6.2 Interception

Interception can only be loosely defined as a catchment characteristic as it is the combined effect of several influential factors such as rainfall, climate and vegetation cover. However, in other respects it falls conveniently into this chapter and so is discussed here.

Losses from interception, the rainfall that collects on vegetation and is re-evaporated, can be highly variable and depends mostly on vegetation type (size, shape and disposition of leaves and branches); rainfall amount, intensity and drop size; wind speed, temperature and eddying. Interception is difficult to measure, especially for crops. It can be attempted by placing rain gauges under vegetation either randomly to sample average interception, or by the selection of specific target areas. In wooded catchments, rain gauges should be attached to tree trunks to assess stem flow, as in Figure 6.6 below, but with multi-stemmed vegetation this is very difficult.

Figure 6.6: Stemflow Measurement on trees

In Figure 6.6, in addition to free-standing gauges under the canopy, a peripheral collector is wrapped around the trunk to direct flow into a single rain gauge that is covered.

Empirical work has led to estimates of losses of 10 - 20% of seasonal rainfall and deduced storage capacities of 0.8 to 1.5 mm of rain per storm. Equation (6.1) describes an empirical interception relation and Table 6.3 gives examples for various crops for a 25 mm rainfall.

I = (S_i + Et_r) (1 - e^-kP) where (6.1)

I = total interception
S_i = storage capacity per unit of the area
E = evaporation rate
t_r = duration of rainfall
P = amount of rain k = 1/ (S_i + Et_r)
e = base of natural logs.

In terms of runoff studies, the situation regarding interception is even more complex. It is usually lumped with rainfall storage due to ponding and infiltration for runoff modelling purposes, where it is assigned a purely notional value.

Table 6.3 Interception Losses from a 25 mm Rainfall