Drainage Basin System -Interception, Stemflow, Canopy Drip,

Interception

storage of water above the ground surface, mostly in vegetation
where vegetation is present, precipitation consists of

gross rainfall measured above the canopy or in openings in a forest
throughfall: water that falls between plants,
drippage: of water from the plants to the grounds, and
stemflow: flow of water down stems and trunks

therefore, interception significantly reduces precipitation intensity as water is first temporarily sorted and much is lost
interception loss
that part of the precipitation on the canopy that doesn't reach the ground, because it evaporates from the canopy (canopy interception loss) and from near-ground plants and leaf litter (litter interception loss) or, to a lesser extent, is absorbed by plants
Controls on interception rate, capacity and loss

vegetation characteristics

interception increases exponentially during a storm until the interception capacity is achieved and the weight of more rain overcomes the surface tension holding the water on the plants
also snow periodically sluffs off plants when the capacity to intercept snow is achieved
interception capacity is a function of
growth form: trees, shrubs, grasses, forbs

coniferous trees intercept 25-35% of annual precipitation
deciduous trees intercept 15-25% of annual precipitation, but just as much as coniferous trees during the growing season
trees also have greater interception capacity because they project above ground and into the wind, creating turbulence which drives water on the lee side and into the interior of the tree
grasses and forbs have high interception capacity during the growing but then either die (annual plants) or loose mass (perennial plants); also they are grazed and harvested (spring wheat intercepts 11-19% of precipitation before harvest)

plant density

biomass data (mass/unit area) are a poor indication of interception capacity, rather the extent of ground cover and canopy closure are the important aspect of density

plant structure: number, size, flexibility, strength and pattern of branches; texture, surface area and orientation of leaves

trees native to regions of heavy snowfall have flexible branches and trunks to support and shed heavy snow loads (10-20 kg/m2 for wet snow)
thus forest mensuration data (tree heights, diameters and volumes) are a poor indication of interception capacity since they don't convey tree structure

plant community structure

secondary interception occurs in stratified forest communities where water drips from the canopy and is intercepted by lower plants
in short vegetation, interception storage merges with surface storage, especially if the plants are flexible and bed under the weight of water (e.g., the lodging of crops, which can substantially reduce yields)
snowcover on shrubby vegetation and tall grasses is very irregular with large void spaces representing up to 40% of the snowpack
meteorological factors

precipitation intensity

water can be delivered too quickly for the plants to accommodate
a larger proportion of low intensity precipitation will be intercepted as the storage capacity is created by drippage and stemflow

precipitation duration

absolute interception storage increases with increasing storm duration
but, because interception decreases exponentially, a larger proportion of short duration precipitation is intercepted than is the water from a long storm which is shed once a steady state (interception capacity) is achieved
nearly all the precipitation from a very short storm can be intercepted, i.e., there is no drippage or stemflow

wind speed

promotes interception loss by evaporation
inhibits interception until an initial layer of water or snow forms to support further storage
increases interception by blowing water into the interior of plants and plastering wet snow against trees and shrubs
thus the influence of wind is complex and depends on wind speed and type of precipitation

type of rainfall: rain versus snow

liquid water has high surface tension and forms an initial layer (sooner than snow) to which subsequent rain coheres
at temperatures around 0oC, rain can freeze to plants
snow is more easily blown off or away from plants, but once it sticks, snowflakes (depending on their size, shape and liquid water content) can bridge that gap between leaves, stems and branches; thus the interception of wet snow can be considerable

precipitation frequency

a very important factor as in wet vegetation part of the interception capacity is already occupied before a storm
therefore, maximum interception capacity occurs with short duration precipitation events that are spaced sufficiently far apart that vegetation dries out
however, infrequent precipitation is not conducive to plant growth, so there may be less vegetation under these climatic conditions
horizontal interception

fog drip and rime, where water is filtered out of fog as it passes through vegetation
concentrated along the windward edge of coastal forests, where precipitation may be 2-3 times higher than measured by rain gauges
all precipitation is drippage (fog drip) and stemflow, since the precipitation is induced by the vegetation and otherwise would not occur
Significance of Interception and Interception Loss

By how much does interception loss reduce inputs to the basin hydrological cycle?
not much because interception loss is offset by decreased transpiration
the amount of solar energy for evaporation and transpiration is constant for any time and place, so evaporation of intercepted moisture simply replaces the evapotranspiration that would have occurred in the absence of precipitation
experiments have shown decreased transpiration from wet foliage, thus
interception loss represents a net loss of water
the rate of evaporation of intercepted water exceeds rates of transpiration, because transpiration is limited by soil moisture conditions and rate at which water is transferred to leaves and interception loss can occur from dead (non-transpiring) vegetation
evaporation of intercepted water cools the plant, suppressing transpiration and causes a heat flux from the air which contributes to further evaporation
horizontal interception represents an additional input of water to coastal forests
thus interception loss from live vegetation usually represents a net loss to the basin hydrological cycle, so logging tends to increase runoff by reducing interception loss and transpiration
Summary - importance of interception

usually results in a net loss of water available to the basin hydrological cycle
lowers the intensity of precipitation
washes solid particles and dissolved carbon from leaves affecting soil and water chemistry and weathering processes