Site Selection, Survey and Treatment Plan - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 66

hectare of land, ultimately the final population that survives tends towards a charac-
teristic natural limit, perhaps caused by competition for moisture and space in the
rhizosphere, and other such factors peculiar to that ecosystem.
Pits should be so aligned that two neighbouring pits fall on a contour line, an
arrangement that is essential for equal availability of rain water to plants. In most
cases, pits are used in combination with other conservation measures—most of them
linear and aligned along the land contours. In such a combination method, the pits
are aligned between two contour trenches or contour V-ditches, as the case may be,
in a staggered fashion.
Sometimes pits may be altogether dispensed with. In case of bare-root plants of
fuelwood species, for example, crowbar holes can be made for planting. This saves a
substantial amount of cost, and the results may not be much different from those of pit
planting in areas of poor soils. A common species for bare-root planting in arid areas is
Prosopis juliflora which is extremely hardy and drought resistant—once established
it will provide protection to the watershed and ensure lasting fuelwood supplies.
Harvesting of rainwater and reduction of runoff is of critical importance in arid
afforestation. Various measures for in situ moisture harvesting and conservation
include contour trenches, V-ditches, gradoni, bench terraces, and contour furrows.
Contour trenches are preferred when the slope in the area is more, whereas V-ditches
are suitable when the slope is less. In a given site, different slopes could exist in
different parts and the choice of soil working method will be governed accordingly.
In areas where soil is nearly absent or is of very little depth, it will not be possible to
dig trenches. In such areas other measures like gradoni, contour furrows, or contour
dikes may be opted for.
Continuous contour trench is the most effective technique of S&WC. Alignment
of the trench should be done with the help of a precise levelling instrument (e.g. HSL,
see Sect. 3.6). The spacing between two contours, measured along the slope of the
ground, should be 7-10 m on the upper slopes with higher rate of runoff, and 15-20 m
in the lower slopes. For more accurate spacing, the section and the spacing should
be so matched that for a given storm of precipitation all runoff is intercepted in the
trenches.
A contour V-ditch is the same as a contour trench, except that the section is spread
out in V-shape (rather like √ mark) so that it can be used in soils of less depth and
gentle slope—generally in a terrain with a slope less than 1 %. Spacing of contour
lines for the V-ditch can vary from 30 to 50 m depending upon slope of the ground,
texture of the soil, and the rate of runoff and the rate of precipitation in a given storm.
In a hilly terrain where soil is absent or skeletal, the contour dike-cum-terrace
is preferred. The dike is a wall of dry stone masonry having a suitable section, and
preferably topped with mortar or mud coping to prevent it from washing away. Soil
from nearby area is collected and stacked on the upper side of the dike, thus making
it into a terrace. This terrace acts as a barrier to movement of water and soil. Since
more soil is likely to be collected on it, the level of soil filled in terrace should be
lower than the top of the dike. The spacing of contour lines for such dikes can be
from7to10m.