New Updates to FEH methods

In Summer 2025, the flood estimation team have released an update to the statistical flood frequency estimation methods (the FEH methods), including changes to QMED estimation, donor transfer, the pooling method, and urban adjustments. These updates, the first major updates in some time, are based primarily on a set of new catchment descriptors to complement existing ones, and a large increase in the amount of available peak flow data.

BHS Webinars on FEH 2025 Statistical Method Updates

New FEH Catchment Descriptors for 2025

As part of the NRFA Peak Flow dataset V14, four new catchment descriptors are being added to describe the current state of UK catchments.

  • SAAR9120: An update to SAAR6190, which describes the average annual rainfall for the 30-year period 1991-2020, based on the Met Office's HadUK gridded datasets (OGL).
  • FARL2015: An update to FARL, which describes the attenuation of peak flow due to reservoirs and lakes. This makes use of OS data and the UKCEH Land Cover Map 2015.
  • URBEXT2015: An update to URBEXT2000, which describes the proportion of urban and suburban land in catchments, making use of UKCEH Land Cover Map 2015.
  • BFIHOST19SCALED: A minor update to adjust BFIHOST19 which removes water bodies from the calculation. Uses the same model coefficients at BFIHOST19. Based on HOST soil classification data.

A report has been written discussing how the new descriptors were derived, what the big changes are, and how the new catchment descriptors should be used.

Download PDF of new FEH Catchment Descriptors Report

The FEH 2025 Statistical Method

Along with the catchment descriptors, there has been an update to most areas of the FEH Statistical Method, which gives estimates of peak flow return periods for gauged and ungauged locations on the UK river network. The key updates for 2025 are as follows:

  • QMED equation. This equation uses catchment descriptors to estimate the 2-year return level at an ungauged location. The 2025 update makes use of the new catchment descriptors, and better estimates of gauged QMED from longer records.
  • Donor transfer. The QMED equation has biases which have strong spatial patterns. To correct this, a donor transfer method is applied which uses proximate stations of good quality to correct for this bias. The new method recommends using more stations in the method (8, up from 6).
  • Pooling method. This method is to improve estimates of rarer event magnitudes by combining the data from a set of hydrologically similar stations. The new method adds BFIHOST19SCALED to the similarity metric, and recommends the use of more station-years in calculations.
  • Urban adjustment. The methods above generate "as-rural" estimates, and there are methods to account for urbanisation in target catchments. The update makes use of URBEXT2015 in both the QMED and Pooling methods.

A report has been written which discusses how the methods were updated, what the big changes are, and how they should be applied in practice. The methods in these reports should be followed in adherence with guidance from local and national authorities, make sure to check these before using the new methods.

 

Download FEH 2025 Statistical Method Update Report

Data and Software

The new catchment descriptors will be available for NRFA stations through the Peak Flow V14 dataset:
https://nrfa.ceh.ac.uk/data/peak-flow-dataset

and through the FEH Web Service:
https://fehweb.ceh.ac.uk/Map

The new methods will be available in an update to WINFAP 5.3, software from Wallingford Hydrosolutions:
https://www.hydrosolutions.co.uk/software/winfap-5/