It is possible to use the FSR to predict the depth of rainfall from a storm of a given duration and return period. The FSR includes values for two key variables mapped across the UK: the M5-60 minutes rainfall, and the ratio "r".
*M5-60 minutes rainfall is the expected depth of rainfall in millimetres (mm) from a storm lasting 60 minutes (1 hour) with a return period of 5 years (hence M5).
*M5-2 days rainfall is the expected depth of rainfall from a storm lasting 2 days (48 hours) with a return period of 5 years.
*The dimensionless ratio "r" is the M5-60 minutes value divided by the M5-2 days value.
*Factor Z1 is interpolated from figures based on the values of M5-2 days and "r".
*Factor Z2 (the growth factor) is found from the M5 rainfall depth, and depends on the return period.
*The Areal Reduction Factor (ARF) takes the catchment area into account. For small catchments (below 1 km) the ARF is not required.
To find the depth of a rainfall of duration D and return period T at a given location in the UK, the following should be carried out:
# Find M5-60 minutes rainfall depth and "r" for the location using FSR maps.
# Divide this rainfall depth by "r" to get the M5-2 days depth.
# Multiply the M5-2 days depth by factor Z1 to find the M5-D depth.
# Multiply the M5-D depth by factor Z2 (the growth factor) to find the MT-D depth.
# Multiply the MT-D depth by the Areal Reduction Factor (ARF).