Agriculture Reference
In-Depth Information
Fig. 12.8
Rock catchment dam
12.3.2.2 Rock Surface
Rock surfaces can also be used as collection
catchments. Bedrock surfaces found within rocky
top slopes or exposed rock outcrops in lowlands
often have natural hollows or valleys which can
be turned into water reservoirs by building a dam
(Fig.
12.8
). Developing a rock catchment area
typically involves clearing and cleaning the site
from vegetation and marking out the catchment
area to be enclosed with gutters. Rock surfaces
should not be fractured or cracked, as this may
cause the water to leak away to deeper zones or
underneath the dam. As with ground catchments,
water is generally of lower quality than direct
rainfall collection. Water quality can be improved
if access to the area (e.g., by animals and chil-
dren) is limited.
Several types of conveyance systems exist for
transporting water from the catchment to the stor-
age device, including gutters, pipes, glides, and
surface drains or channels. Larger-scale convey-
ance systems may require pumps to transfer
water over larger distances. These should be con-
structed from chemically inert materials, such as
wood, bamboo, plastic, stainless steel, aluminum,
or fi berglass, in order to avoid negatively affect-
ing on water quality (UNEP
1997
). In the case of
rock catchments, gutters can be constructed from
a stone wall built with rough stones/hardcore and
joined with mortar.
Storage devices are used to store the water that
is collected from the catchment areas and are
classifi ed as (1) aboveground storage tanks and
(2) cisterns or underground storage vessels.
These facilities can vary in size from one cubic
meter to up to hundreds of cubic meters for large
projects. For storing larger quantities of water,
the system will usually require a bigger tank or
cistern with suffi cient strength and durability.
Typically these tanks can be constructed out of
bricks coated with cement. For water captured
from a rock catchment, a dam is the more com-
mon form of storage device.
Climate change is disrupting global rainfall
patterns meaning some parts of the world are suf-
fering from a drastic drop in precipitation leading
to a fall in water levels in many reservoirs and riv-
ers. In sub-Saharan Africa where two-thirds of the
region is desert and dryland, the need for improv-
ing water management in the agriculture sector is
particularly critical. Rainwater harvesting repre-
sents an adaptation strategy for people living with
high rainfall variability, both for domestic supply
and to enhance crop, livestock, and other forms of
agriculture (UNEP and SEI
2009
).
Generally, the amount of water made available
through rainwater harvesting is limited and
should be used prudently to alleviate water stress
during critical stages of crop growth.
Supplemental irrigation is a key strategy and can
help increase yields by more than 100 %. A small
investment providing between 50 and 200 mm of
extra water per hectare per season for supplemen-
tal irrigation, in combination with improved
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