Site Selection, Survey and Treatment Plan - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 67

A contour bund is a water-conservation-cum-water-spreading device used in areas
with gentle slope and deep soil. These are the standard structures for water conserva-
tion in arable lands and have limited potential in case of lands used for afforestation
which are often sloping and have poor soil depth. Bunds are made in standard sec-
tions and are spaced wider—say, from 50 to 100 m. Often masonry structures such
as weirs must be constructed at suitable points for safe passage of excess water (sur-
plussing arrangement). The average density of bunds will vary depending upon the
soil type, the ground slope, and the rate of precipitation, but in arid and semiarid
areas a bunding density of 240 m/ha may be considered adequate.
Bench terracing involves cut-and-fill type of excavation to build up the hill slope
into a series of terraces. This costly method is suitable only for production forestry
where the economic interests are stronger—namely, horti-silvicultural plantations,
plantations for oilseeds, medicinal products, and bamboo.
Having decided upon the type and the specifications of the contour-aligned S&WC
structures, the length of such structures should be calculated using the site plan,
the slope data, and other relevant data. As an average guide, it will be seen that
500-1000 m of contour trench or V-ditch, or both put together, would be required,
per hectare, to effectively arrest runoff, assuming a medium slope and moderate
rainfall—slope ranging from 2 to 10 % and rainfall of 500 mm per annum with no
1-h storm exceeding 25 mm—if sectional area is 0.15 m 2 . The total estimated length
of each type of structure should be recorded in the appropriate place in the treatment
plan, and marked schematically on the treatment map.
Most S&WC structures should be turned into vegetative barriers by sowing seeds
of suitable species in the worked soil. Seed sowing not only makes these structures
long-lasting, but also increases productivity of the plantation. Many times plants
raised by direct sowing in field can outperform nursery raised plants. Seeds of grasses,
shrubs, and trees can be sown on the bunds of the contour S&WC structures. Sowing
is often done in two lines, and spacing between two successive dibbles in a line is
kept from 30 to 50 cm. A treatment plan should include all details about the sowing
plan, including the time of sowing which usually is a couple of weeks before the
estimated time of the first rain showers of the season.
Apart from linear S&WC structures on the contour, some point structures may be
required for effective soil and water conservation plan. Check dams and stopdams
are examples of such structures. Use of these structures is complementary to that of
the linear S&WC structures, so that if the latter is appropriately designed, the need
for the former will be so much less.
In a hilly tract with numerous streams and rivulets, soil erosion caused by flow
of water may be quite severe. Gully formation and ravines may result from such
erosion. Flow of water in the streams that originate further upstream—with larger
catchment lying outside the plantation site—should be slowed down using check
dams of dry stone masonry at different points along the length of the streams. Check
dams should ideally be wedge-shaped structures, so positioned that the entire stream
bed is converted into terraces.
Sometimes it is advantageous to have a water body within the plantation site.
Apart from being a source of water for a short while, water from the pond will