The National River Flow Archive calculates flow statistics for individual flow records / gauging stations to aid selection and analysis of river flow data. Such statistics are displayed on the data tab of the station webpages. Unless otherwise stated all flows statistics are calculated using the period of record Gauged Daily Flow series. Users should note that statistics are not available for all flow records / gauging stations. The following flow statistics are commonly calculated by the NRFA:

## Mean Flow

Period of record mean gauged flows at gauging stations are calculated by the average, weighted to account for the different number of days per month, of the mean monthly flows for the period of record.

## Flow Percentiles

Percentiles are computed using gauged daily flow data only for those years with five days, or less, missing on the NRFA.

- Q10 (the 90 percentile flow): The flow in cubic metres per second which was equalled or exceeded for 10% of the specified term – a high flow parameter which, when compared with the Q 95 flow provides a measure of the variability, or ‘flashiness’, of the flow regime.
- Q50 (the 50 percentile flow): The flow in cubic metres per second which was equalled or exceeded for 50% of the flow record. Q 50 flows greater than zero but less than 0.005 m
^{3s-1}appear as ‘>0.0’. - Q70 (the 30 percentile flow): The flow in cubic metres per second which was equalled or exceeded for 70% of the flow record. Q 70 flows greater than zero but less than 0.005 m
^{3s-1}appear as ‘>0.0’. - Q95 (the 5 percentile flow): The flow in cubic metres per second which was equalled or exceeded for 95% of the flow record. The Q 95 flow is a significant low flow parameter particularly relevant in the assessment of river water quality consent conditions. Q95 flows greater than zero but less than 0.005 m
^{3s-1}appear as ‘>0.0’. Q95 values should be used with caution in view of the problems associated with both the measurement of very low discharges and the increasing proportional variability between the natural flow and the net impact of artificial influences, such as abstractions, discharges, and storage changes as the river flow diminishes.

## Mean Annual Catchment Runoff

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

Runoff in mm = Mean Flow (m^{3s-1}) x 86.4 x 365 Catchment Area (km^{2})

The mean annual runoff is rounded to the nearest millimetre.

As a consequence of missing data there will not be full equivalence between the mean annual rainfall and the mean annual runoff for some catchments. Runoff statistics and the corresponding mean flow are computed on the basis of naturalised flows for the small minority of catchments where sensibly continuous daily, or monthly, naturalised data are held on the NRFA. The uncertainty in the magnitude of the necessary adjustments to the gauged flows may be considerably greater than the uncertainty associated with the gauged flows themselves.

The net impact of abstractions and discharges may result in unrepresentative mean annual runoff figures. More commonly, a lack of coincidence between the topographical catchment divide and the true extent of the contributing area (which may be substantially different for permeable catchments) can produce anomalous mean annual runoff totals. Note also that measurement limitations – especially precipitation assessments in very wet upland catchments – may give rise to runoff which approaches, or even exceeds, the corresponding catchment rainfall. Guidance as to how representative the mean annual runoff is of the natural flow regime may be found in the Factors Affecting Runoff (F.A.R.) codes and the Station Descriptions.

## Mean Annual Loss

The mean annual loss is the difference between the mean annual catchment rainfall and the mean annual catchment runoff. Entries are confined to catchments where there is good agreement between the periods for which rainfall and runoff are held on the National River Flow Archive. The mean annual loss provides a guide to average annual evaporative losses but limited precision in the rainfall and runoff figures, the net effect of artificial influences on the mean runoff and, particularly, a lack of congruency between the topographic and the true catchment areas may all combine to produce unrepresentative mean losses. For those few catchments where computed mean runoff exceeds computed mean rainfall no mean annual loss is given. The F.A.R. codes and the relevant Station Descriptions of the Register should be consulted to assess the credibility of the featured mean annual losses.

## Base Flow Index

The Base Flow Index (BFI) was developed at the Institute of Hydrology (now Centre for Ecology & Hydrology) during the Low Flow Study to help assess the low flow characteristics of rivers in the United Kingdom (for details of the procedures used to compute the BFI, see Gustard et al 1992^{REF}). The BFI has been computed using the archived record of gauged daily mean flows.

The BFI may be thought of as a measure of the proportion of the river runoff that derives from stored sources; the more permeable the rock, superficial deposits and soils in a catchment, the higher the baseflow and the more sustained the river’s flow during periods of dry weather. Thus the BFI is an effective means of indexing catchment geology. For instance, rivers draining impervious clay catchments (with minimal lake or reservoir storage) typically have baseflow indices in the range 0.15 to 0.35 whereas most Chalk streams have a BFI greater than 0.9 as a consequence of the high groundwater component in the river discharge.

BFI values computed using less than five years of flow data should be regarded as provisional.

A BHIHOST statistic is also available for some gauged catchments.

REF Gustard, A., Bullock, A., Dixon, J. M.. 1992. Low flow estimation in the United Kingdom. Wallingford, Institute of Hydrology, 88pp. (IH Report No.108).

## Percentage Data Complete

The percentage completeness of the daily gauged flow time series over the given period of record. For some stations, a relatively low percentage completeness may reflect large gaps in the record (e.g. where a station has been recommissioned after several years without active monitoring).

## Period of Record

Shown on the ‘Daily flow data’ page for each station, the month and year for which daily river flow records start and end. Data for periods preceding the first year, often of a sporadic nature or of poorer quality, may occasionally be available from Measuring Authorities or other sources. Areal rainfall data may not be available for the full period of record.