The mean Gauged Daily Flow (GDF) archive forms the main part of the NRFA data holdings.
Definition
The Gauged Daily Flow is the mean river flow in cubic metres per second (abbreviated to m3s-1 and sometimes also referred to as 'cumecs') in a water-day, (09.00 to 08.59 GMT, for example; the flow on the 1st December is the mean flow from 09:00 1st December to 08.59 2nd December) or, where indicated, a calendar day.
Data Update Schedule
The period of record available for each gauging station is shown on each station page. The river flow data from operational gauging stations are provided to the NRFA by Measuring Authorities through an annual submission programme. Data for each water year are normally submitted to the NRFA the following spring, after which it undergoes detailed quality control before being added to the national archive. River flow data for each water year are normally available on the archive by the late-summer. For example, river flow data for water year 2020/2021 will not normally be available on the NRFA until late-summer 2022. Where more recent river flow data are required, users may be able to source this by contacting the relevant measuring authority.
Data Precision
Gauged Daily Flows are given to four significant figures or three decimal places, which ever represents the larger flow.
Computation and Accuracy
An overview of river flow measurement techniques and hydrometric practice within the UK is provided here. Within the UK, measuring authorities typically calculate river flows on the basis of measurements at 15-minute intervals. These high resolution data are used to calculate the mean Gauged Daily Flow. These mean daily flow data are submitted to the National River Flow Archive.
The NRFA exists to provide not only a central database and retrieval service but also an extra level of hydrological data validation. To further this aim, project staff at the UK Centre for Ecology & Hydrology liaise with their counterparts in the Measuring Authorities on a regional basis and, by visiting gauging stations and data processing centres, endeavour to maintain the necessary knowledge of local conditions and problems which is essential to help identify and rectify anomalous flow data.
In collaboration with the measuring authorities considerable effort is devoted to maintaining the quality of nationally archived river flow data. Data submitted to the NRFA are systematically submitted to a sequence of quality control checks. However, the ability of individual gauging stations to provide accurate and representative flows on a sensibly continuous basis, varies greatly from station to station and, often, through time at the same site.
Flow data for some stations may include estimated values provided by the Measuring Authorities or derived by the UK Centre for Ecology & Hydrology.
