When permeable catchments (defined as SPRHOST <=20) are present in the pooling group a ‘permeable adjustment’ should be applied to the gauging station AMAX. This accounts for the fact that in permeable catchments there may be ‘non flood’ years; these are defined as years in which the AMAX is less than half of the QMED. The L-CV and L-Skew should be adjusted to account for the non flood years. The method is described within the Flood Estimation Handbook Volume 3 (Chapter 19).
WINFAP 4 does not currently support this adjustment method. WHS have, however, produced the ‘WHS Permeable Adjustment Worksheet’ which can be used to implement this process. This can be downloaded from https://www.hydrosolutions.co.uk/software/free-downloads/.