Environmental Engineering Reference
In-Depth Information
cropping moves beyond the reach of seasonal floods that
saturate alluvial soils and allow for the maturation of one
crop, or as soon as the growing population requires the
planting of a second alluvial crop during the low-water
season. Such obligatory irrigation marked the gradual in-
tensification of farming, first among the Sumerians, later
in Egypt. A second type of traditional irrigation evolved
in response to seasonal water deficits, especially in the
northern parts of monsoonal Asia (Punjab, North China
Plain). And, of course, rice cultivation required its own
ingenious arrangements for flooding and draining the
fields and for lifting irrigation water.
Gravity-fed irrigation is energetically most desirable,
but in river valleys with minimal stream gradients and on
cultivated plains, it is necessary to lift large volumes of
surface or underground water, be it only 50 cm into the
bunded fields or several meters from the steeply banked
streams or wells. Even if only 50% of total need were sup-
plied by lift irrigation, it would be necessary to raise at
least 3,000 m
3
/ha for a typical grain crop. Irrigation effi-
ciencies (at best 50%, more likely 35%) at least doubled
or tripled the theoretical need. Lifting 6,000 m
3
just
1 m needs roughly 30 MJ or (with 20% labor efficiency)
some 150 MJ. A steadily working laborer (at 60 W)
would need almost 700 h to accomplish the task. This is
an extraordinary burden. A single laborer can hoe 1 ha of
wheat field in 12-20 days of steady work, cut with a
cradled scythe in 8 h, but supplying half of the field's
evapotranspiration would take nearly three months of
8-h days.
Not surprisingly, traditional agricultures tried to do
with as little irrigation as possible, or they employed a va-
riety of ingenious mechanical devices (Ewbank 1870;
Molenaar 1956; Forbes 1965; Needham 1965; Oleson
1984; Fraenkel 1986). The simplest ones were tightly
woven or lined shovel-like scoops, baskets or buckets
slung on ropes and handled by two people facing each
other, dipping the device, and swinging it over a ridge
never higher than 1 m. Worked by two pairs of laborers
alternating in 2-h spells, the best performance with a typ-
ical 60-cm lift was about 5 m
3
/h. A scoop or bucket sus-
pended by a rope from a tripod was more effective, lifting
about 8 m
3
/h to a height of 1 m. But the oldest
and simplest water-lifting mechanism that achieved
widespread diffusion was the counterpoise lift (swape or
well-sweep), perhaps best known as the Arabic sh
¯
d
¯
f.
Recognizable first on a Babylonian cylinder seal of 2000
B
.
C
.
E
. and widely used in ancient Egypt, it reached China
by about 500
B
.
C
.
E
. and eventually spread all over the
Old World.
This simple machine was easily made and repaired, just
a long wooden pole pivoted as a lever from a crossbar or
a pole with a bucket dipper suspended from its longer
arm and counterpoised by a large stone or a ball of dry
mud to balance the weight of the full bucket. Its effective
lift was 1-3 m, but serial deployment of the devices in
two to four successive levels was common. With two
workers spelling each other in 2-h shifts, hourly perfor-
mance was 6 m
3
with a 2-m lift; in Egypt a single worker
usually lifted 3 m
3
/h a distance of 2.5 m. Whereas the
sh¯d¯f required downward pulling on a rope to lift the
counterweight, the Archimedean screw (Arabic tanb¯r,
Roman cochlea) needed tiresome cranking to rotate a
wooden double helix inside a cylinder (150-250 cm
long, 40-55 cm in diameter). Only low lifts were possi-
ble, and maximum capacities were about 30 m
3
/h with
a 25-cm rise when powered by two men, or 15 m
3
/h
with 75-cm lift.
Hand- or foot-operated paddle wheels were very inef-
ficient until the lower parts of paddles were enclosed in a
