Interception and depression storage losses

Interception loss

When rain falls on vegetation, part of it is held on leaves, stems and branches, and later evaporates without reaching the ground. This is interception. It depends on the type and density of vegetation, season (leaf cover), rainfall intensity and wind. It is about 10-20% of annual rainfall in forests and may be 100% for a light shower.

Depression storage

Rain that reaches the ground fills small hollows, puddles and ditches. This water cannot flow as runoff; it is lost by evaporation and infiltration. It depends on the slope, soil, land use and the rainfall excess. Typical values: 5 mm for sand, 2.5 mm on slopes, up to 10-15 mm in flat farmland.

Estimation in hydrological analysis

• Interception:
  • Measured as the difference between rainfall above the canopy and the throughfall plus stemflow below it.
  • Empirical form (Horton): $S_i = a + bP^n$ with $a$, $b$, $n$ as vegetation constants.
  • In design, 1-3 mm is often lumped as initial loss.
• Depression storage (Horton):

$$V_d = S_d\left(1 - e^{-kP_e}\right)$$

where $V_d$ is the storage filled, $S_d$ the maximum storage capacity, $P_e$ the rainfall excess and $k$ a constant. Alternatively, the initial abstraction $I_a$ (the sum of interception, depression storage and early infiltration) is subtracted from the storm rainfall (SCS: $I_a = 0.2S$).