Soil and Water Conservation Works - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 95

Bunds impound water above ground level, contrary to the other linear S&WC
structures which store water below ground level. In order to retain surface water, it
is necessary to have bunds in a closed circuit with some arrangement for surplussing
the excess water. Such surplussing structures may be built in block stone or dry
stone masonry. A clearfall stone weir is often the preferred structure, though a pipe
outlet may be simpler. While constructing such engineering structures, the same
considerations may be kept in view that are relevant in construction of stop dams
(see Sect. 5.10).
Contour furrows are constructed using tractor drawn discs, followed by manual
dressing to form adequate section and manual compaction of the ridge to make it
stable. The furrows are designed to run along contour lines at a spacing of 1-2 m.
Sowing of grass seeds on ridges, with CVH species on every fifth or tenth furrow,
should be done.

5.8 Vegetative Barriers

Establishing vegetative barriers on contour-based linear structures such as contour
trench, contour V-ditch, and contour dike-cum-terrace improves effectiveness of
the afforestation work in general and of the S&WC measures in particular. Correct
choice of species, proper design of the barrier, proper timing and technique of sowing
and planting, and meticulous maintenance operations, are some of the essential
ingredients for successful establishment of vegetative barriers.
The commonest and easiest method of raising vegetative barriers is by sowing of
seeds. Seeds with hard coats should be pretreated in appropriate manner before being
sown. Sowing should be carried out by dibbling seeds in lines at a spacing of 0.30
m in case of shrubs, and 0.15 m in case of grasses. Grass seeds should be dibbled in
groups, that is, 4-5 seeds should be held in a pinch and sown at a single dibble. In
case of a contour trench or a V-ditch, the sowing work should be done in two to three
rows on the bund. One row should be positioned on the upper side of the bund and
the other one on its lower side. If a third row is chosen, it can be positioned on top of
the bund. In case of a dike-cum-terrace, seeds should be sown in two rows, one at a
distance of 0.15 m from the dike and the other at a distance of 0.50 m from the first
row. If ground slope is too steep and the width of the terrace is less than a metre, then
only one row should be sown. In case of contour bunds, seeds of a suitable perennial
grass—such as Cenchrus ciliaris or Dicanthium annulatum —should be sown on the
side slopes and seeds of shrubs and small trees should be sown on top.
Timing of seed sowing is critically important. If sown too early, most seeds will
be eaten by rodents, blown away by wind, or destroyed by termites. If sown too
late, these may miss the first showers of rain, and thus lose valuable growth time.
Ideally, sowing should be complete by the week before the estimated arrival of the
rainy season.
Many times, the seeds sown will fail to germinate. This may be a result of poor
rainfall, or bad seeds, or destruction of seeds by pests. Occasionally, seeds may
germinate in the first rains but will dry up subsequently because of a long dry
spell. Therefore, resowing will have to be considered in almost all cases. Resowing