Agriculture Reference
In-Depth Information
to form the store of water available to the plants (Ss). That part of the water entering
the profile that exceeds its water holding capacity will be lost by drainage (D)‚ either
laterally to adjacent sites or to deeper levels in the profile‚ beyond the range of plant roots.
At sloping sites‚ surface runoff and sub-surface drainage waters from areas upslope of
the site of interest also contribute to the local water balance.
Neglecting capillary rise‚ the water balance may be presented as follows:
where the symbols have the meaning expressed in the paragraphs above. The term
evapotranspiration is often used to include the sum of the terms EC‚ El‚ Es and T in
the above equation.
In temperate France‚ Fardjah
et al.
(1980) compared the water balance of a bare soil
with two vegetation types‚ a beech wood (
Fagus sylvatica
) and an area colonised by
the grass
Calamagrostis epigeios.
The soil‚ a spodosol‚ was similar at all sites and
the study area was horizontal; thus no runoff or run-on effects were present. Table I.13
presents the major fluxes of water in the study areas and their partitioning over a period
of one year. The increased drainage component from the bare soil is due to the lack of
evaporation and transpiration from the canopy and is typical of unvegetated areas; it is
not compensated for by evaporation from the soil surface alone. The substantial
interception losses from the forest are characteristic of this vegetation type and are due
to the considerable turbulence that occurs at the top of the canopy.
2.3.2
VEGETATIONAL AND SEASONAL EFFECTS ON SOIL WATER
Figure I.28 presents the seasonal changes in the estimated reserves of plant-available
water taking place over nine months for the three plots studied by Fardjah
et al.
(1980).
The total‚ nominally 'plant available' soil reserve (
i.e
.‚ that held between water poten-
tials of -0.03 and -1.5 MPa) was estimated as approximately 127 mm over all plots.
Figure I.28 shows the effect of the predominantly winter rainfall in recharging the store
of soil water depleted by evapotranspiration over the summer period.
