- 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
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