Nursery Techniques - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 17

irrigating 100,000 plants (considering peak demand in summer, assuming sprinkler
method of irrigation). It will be better to have at least 20 % extra water for contingency
and to account for losses involved in the irrigation method.
A tube well (bore well) is an ideal source of water (Fig. 2.2 ) for a nursery. Water
from a tube well can be directly pumped to an OHR to drive sprinklers or to feed
a network of pipes fitted with hydrants (Fig. 2.6 ). The cost of a typical tube well
for a nursery having a capacity of producing 300,000 plants per year could be 5,000
mdeq. Of course, this can vary greatly, depending upon the type of strata found in
drilling and other factors.
An open well (dug well)—or a dug-cum-bore (DBC) well—can be a better al-
ternative to a tube well in many cases. When a tube well is not feasible or is too
expensive, this is the second best choice. Such a well should be fitted with an electric
motor coupled with a pump (Fig. 2.3 ) or a diesel pumping set of sufficient discharge
and head. Many types of pumps are available to choose from, although monoblock
pumps of centrifugal type and multistage submersible pumps are the best alternatives.
Wherever surface water is available, it should be used without exploring alternative
sources of water. Although it is less likely to be feasible in arid areas, yet many times
a canal or a tank with perennial storage is available near nursery site. In such a
case, only a pumpset of mild power is required, and the quality of water is assured.
The need for pumping may be obviated altogether if water can be drawn by siphon
or gravity. It should be assured, however, that water supply will not be interrupted
during any part of the year. A closure in the canal or a sudden dry-up in the tank
due to insufficient rain may bring all the efforts to a nought or else the expensive
alternative of transporting water from distance should be kept at hand.
Choice between a GLR and an OHR will be based on the twin criteria of the
cost and the necessity of using a sprinkler system. Large nurseries will have beds
spread over too great a distance to be irrigated through a single GLR. In the case of
a sloping terrain and locating the GLR at the point of highest elevation, it will be
feasible to irrigate plants by gravity flow in pipes. In other cases, a number of GLRs
should be located in different parts of the nursery, each located higher than the beds
it irrigates. If the cost of labour required for manual irrigation is high, installation
of a pressure-driven sprinkler system driven by an OHR of sufficient head will be a
better alternative.
A GLR should be located at the point of greatest elevation within the nursery
premises in order to have good pressure of water at all points. Its construction will
depend upon local conditions, but the one of masonry (Fig. 2.4 ) will be simpler
and cheaper, unless a very large capacity is required. A reinforced cement concrete
(RCC) structure should be opted when the masonry alternative is nearly as expensive
or where a storage capacity of more than 100 m 3 is required. Normally, a GLR should
be of sufficient capacity to serve as a backup source of water even if the tube well is
out of working order for a week. If the mean time between failures of the tube well
is low and the mean time to repair is high, the GLR should be designed for an even
larger capacity.