The urban expansion factor is detailed in Bayliss, A. Black, K. Fava-Verde, A. Kjeldsen, T. (2006). URBEXT2000 - A new FEH catchment descriptor: Calculation, dissemination and application. R&D Technical Report FD1919/TR, DEFRA, CEH Wallingford. The urban expansion model assumes a national average expansion as a function of year. This means that on some catchments the value will be overestimated (primarily on rural ones) and on others the value will be underestimated (primarily on urban ones).