FEH catchment descriptors

Most of the catchment descriptors and statistics are based, directly or indirectly, on the Integrated Hydrological Digital Terrain Model (IHDTM) and may differ slightly from corresponding descriptors derived using different digital terrain models. For a few gauging stations where the drainage paths are difficult to determine (e.g. for some spring sources) or do not reflect local topography (e.g. artificial channels used for water transfers), realistic catchment descriptors cannot be derived. 

The full list of FEH Catchment Descriptors and a description are provided below. Details of the derivation and utility of these descriptors are given in Volume 5 of the Flood Estimation Handbook (FEH). 

For information on new catchment descriptors (FARL₂₀₁₅, SAAR₉₁₂₀, BFIHOST_19SCALED, URBEXT₂₀₁₅), further information is available in the New FEH Catchment Descriptors 2025 report.

Mean catchment altitude (m above sea level), derived from the IHDTM.

Index representing the dominant aspect of catchment slopes (°).

Index representing the invariability in aspect of catchment slopes (°).

Catchment drainage area (km²), derived from the IHDTM.

This base flow index is a measure of catchment responsiveness derived using the 29-class Hydrology Of Soil Types (HOST) classification. The HOST dataset is available as a 1km grid, which records, for each grid square, the percentage associated with each HOST class present. Using IHDTM boundaries for each gauged catchment, the soil characteristics of the catchment can be indexed, and by exploiting the relationship between soil typologies and runoff response, an aggregated assessment of BFIHOST for the catchment can be derived.

Note: there is a strong general association between BFIHOST and the Baseflow Index derived using the hydrograph separation approach, but no close equivalence can be expected where the natural flow regime is substantially disturbed, e.g. by compensation flows or major augmentation from sewage effluent.

The estimate of the base flow index (BFI) based on the Hydrology of Soil Types (HOST) classification, BFIHOST, provides a measure of catchment responsiveness. The new BFIHOST₁₉ descriptor addresses several issues in the original BFIHOST developed in 1995, including:

• The BFI calculated through base flow separation tended to be underestimated in clay-dominated catchments,
• the calculation technique performed poorly in ephemeral catchments or those with missing data, and
• The pragmatic bounding of BFI coefficients for permeable soils overlying aquifer outcrops was also problematic for small catchments.

A minor update to the Base Flow Index based on the Hydrology of Soil Types 2019 update (BFIHOST₁₉) that excludes water bodies as a HOST class from the calculation to reduce correlation with FARL and FARL₂₀₁₅. This descriptor was first used in the Peak Flow Dataset Version 14 (August 2025).

Centroid of the catchment, in kilometres. This descriptor was first used in HiFlows-UK Version 3.

An estimate of the depth of precipitation for a specified duration and frequency or recurrence interval.

Mean of distances between each node on the IHDTM grid and the catchment outlet, in kilometres. Used to characterise catchment size and configuration.

This landform descriptor (mean Drainage Path Slope) provides an index of overall catchment steepness. It was developed for the Flood Estimation Handbook and is calculated as the mean of all inter-nodal slopes (derived using the IHDTM) for the catchment. The index is expressed in metres per kilometre with values ranging from >300 in mountainous terrain to <25 in the flattest parts of the country.

A descriptor that gives information on the drainage density of the catchment. It is defined by dividing the length of rivers (km) by the contributing catchment area (km²). Higher drainage density indicates a catchment with many closely spaced streams. This often occurs in areas with impermeable soils, high annual rainfall, and steep slopes. Low drainage density suggests a catchment with fewer, more widely spaced streams. This is common in areas with flat terrain, low annual rainfall and permeable soils. These values are approximations, intended primarily for comparing catchment characteristics rather than giving precise measures. The DRAINDENS descriptor is not used in any FEH methods. This descriptor was first used in the Peak Flow Dataset Version 14 (August 2025).

Any reservoirs or lakes within a catchment will tend to have some effect on flood response, but it is those directly linked to the stream network that are most likely to produce an attenuation effect. The Flood Attenuation by Reservoirs and Lakes (FARL) index, developed for the Flood Estimation Handbook, provides a guide to the degree of flood attenuation attributable to reservoirs and lakes in the catchment above a gauging station. Values close to unity indicate the absence of attenuation due to lakes and reservoirs, whereas index values below 0.8 indicate a substantial influence on flood response.

An update to the previous version of Flood Attenuation due to Reservoirs and Lakes (FARL), now using 2015 Land Cover Map data. This descriptor was first used in the Peak Flow Dataset Version 14 (August 2025).

The mean depth of water on floodplains in a 100-year event. This descriptor was first used in HiFlows-UK Version 3. 

The floodplain extent is defined as the fraction of the catchment that is estimated to be inundated by a 100-year flood. This descriptor was first used in HiFlows-UK Version 3. 

The location of floodplains within the catchment is described using the same principles employed to derive values of the FEH index URBLOC. This descriptor was first used in HiFlows-UK Version 3. 

Longest drainage path (in kilometres), defined by recording the greatest distance from a catchment node to the defined outlet.

Dry soils tend to inhibit flood formation whilst, in contrast, saturated soil conditions precede and contribute to many large flood events. This catchment wetness index (PROPortion of time soils are WET), developed for the Flood Estimation Handbook, provides a measure of the proportion of time that catchment soils are defined as wet (in this context, when soil moisture deficits are less than 6mm). PROPWET values range from over 80% in the wettest catchments to less than 20% in the driest parts of the country.

Average annual rainfall in the period 1941-1970) in millimetres.

Average annual rainfall in the period 1961-1990 in millimetres. Based on Met Office data (not exactly HadUK; subject to different averaging methods)

An update to the previous version of Standard-period Average Annual Rainfall for 1961-1990 (SAAR). SAAR₉₁₂₀ uses 1991-2020 Met Office HadUK-Grid 1.3.0.0 in Great Britain, Northern Ireland, and the Republic of Ireland within approximately 25 km of the border, with a small amount of infilling using CEH-GEAR in areas more than 25 km from the border that drain through Northern Ireland. This descriptor was first used in the Peak Flow Dataset Version 14 (August 2025).

Standard percentage runoff (%) associated with each HOST soil class. This can be used to derive SPRHOST over a catchment. SPRHOST can be derived from flow data where available.

Index of urban and suburban land cover in 1990 expressed as a fraction.

Index of urban and suburban land cover in 2000 expressed as a fraction.

An update to the previous versions of catchment urban extent (URBEXT₁₉₉₀ and URBEXT₂₀₀₀), now using 2015 UKCEH Land Cover Map data. This descriptor was first used in the Peak Flow Dataset Version 14 (August 2025).

Index of the concentration of urban and suburban land cover in 1990 / 2000 expressed as a fraction.

Index of the location of urban and suburban land cover in 1990 / 2000 expressed as a fraction.