Areal rainfall is calculated at a daily timestep for 5 areal units which cover London using the Soil Moisture Model developed and run by the Environment Agency.

For each areal unit, the areal rainfall and soil moisture deficits (SMDs) are calculated.

Areal rainfall – This is calculated using rainfall data collected at one or more rain gauges situated within each areal unit. This is then averaged and weighted across each areal unit.
Soil moisture deficits (SMDs) are calculated using the areal rainfall data and the potential evaporation (PE) estimated for each unit.Realistically, the SMD would vary slightly during the day given showers/thunderstorms etc and the PE would also vary with the time of day and temperature.This sort of detail is not available and unnecessary for most water resources studies, so the SMD is referred to as the day end value.
All areal rainfall and SMD data provided by the Environment Agency use the best available data at the time of running the soil moisture model and is subject to periodic updates and verification.

For SMD, best practice is to refer to end-of-month values and compare with long term average end-of-month values (1961-90).