the Archive.
The level to the nearest metre of the highest point in the catchment area.
The mean 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. The naturalised discharge is the gauged discharge adjusted to take account of net abstractions and discharges upstream of the gauging station. Throughout the River Flow Data section flows are given to four significant figures. Flow data for some stations may include estimated values (highlighted in blue) provided by the measuring authorities or derived by the Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology).
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 = Average Flow (m3s-1) x 86.4 x n / Catchment Area (km2)
where n is the number of days in the month. The runoff total is rounded to the nearest millimetre.
Runoff is computed on the basis of naturalised flows (see 'Factors Affecting Runoff') for the small minority of catchments where daily, or monthly, naturalised flows are available.
The rainfall over the catchment in millimetres for each month. Each areal rainfall total is derived from a one kilometre square grid of rainfall values generated from all available daily and monthly rainfall data. A computer program calculates catchment rainfall by averaging the values at the grid points lying within the digitised catchment boundary. Validation procedures allow for the rejection of obviously erroneous raingauge observations prior to the gridding exercise. Normally the catchment rainfall figures are derived several months after the corresponding runoff figures are posted on the Web. The bulk of the rainfall data for individual raingauges are provided by the Met Office.
With few exceptions, areal rainfall assessments are based on monthly totals for standard raingauges. Research has demonstrated that ground level gauges (relatively few of which are currently used outside of research catchments) provide a more accurate measure of rainfall amounts. Standard raingauges systematically underestimate rainfall totals; on an annual basis this underestimation may be in the 3%-10% range.
Where, as for instance in some small mountainous catchments, raingauges are few and their siting and exposure are not ideal, great precision in the areal rainfall estimates cannot be expected. Changes in the raingauge network (and in the method used to compute the catchment average) can also affect the homogeneity of monthly areal rainfall time series.
Only complete monthly records are used in the derivation of the average, low and high values of river flow, runoff and rainfall. The rainfall and runoff statistics are normally directly comparable but full equivalence will not obtain where the pattern of missing data differs between the archived rainfall and runoff data sets.
Where applicable, a guide to the amount of missing data is given following the section heading. Some slight variations from the statistics held by the measuring authorities may occur; these may be due to different methods of computation or the need for uniformity in presentation.
Current year flow statistics are tabulated alongside the corresponding values for the previous record. Where appropriate, the current year figures are expressed as a percentage of the preceding average. (As a consequence of leap years, the runoff and mean flow percentages may not be identical.)
The average of all available daily mean flows during the term indicated.
The value and date of occurrence of the lowest mean flow in cubic metres per second in a water-day during the term indicated. In a record in which the value recurs, the date is that of the last occasion.
River flow measurement tends to become more imprecise at very low discharges. Very low velocities, heavy weed growth and the insensitivity of stage-discharge relations combine with the difficulty of accurately measuring limited water depths to reduce the accuracy of computed flows. The reliability of both the lowest daily mean flow and the 95 percent exceedance flow (see below) as representative measures of low flow must be considered carefully and the values used with caution in view of the increasing proportional variability between the natural flow and the artificial influences, such as abstractions, discharges and storage changes as the river flow diminishes.
The flow in cubic metres per second which was equalled or exceeded for 10 per cent of the specified term - a high flow parameter which, when compared with the mean may give a measure of the variability, or 'flashiness', of the flow regime. The 10 per cent exceedance value is computed using daily flow data only for those years with ten days, or less, missing on the National River Flow Archive.
The flow in cubic metres per second which was equalled or exceeded for 50 per cent of the specified term - the median value. The same conditions for completeness of the annual records apply as for the 10 per cent exceedance flow.
The flow in cubic metres per second which was equalled or exceeded for 95 per cent of the specified term - a significant low flow parameter relevant in the assessment of river water quality consent conditions. The same conditions for completeness of the annual records apply as for the 10 per cent exceedance flow.
An indication of the various types of abstractions from, and discharges to, the river operating within the catchment which alter the natural flow is given by a standard set of abbreviated descriptions. An explanation of the abbreviated descriptions is given below. With the exception of the induced loss in surface flow resulting from underlying groundwater abstraction, these codes and descriptions refer to quantifiable variations and do not include the progressive, and difficult to measure, modifications in the regime related to land-use changes. Except for a small set of gauging stations for which the net variation, i.e. reservoir storage changes and/or the balance between imports and exports of water to, or from, the catchment, is assessed in order to derive the 'naturalised' flow from the gauged flow, the record of individual abstractions, discharges and changes in storage as indicated in the code above is not held centrally.
CODE EXPLANATION
| N | Natural, i.e., there are no significant abstractions or discharges or the variation due to them is so limited that the gauged flow is within 10% of the natural flow at, or in excess of, the 95 per cent exceedance flow. |
| S | Storage or impounding reservoir. Natural river flows will be affected by water stored in a reservoir situated in, and supplied from, the catchment above the gauging station. |
| R | Regulated river. Under certain flow conditions the river will be augmented from surface water and/or groundwater storage upstream of the gauging station. |
| P | Public water supplies. Natural river flows are reduced by the quantity abstracted from a reservoir or by a river intake if the water is conveyed outside the gauging station's catchment area. |
| G | Groundwater abstraction. Natural river flow may be reduced or augmented by groundwater abstraction or recharge. This category includes catchments where minewater discharges influence the flow regime. |
| E | Effluent return. Outflows from sewage treatment works will augment the river flow if the effluents originate from outside the catchment. |
| I | Industrial and agricultural abstractions. Direct industrial and agricultural abstractions from surface water and from groundwater may reduce the natural river flow. |
| H | Hydro-electric power. The river flow is regulated to suit the need for power generation. |
A comprehensive set of gauging station and catchment descriptions is provided in the 'Hydrometric Register and Statistics 1991-95'. Further details of the net impact of abstractions and discharges on river flow patterns are given in: Gustard, A., Bullock, A. and Dixon, J.M. 1992. Estimating Low River Flows in the United Kingdom. Institute of Hydrology Report number 108. (For details of these and other publications visit the Centre for Ecology and Hydrology's online bookshop.)
A summary of any important factors influencing the accuracy of the current year's flow data specifically; for instance, the reconstruction of a gauging station or the use of extrapolated stage-discharge relations during periods of very low or very high flows. A short commentary providing a guide to the characteristics of the station, its flow record and the catchment it commands. The principal objectives of this summary information are to assist data users in the selection of gauging station records appropriate to their needs and to assist in the interpretation of flow variability at individual gauging stations particularly where the natural flow pattern is significantly disturbed by artificial influences.