422
Section 4 Technological Contexts
The source of water is usually a
lake, a river, or an underground source
(aquifer). A dam at its outlet often controls
water in a lake. This kind of barrier backs
up river water to form a reservoir. See
Figure 19-15. A well and pump obtain
underground water. A series of canals and
pipes moves the water from the source to
farm fields. One of three basic methods is
used to apply the water to the land.
Flood irrigation is used on level fields
where there is a lot of water available. A
sheet of water advances from a ditch across
the field. Lateral ditches and pipes with
holes along their lengths supply the water
to one side of the field. Gravity causes the
water to flow across the field. Other ditches
or pipes might carry off excess water. In
row crops, furrows (small ditches) between
rows of plants are used to move the water
from one end of the field to the other. See
Figure 19-16. Pipes or tubes are used to
control the water entering each furrow.
Sprinkler irrigation is used to better
control the amount of water used and the
area watered. These systems involve a
water source, a pump, main (distribution)
lines, lateral (sprinkler) lines, valves, and
sprinkler heads. The pump forces water
into the main distribution lines. The water
flows through them to lateral pipes, which
have sprinkler heads attached at set inter-
vals. Valves between the main and lateral
lines can shut off or control the water flow.
Figure 19-15. This reservoir behind Grand
Coulee Dam provides water for power generation
and irrigation.
Figure 19-16. These strawberry plants are being flood irrigated. (U.S. Department of Agriculture)