Catchment Description
The National River Flow Archive provides concise station and catchment descriptions for gauging stations held on the archive. The descriptive material has been developed in collaboration with the principal UK measuring authorities; for a few stations descriptive components may not be present. Reference to the Glossary should be made for an explanation of technical terms, abbreviations and acronyms used. The Station and Catchment Descriptions are under continuing review – reflecting the availability of more information, changing hydrometric conditions at individual gauging stations, and changing land and water usage patterns within the catchment.
Catchment Area
Catchment areas for NRFA stations are based on information provided by the relevant measuring authority, and may have been calculated or estimated using a range of methods. Consistent digitally derived catchment areas are also calculated from the Integrated Hydrological Digital Terrain Model (IHDTM), a hydrologically consistent 50m gridded representation of topography for the UK, which allows the computation of drainage paths. This IHDTM Area is used within Flood Estimation Handbook methods, and within Peak Flows data files, and is displayed alongside other FEH catchment descriptors on the peak flows tab. Catchment polygons are derived using the IHDTM and will generally have an area consistent with the IHDTM Area. For a small number of stations, it is not possible to derive a digital catchment boundary using the IHDTM (e.g. for combined stations, or for very low relief areas) and catchment boundaries may be manually defined, and IHDTM Areas estimated.
Factors Affecting Runoff
The Factors Affecting Runoff (F.A.R.) codes provide an indication of the various types of artificial influences operating within the catchment which alter the natural runoff. For some areas the allocation of F.A.R. codes is incomplete and for all catchments the codes are subject to continuing review. The absence of F.A.R. codes does not imply a natural flow regime. An explanation of the code letters 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 flow regimes related to land use changes.
Until recently, assignment of F.A.R. codes has been largely determined by expert local judgement of the magnitude of the impact of artificial influences at individual gauging stations. Access to the Low Flows 2000REF and other databases, is beginning to allow a more objective and quantitative approach to assignment of F.A.R. codes. Low Flows 2000 assessments of artificial influences have been used, generally for stations commissioned during the last decade, to guide F.A.R. designations and is expected to be exploited more fully in future.
Except for a small set of gauging stations for which the net variation (i.e. the sum of abstractions and discharges) 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.
REF: Young, A. R., Grew, R. and Holmes, M. G. R. 2003. LF 2000: a national water resources assessment and decision support tool. Water Science & Technology, 40 (10), 119-126.
Code Explanation
| Code | Explanation |
|---|---|
| N | Natural, i.e. there are no abstractions and discharges or the variation due to them is so limited that the gauged flow is considered to be within 10% of the natural flow at, or in excess of, the Q95 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 runoff is 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 the diminishing number of catchments where mine-water discharges influence the flow regime. |
| E | Effluent return. Outflows from sewage treatment works will augment the river flow if the effluent 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; catchment to catchment diversions may also significantly affect average runoff. |
