Interception of rainfall in a young loblolly pine plantation
Those who have sought refuge during a storm have found a tree to be an effective umbrella for a light shower but leaky in a heavy rain. Even so, it is usually possible to stay slightly more dry under forest canopy than in the open. That is because a portion of the rain is stored on leaves and branches and eventually evaporated back to the air. The term interception is used for both the process and the amount of water thus dissipated. If one watches the rain drops, he can see how the process works. The first raindrops that strike the foliage spatter out, wetting the leaf surface. Finally the leaves are thoroughly wetted and water begins to drip off to the ground. Of course, some raindrops find openings in the tree crown and fall through directly to the ground. A part of the rain falling onto leaves and twigs flows from them to larger branches and finally runs down the trunk to the ground. The portion which is led to the ground down the stem is called stemflow, while that falling directly to the ground or dripping from twigs and leaves is called throughfall because it passes through the canopy. The sum of throughfall and stemflow or the total rain reaching the ground beneath a plant canopy is net rainfall.