The new BFIHOST19 descriptor is now available for catchment and points alongside the original BFIHOST data.

The estimate of the base flow index (BFI) based on the Hydrology of Soil Types (HOST) classification, BFIHOST, provides a measure of catchment responsiveness. BFIHOST is used with other variables to estimate the median annual maximum flood (QMED) in the UK standard Flood Estimation Handbook (FEH) statistical method and is also an explanatory variable in ReFH2, the FEH design hydrograph package.

The new BFIHOST19 descriptor addresses a number of 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.

To resolve such issues, CEH has revised the regression method used to estimate BFIHOST and has improved the classification of rare soil types. The revision has been published in Hydrology Research available here: https://doi.org/10.2166/nh.2019.166.

The catchment descriptor equations that include BFIHOST are being reviewed and recalibrated as required to allow the improved data to be used in flood frequency estimates. The latest FEH rainfall-runoff method (ReFH2.3) uses the BFIHOST19 descriptor.

The BFIHOST19 descriptor is included within the following exports:

• Descriptors for ReFH2.3+ (XML) for catchments and points, and
• Descriptors with BFIHOST19 (CSV) for catchments.