Science
Ph.D.
thesis "Rainfed orchards in semi-arid environments: retaining the water
and the soil"
Click here for the
cover of the thesis:
Abstract:
The spatial distribution and properties of rainfed orchards in
semi-arid environments
result from complex interactions between man and the physical and
economical
environment. This thesis investigates a number of these interactions in
the context
of the mechanisation of management practices since the 1950's. It is
shown how
the practice of clean sweeping (i.e. frequent shallow tillage)
influences the orchard
water balance and how the removal of traditional soil and water
conservation
structures affects the connectivity of overland flow to the river
system. Although
clean sweeping prevents transpiration and competition by weeds, it also
constrains
the root growth in the plough layer, so that the trees cannot access
the water from
small rain events. In addition, clean sweeping promotes accelerated
soil erosion. It
appears that the practice of clean sweeping limits the water
availability in orchards
on loamy soils with an annual rainfall in the order of 300 mm. It is
demonstrated
that the presence and properties of rainfed orchards are related to
spatial patterns
of soil characteristics and climate. The observed decline in
conservation structures
like terraces and check-dams leads to an increase in the connectivity
of water and
sediment to the river system. An alternative for these traditional
techniques to retain
the water and the soil is the application of cover crops. The advantage
of cover crops
is that they do not limit the field size. A drawback in dry areas is
the competition for
water and nutrients between the cover crop and the trees. Field
evidence and water
balance simulations suggest that cover crops are feasible in areas with
an annual
precipitation of 500 mm or more.
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