#### Interception rate and interception storage

Interception rate can be calculated either by a simple
threshold formulation or by an exponential function (see switch InterceptionModel).
The threshold function gives the interception rate, *I (mm
day*^{-1}), by the vegetation
canopy.:

(3.51)

where *P* is precipitation,
*f*_{th,d} is the fraction of
the precipitation that directly reaches the soil surface without being affected
by the vegetation, *S*_{imax} is the interception capacity, and
*S*_{i} *(t-1)* is the interception storage remaining from the
previous time step.

Alternatively, the interception rate, *I*, is calculated by an exponential function (Hedström
& Pomeroy, 1998):

(3.52)

where
*f*_{∆t,snow} is a time step
dependent “snow unloading” coefficient, representing the influence of snow
falling of the canopy during and interception event. It is automatically set to
unity if snow interception is not treated (see switch SnowInterception) and/or
in case of liquid precipitation. For snow, *f*_{∆t,snow} is set to 0.7 for hourly
time steps, and empirically corrected to obtain the same interception rates if
other time steps are chosen.

The interception capacity (maximum storage)
*S*_{imax} is a function of the leaf area index, *A*_{l}:

(3.53)

where *i*_{LAI} and *i*_{base} are parameters. See viewing function Interception storage as a function of LAI.

The change in interception storage, ∆*S*_{i}, is calculated as the
difference between the interception rate, *I*, and the actual interception evaporation,
*E*_{ia}:

(3.54)

where
*U* is the amount of snow falling off the canopy due to a changed
interception capacity i.e. increased air temperature or snow melt in the canopy
(cf. section Interception capacity with snow
interception):

(3.55)