The partitioning of precipitation into interception, throughfall, runoff and infiltration is described Gregory and Smith (1990) and is applied separately on each tile. Essentially, the rainfall rate is assumed to be distributed exponentially across the area. In addition, if the rainfall is ‘convective, then it is assumed to cover only 0.3 of a the area. This part of the JULES model aims to overcome the problem of GCM-drizzle, whereby every time it rains, a small amount of water covers the entire area, is held on the leaves of the vegetation and re-evaporates, without entering the soil matrix. In practice the rainfall can be intense over small areas and this means the rainfall fall through the vegetation canopy and into the ground. It is possible to turn this feature off if observed rainfall is being used to drive the model (where the rainfall is less frequent but more intense).
The model is being developed so that is will include a rainfall-runoff module and a runoff routing scheme. At present these options are not available.
Gregory, D., and R. N. B. Smith, 1990: Canopy, surface and soil hydrology. Unified Model documentation paper 25, Meteorological Offce, London Rd, Bracknell, Berkshire, RG12 2SY.