Canopy Interception

Interception is defined as the process whereby precipitation is retained on the leaves, branches, and stems of vegetation. This intercepted water evapo­rates directly without adding to the moisture storage in the soil.

The interception process is modelled as an interception storage, which must be filled before stem flow to the ground surface takes place. The maximum amount of interception storage, S_{int max}, depends on the vegetation type and its stage of development, which is characterised by the leaf area index, LAI. Thus,

(20.14)   

where C_int is the interception storage capacity that defines the maximum amount of interception storage for the vegetation. A typical value is about 0.05 mm but a more exact value may be determined through calibration.

The interception storage, S_int, is accumulated over time. So, if the amount of precipitation in the time step, plus the current amount of storage, exceeds S_{int max}, then the interception storage will equal S_{int max} and any excess will be added ponded water on the ground surface. If S_{int max} is not exceeded, then the canopy will capture all of the available precipitation in the time step.

If the LAI decreases between time steps (e.g. after a crop harvest), S_{int max} will decrease and any excess interception storage will be added to the ponded water on the ground surface.