Environmental Engineering Reference
In-Depth Information
paddles in a stream, by water current. Discharges varied
from 20-22 m
3
/h with a lift of 1.5 cm to 8-10 m
3
/h
with high lifts around 9 m. The need to lift the buckets
one wheel radius above the level of the receiving trough
was highly inefficient and led to a better arrangement in
the Egyptian tabl¯ya. This improved device included a
double-sided all-metal wheel scooping up water at the
outer edge and discharging at the center into a side
trough. Its best performance (when ox-driven) was about
20 m
3
/h lifting water 1.5 m.
Energetic imperatives in traditional irrigation
meant that a single laborer, working in 2-4-h spells at
rates close to 100 W, could easily power all low-lift
Archimedean screws, low-capacity water ladders, and
counterpoise lifts. Two people were normally needed to
energize high-capacity ladders and some Archimedean
screws; 1-h spells by a single person (at close to 200 W)
could cover the highest counterpoise performances. A
single small ox could take care of a tabl
¯
ya or a low-lift
s
¯
q
¯
ya, but lifts over 3 m required a pair of animals, as
did all other high-lift methods. High-volume deep-well
bucket lifts required three or four oxen (up to 1.6 kW).
The energy costs of irrigation ranged from 100-250 kJ/
m
3
of water for human-powered low lifts to as much as
4.5-6.5 MJ/m
3
for animal-powered medium and high
lifts. Cost-benefit generalizations are precluded because
of differences in crop sensitivities to water supply (inter-
specific variations and different responses to restricted
water supply, with the flowering period the most vulner-
able time).
A single specific calculation demonstrates the consider-
able energy returns of traditional irrigation. Spring wheat
yields would drop 23% with a 20% water deficit spread
over the entire growing period. Supplying this need of
about 80-100 mm brings an additional yield of 300
kg/ha of grain. When irrigated with human-powered
devices the cost, assuming 50% irrigation efficiency,
would be 200-500 MJ/ha of additional food intake or
only about 5%-10% of the yield gain. With animals, costs
(4.5-6.5 GJ/ha) would be about equal to benefits (4.5-
5 GJ/ha), but because oxen were fed solely crop- and
grain-processing residues, energy return in terms of grain
would be almost as favorable as with the human labor.
Some irrigation systems required such enormous expen-
ditures of human labor that their net energy return had
to be very low or negative. Inca canals carved out of
rocks (some main lines up to 10-20 m wide) carried
water over astonishing distances. The main arterial canal
between Parcoy and Picuy ran for 700 km to irrigate pas-
tures and fields (Murra 1980).
Together with adequate water supply, nutrients are the
critical inputs opening the photosynthetic work gates,
and none is more important than nitrogen. Shortages of
nitrogen have been encountered in all traditional agricul-
tures. For example, a harvest of 1 t/ha of wheat would
remove, in grain and straw, about 20 kg N, 4.5 kg P,
4 kg K, 2.5 kg sulfur, and 1 kg each of calcium and mag-
nesium (Laloux, Falisse, and Poelaert 1980). Traditional
farming resorted to three basic strategies to replenish ni-
trogen: returning a part of the phytomass to the soil by
plowing in crop residues; recycling animal and human
wastes and other organic materials; and planting green
manures, mostly leguminous crops, to be incorporated
into the soil to provide nitrogen for the subsequent grain
crop. Of considerable antiquity, these practices also con-
fer other important agroecosystemic benefits like improv-
ing the soil's moisture-holding capacity and tilth (Smil
1983).
Crop residues, above all cereal straws and stalks, are a
large reservoir of recyclable nitrogen. Traditional culti-
