Throughfall

Throughfall is the precipitation that penetrates through the canopy and reaches the soil surface by canopy drip. The amount of water retained by the canopy is termed interception (Interception = Precipitation - [Throughfall + Stemflow]). During the early stages of a precipitation event, much of the precipitation is stored on the canopy (e.g., foliage, branches, stems, and epiphytes) where it may be evaporated back to the atmosphere. As the precipitation continues, these surfaces reach their retention capacities and excess water drains to the soil surface as canopy throughfall. Evaporation of intercepted precipitation may take place throughout a storm but is of primary importance after precipitation ceases. Generally, 10-35% of the annual precipitation is intercepted by the canopy. For an individual precipitation event, the amount of interception depends on the intensity and duration of the event. Interception of snow is often greater than for rainfall.

Large deviations from these average values occur by season (e.g., leaf-on versus leaf-off for deciduous trees) and as a function of the stage of stand development (e.g., leaf area index).

Review Tables 1a and 1b