Agriculture Reference
In-Depth Information
The marked effect of the vegetation present is clear from the greater soil water store
in the unvegetated area and the longer period during which this reserve remained high.
In the vegetated area‚ the available store of soil moisture was exhausted twice during
the summer period. Precipitation after August restored the soil store by early October in
the bare area but not until December for the vegetated sites. Differences between the two
vegetation types are related to the effects of the forest canopy. As indicated above‚
increased turbulence at the top of the canopy meant that considerably less water reached
the soil in the beech wood: net interception was estimated at 15.1 % in the beech wood
and 8 % in the grassed area. Evaporation from the soil surface is similar in both vegetation
types and‚ in the grassed area‚ the low evaporation rate was due to the substantial litter
layer present.
2.3.3
EFFECTS OF TOPOGRAPHY ON SOIL WATER
Except for those in the highest parts of the landscape‚ soils at a particular site are
usually influenced by those surrounding. Normally‚ they acquire materials and water
from sites upslope and supply them‚ often in a modified form‚ to those downslope; such
suites of related soils are known as toposequences or catenas (Chapter II.4.5). The water
that flows to sites lower in the landscape by either surface runoff or subsurface lateral
flow profoundly influences their hydrological regimes.
Figure I.29 presents seasonal changes in moisture status at three such sites located
sequentially down a hillslope in a humid (1276 mm annual average precipitation)
savanna in the Côte d'Ivoire (Lavelle‚ 1978). The highest plateau site is dominated by
perennial grasses with a few woody shrubs‚ the sloping site has a similar cover with
more woody vegetation and the next lowest site has a near-complete herbaceous cover
with some palms. The soil of this latter site is described as hydric. That is‚ a soil whose
