Interception of rainfall is calculated for vegetation and for houses (roof interception can be switched on or off by the user). The principle is simple: the canopy is seen as a fixed storage that fills up and overflows. De filling upmprocess is exponential and depends on canopy characteristics.
It is important to realize that the rainfall that is not intercepted reaches the surface with the same intensity as the rainfall. This is not the case in reality but throughfall intensities are only known for certain tree species and not for crops or low vegetation.This has an effect on the infiltration process. However it does not have an effect on the kinetic energy for splash erosion, that is assumed to depend on the falling heuight of the throughfall only (see splash erosion LINK).
The actual canopy interception Cs (mm) at a given moment, is calculated from the maximum canopy storage Smax (mm) and the cumulative rainfall since the beginning of the event Pcum (mm):
Cs = Smax*(1-exp(-k*Pcum/Smax))
where k is a parameter related to the canopy openness (co) and determines how fast the canopy fills up. The parameter k depends on the Leaf Area Index (LAI in m2/m2):
k = 1-exp(-co*LAI)
The parameter co can be set in the interface and has values of 0.45 for very open canopies such as Eucalypt, to 0.9 for for instance Douglas Fir species or dense crops. A higher value causes the canopy to fill up faster. Below is a graph that shows how fast a canopy fills up for different LAI values, from 1 to 6 m2/m2. It is assumed here that Smax is 0.2*LAI. The canopy storage Smax can be directly supplied as a map by the user, or it is derived from a map with LAI values using a set of empirical equations based on a literature research (REF de jong jetten):
These parameters all describe the interception of the vegetated part of the gridcell! The interception amount per gridcell is the interception of the canopy (Cs) multiplied by the cover fraction of the plants. Therefore the LAI must give the average LAI of the plants in a gridcell, NOT the average LAI of the gridcell itself.