Canopy Interception and Storage Capacity

Precipitation falls on foliage, branches, and bark, where it collects and readily evaporates back to the atmosphere. The amount collected depends on rainfall intensity, frequency, and duration. Most plants have a small capacity to store water. Early studies of interception suggested that trees, shrubs, and grasses can store about 1.3 mm of liquid water on their foliage (Zinke 1967). A slightly larger amount of water (3.8 mm) can be stored as snow in trees. This storage capacity varies with the leaf area index (L) of the canopy. Studies find that storage capacity (mm) ranges from 0.15L to 0.3L, and may be as large as 0.5L in some forests (Carlyle-Moses and Gash 2011). The interception capacity and amount of water held in the canopy has a significant effect on climate because it readily evaporates (Davies-Barnard et al. 2014).

During brief, moderate storms, this storage capacity may not be exceeded and most of the rainfall is intercepted. In long, intense storms, the storage capacity is quickly exceeded and water drips to the ground. Figure 10.2 shows typical relationships among rainfall amount and throughfall and stemflow. Stemflow is typically minor. Throughfall is the dominant component. For example, a deciduous forest in full leaf may allow 18 mm of rainfall to reach the ground during a 20 mm storm. Of this, 17 mm reaches the ground as throughfall and 1 mm is stemflow. The remaining water (2 mm) is intercepted. Intercepted water readily evaporates because it is held externally on foliage and wood. One study found that in pine forests the rate of evaporation of intercepted water was three times the rate of transpiration under the same radiation conditions (Stewart 1977).

Plant canopies generally intercept about 10-20 percent of annual precipitation (Carlyle-Moses and Gash 2011). Lowland tropical forests intercept, on average, 15 percent of annual precipitation (Krusche et al. 2011). Satellite-based estimates of interception loss range from 13 percent of annual rainfall for broadleaf evergreen forest to 19 percent for broadleaf deciduous forest and 22 percent for needleleaf forests (Miralles et al. 2010). One determinant of interception is vegetative cover; the denser the foliage the greater the amount of water stored. The seasonal emergence and senescence of leaves also matters. Deciduous forests intercept less rainfall when leafless than during the growing season when leaves are present (Figure 10.2).

When in full leaf, grasses have the same storage capacity as trees, but because they have no woody material when dormant they intercept considerably less precipitation annually than trees. The type of leaf also determines the amount of interception. Broad-shaped leaves, such as found on deciduous trees, allow water droplets to run together, forming larger drops that readily drip off the leaf. Needle-shaped foliage, such as on coniferous trees, does not facilitate dripping because the separated small needles do not allow water to coalesce into large droplets.