The Wallingford Procedure includes a factor called the Urban Catchment Wetness Index (UCWI) which is designed to allow for the likely saturation level of the soil before the storm event being considered. A higher UCWI value will lead to a higher runoff value as the soil will become saturated more quickly during the design storm event.

Urban Catchment Wetness Index Calculation

For design purposes, the CivilWeb Rainfall & Runoff spreadsheet uses a correlation between the Standard Annual Average Rainfall (SAAR) value and the UCWI value, which is shown in the graph below. This correlation is appropriate because the SAAR value is a reasonable measure of the total wetness of the catchment. The wetter the catchment, the more likely the design storm event will occur when the soils are already partially saturated, and therefore the runoff will be higher.

The choice of winter or summer in the spreadsheet determines which curve is used to calculate the UCWI. Winter is always the worst case as can be seen from the graph below.