The descriptors, main pooling group and QMED equations as well as urbanisation precodures used in the 2008 method are presented below.

Legacy Descriptors

The 2008 method uses a number of different descriptors to those used for the 2025 method. These are listed below:

SAAR6190: Standard Annual Average Rainfall (SAAR) in mm for the period 1961 to 1990.
FARL: an index of flood attenuation attributable to reservoirs and lakes.
URBEXT2000: the extent of urban and suburban landcover based on LCM 2000, Bayliss et al., 2006).
BFIHOST19: Base Flow Index derived using the UK Hydrology of Soil Types (HOST) classification (Griffin et al., 2019).

Growth curve estimation

The 2008 method uses a default list of 500 years of data.

The standard FEH similarity distance measure (SDM) equation used in 2008 in the method as described in Kjeldsen et al., 2008 is as follows:

$SDM = \sqrt{3.2 \bigg[ {{Ln(Area_i) -Ln(Area_j)} \over {1.28}} \bigg]^2 + 0.5 \bigg[ {{Ln(SAAR_i) -Ln(SAAR_j)} \over {0.37}} \bigg]^2 + 0.1 \bigg[ {{FARL_i -FARL_j} \over {0.05}} \bigg]^2 + 0.2 \bigg[ {{FPEXT_i -FPEXT_j} \over {0.04}} \bigg]^2 }$

For small catchments (under 40km²) an alternative SDM equation can be used. For the 2008 method, within WINFAP, the option to 'use small catchment recommendations as default' is selected automatically for catchments under 25km².

The equation to estimate the SDM within small catchments is described in Stewart et. al., 2019:

$SDM = \sqrt{\bigg[ {{Ln(Area_i) -Ln(Area_j)} \over {1.264}} \bigg]^2 + \bigg[ {{Ln(SAAR_i) -Ln(SAAR_j)} \over {0.349}} \bigg]^2 }$

Note that within the 2008 method, where at-site data is included in the pooling group, 'additional' weighting was provided to the at-site data. This was referred to as the Enhanced Single Site (ESS) procedure. This additional weighting is not present in the 2025 method however this is still referred to as an ESS estimate as it uses at-site data.

QMED estimation

QMED from Catchment Descriptors

The equation to estimate QMED from the Catchment Descriptors, developed by Kjeldsen et al., 2008, is presented below.

$QMED = 8.3062 \times AREA^{0.8510} \times 0.1536^{ \big( { 1000 \over SAAR} \big) } \times FARL^{3.4451} \times 0.0460^{(BFIHOST19)^2}$

The Catchment Descriptor estimate of QMED can be refined using Donor Adjustment, as detailed in Kjeldsen et. al., 2014.

Where the 'use small catchment recommendations as default' setting is selected then only 1 donor catchment will be used by default. Where this is not selected 6 catchments are used by default. You can modify these as appropriate.

Urbanisation Procedures

Urban Adjustment to QMED

The equations used in the 2008 methods for the urban-adjusted estimate (QMED_URBAN), the Urban Adjustment Factor (UAF) and the differences between percentage runoff within a rural and urban catchment (PRUAF) are as follows:

$QMED_{URBAN} = QMED_{RURAL} \times UAF$

$UAF = (1 + IF.URBAN)^{1.25} (PRUAF)^{1.33}$

$PRUAF = 1 + IF.URBAN. \bigg( { PR_{IMP} \over 69.366-(65.686 \times BFIHOST19)} -1 \bigg)$

Urban adjustment of L-moments for use in derivation of the growth curve

Equations for the L-CV_URBAN and L-SKEW_URBAN used in the 2008 method are presented below:

$L{-}CV_{URBAN} = (L{-}CV_{POOLED}) (0.68654)^{URBAN} $$ $$L{-}SKEW_{URBAN} =(L{-}SKEW_{POOLED} +1) (1.096017)^{URBAN}-1$

See the WINFAP 4: Urban Adjustment Procedures Technical Note for more details Urbanisation and urban adjustment procedures used in the 2008 method.