The development of the BFI model is described within WHS, 2025). This also summarises the development of the spearman rank. The spearman rank is referred to as the SIMINDEX when assigning TSEP sites.

The BFI is estimated for an ungauged site based on descriptors for the catchment.

Note that, due to incomplete descriptor datasets, within catchments where more than 10% of the catchment is within Northern Ireland the BFI is not calculated. As the BFI is part of the spearman rank equation, used within Qube to assigning analogue gauging stations to spot gauges and for identifying TSEP sites, use of catchments with more than 10% in Northern Ireland for these purposes is not possible. Where the BFI cannot be calculate a warning 'Insufficient data at this location. See user guide' will be provided.

The descriptors used for calculation of the BFI include:

• Soils. Peat, Poorly Drained and AlluvMIN.
• SubSoils. Low, Medium and High permeability.
• FARL. Index of Flood attenuation by reservoirs and lakes.
• SAAR9120. Standardised average annual rainfall 1991 - 2020.
• Drainage Density (km of river per km² of catchment area)
• Stream Frequency. The stream frequency, the number of ‘segments’.

A beta regression model, based on the redevelopment of BFI in the UK, Griffin et. al., 2019, was used to estimate the BFI based on these descriptors.

where $\beta_i$ are the fitted model coefficients and $h_i$ is the proportion of soils or value of descriptor for the catchment.