Principles of terrestrial ecosystem ecology.pdf

80 4. Terrestrial Water and Energy Balance
The bark texture and architecture of stems
and trunks influence the amount and direction
of stem flow.Trees and shrubs with smooth bark
have greater stem flow (about 12% of precipitation)
than do rough-barked plants such as
conifers (about 2% of precipitation) (Waring
and Running 1998). In the Eucalyptus mallee in
southwestern Australia as much as 25% of the
incoming precipitation runs down stems, due to
the parachute architecture of these shrubs. The
stem flow then penetrates to depth in the soil
profile through channels at the soil–root interface
(Nulsen et al. 1986).
Water Movement Within the Soil
Pressure gradients associated with gravity and
matric forces control most water movement
through soils. The rate of water flow through
the soil (Js) depends on the driving force (the
gradient in water potential) and the resistance
to water movement. This resistance, in turn,
depends on the hydraulic conductivity (Ls) of
the soil, and the path length (l) of the column
through which the water travels.
DYt
Js = Ls l
(4.6)
This simple relationship describes most of
the patterns of water movement through soils,
including the infiltration of rainwater or snow
melt into the soil and the movement of water
from the soil to plant roots. Soils differ strikingly
in hydraulic conductivity due to differences
in soil texture and aggregate structure
(see Chapter 3). For this reason water moves
much more readily through sandy soils than
through clay soils or compacted soils. The rate
of water flow in saturated soils, for example,
differs by three orders of magnitude between
fine and coarse soils (less than 0.25 to greater
than 250mmh -1 ).
Infiltration of rainwater into the soil depends
not only on hydraulic conductivity but also on
preferential flow through macropores created
by cracks in the soil or channels produced by
plant roots and soil animals (Dingman 2001).
Variation in flow paths in the surface few
millimeters of soil can have large effects on
infiltration. Impaction by raindrops on an
unprotected mineral soil, for example, can
reduce hydraulic conductivity dramatically. For
this reason the presence of a surface moss
or litter layer, which prevents impaction by
raindrops, is one of the most important factors
determining whether water enters the soil
or flows over the surface. Any time that
precipitation rate exceeds the infiltration rate,
water accumulates on the surface and overland
flow may occur, leading to erosional loss of
soil.
Some soils have horizons of low hydraulic
conductivity that prevent water percolation to
depth. For example, calcic (caliche) layers in
deserts, permafrost in arctic and boreal ecosystems,
and hardpans in highly weathered soils
are horizons with such low hydraulic conductivity
that the water table remains close to
the surface, rather than moving into a deep
groundwater pool (see Chapter 3).
Once water enters the soil, it moves downward
under the force of gravity until the matric
forces, which account for the adsorption of
water to soil particles, exceed the gravitational
potential. Water that is not retained by matric
forces drains through the soil to groundwater.
The field capacity of a soil is the quantity of
water retained by a saturated soil after gravitational
water has drained. The large surface
area per unit soil volume in fine-textured soils
explains their high field capacity.A clay soil, for
example, with its high proportion of small
particles (Table 4.4), holds four times more
water than a sandy soil. Organic matter also
enhances the field capacity of soils, because of
its hydrophilic nature and its effects on soil
structure. For this reason the soils beneath
shrubs in deserts, which have higher organic
Table 4.4. Typical pore size distribution of different
soil types.
Pore space (% of
soil volume)
Particle size (mm) Sand Loam Clay
>30 75 18 6
0.2–30 22 48 40