Geoscience Reference
In-Depth Information
Within a plant canopy, moisture exchanges are
extensive and of vital importance to the well-being of the
crop. In reality these processes are highly complex, but we
can get an idea of the exchanges by constructing a simple
model of the water balance.
Figure 8.6
shows the inputs
and outputs of moisture we might expect with an ideal
crop. The major input of most climatic regimes is
precipitation in the form of rain or snow, but hail, dewfall,
frost and fog can add small amounts. Some of this
moisture is intercepted by the leaves. Depending upon the
intensity and duration of the precipitation and the nature
of the leaf, the water may drop off the leaves, or be directly
evaporated without ever having reached the ground
surface. This effect is greatest when the rainfall is light and
the leaf density high. Small quantities of moisture may
flow down the stems of the plants, but with heavy or
prolonged rain some droplets will fall right through the
crop to moisten the soil surface, eventually reaching the
plant roots.
The output of the system is primarily through
transpiration from the leaves and evaporation from both
soil and leaves. Moisture is extracted from the soil to
maintain transpiration, but if the soil becomes too
dry during droughts the plants may wilt or even die.
During periods of rain, input is usually far higher than
evapotranspiration alone. This surplus goes to recharge
moisture in the soil or it becomes run-off - the horizontal
flow of water on the soil - which eventually forms part of
the river system.
So far we have been dealing with the microclimate either
on, or at least close to, the ground surface. From it we have
been able to illustrate the processes controlling the climate
at that level. As the crop or vegetation gets larger, so the
degree of modification increases and the active zone
extends from the higher canopy down to the soil surface.
The extreme example of this effect is seen in mature
forest. So much has been written about the microclimate
within a forest that the term
forest climate
is frequently
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287387
Precipitation (p)
absorption, dewfall
Evapotranspiration (E)
Transpiration
Evaporation
Interception
storage
Guttation
Distillation
Downflow
Throughfall
(direct + leafdrip)
Run
-off (
)
r
)
Surface storage
Root
uptake
Root
uptake
Figure 8.6
The hydrological cascade in a
soil-plant-atmosphere
system.
Source: After Oke (1987)
Soil moisture
storage and
ground water
(
)
S)
Capillary
rise
