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.:
where P is precipitation, fth,d is the fraction of the precipitation that directly reaches the soil surface without being affected by the vegetation, Simax is the interception capacity, and Si (t-1) is the interception storage remaining from the previous time step.
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.
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):