Environmental Engineering Reference
In-Depth Information
MJ/kg, 5 t for a tractor and its implements, 15%
markup for maintenance, and average 12 years of
service).
Traditional agriculture provided the needed water by
simple open-ditch irrigation fed by gravity flows or by a
variety of human- or animal-powered devices (see sec-
tion 6.3). Modernizing agricultures retain the inefficient
ridge-and-furrow arrangements and supply them with
simple mechanical pumps. Energy-intensive solid-set,
big-gun, and center-pivot sprinklers—patented by Frank
Zybach in 1952 and distributing water through a series
of impact sprinklers by a row of mobile towers—now
dominate in the United States, and trickle irrigation of
high-value crops is even more efficient (Keller 2000).
The total volume of water to be delivered depends on
net irrigation needs (evapotranspiration plus leaching mi-
nus soil water stores plus precipitation) and on applica-
tion efficiencies. These may be as high as 95%; good
field practices should average 65%-75%, and furrow irri-
gation may be only 30%-40% efficient.
Consequently, an old diesel-driven unit (50% pump,
25% motor efficiency) in a Chinese wheat field (35% ap-
plication efficiency) may consume 4.5 times more fuel
than a well-run diesel-powered sprinkler in Nebraska
(75% pump, 33% motor, 80% irrigation efficiency) while
delivering the same effective volume of water from the
same depth. With well depths of 30 m and water require-
ments of 30 cm/ha, electricity-powered irrigation may
need about 1.5 GJ/ha, or about 4.3 GJ/ha of primary
energy for thermal electricity. A natural gas-fueled en-
gine performing the same task will use 6.3 GJ/ha, but
with a 60-m-deep well the same engine would need
12.6 GJ/ha. These needs may surpass the total of all
other energy subsidies. The global dependence on irriga-
tion has trebled since the end of WW II, when about 75
million ha of cropland were watered. A generation later
the total was 140 million ha, and by 2000 the figure
topped 275 million ha, with three-fifths in Asia and
nearly one-fifth in China alone (FAO 2006). With half
the land mechanically irrigated at an average cost of 2
GJ/ha, global operating costs at the turn of the twenti-
eth century were about 275 PJ, of which 50 PJ may go
each year into installing and replacing the necessary
pumps, motors, and pipes.
Chemical fertilizers represent the largest indirect
energy subsidy in nonirrigated farming. No other innova-
tion has contributed so much to increased yields as
the three macronutrients N, P, and K, available in com-
mercially produced inorganic compounds (Smil 2000b).
Chilean nitrates (mostly NaNO
3
), discovered in 1809,
and rapidly depleted deposits of Pacific guano were the
only source of inorganic N used to supplement legumes
and organic recycling. Liebig's suggestion of treating
bones with diluted H
2
SO
4
increased the availability of
P bound in hydroxyapatite, but the process was limited
by the availability of animal skeletons. European potash
deposits were abundant but undeveloped. By the second
decade of the twentieth century all these limits had been
removed.
The treatment of phosphate rocks by diluted H
2
SO
4
,
pioneered by Sir John Bennett Lawes and producing
ordinary superphosphate, started to diffuse by the
1870s, and rich phosphate deposits were discovered in
Florida in 1888 and Morocco in 1913. Potash mining
was expanded both in Europe and North America, and
Haber-Bosch synthesis of ammonia removed the most
important nutritional limit. WW I, the global economic
slowdown of the 1930s, and WW II postponed the onset
of large-scale fertilization until the early 1950s. Then the
rapid diffusion of hybrid corn (begun in the United
