Page 261 - Encyclopedia Of World History Vol V

2038 berkshire encyclopedia of world history

The Ganges river near
Serampore has long
been of major economic
and religious
significance to Indians.

sticks, shovels made of wood,
and hoes made of stone to
loosen the dirt. Such tools are
old and widespread. People
use baskets to move the loos-
ened dirt. Because water
responds so quickly to gravity,
people can easily test whether
a ditch has the right slope and
change the slope if they have
made errors.
Most irrigation systems are
what are called “run of the river,” meaning that the water Early irrigation systems built with simple tools were
available to them is what is in the river (the source) at the probably widely distributed.Their existence is difficult to
time.The construction problems are obvious (water must document because all subsequent irrigation systems in
flow downhill, dirt must be moved) and the solutions the same place used the same routes for the ditches.Thus,
have been invented many times. A significant problem researchers have difficulty finding and dating the earliest
with irrigation systems is variation in environmental occurrence. However, scholars think they have evidence
moisture. For example, a drought can reduce a river’s of irrigation early in all of the world’s civilizations. Irri-
water supply to a trickle, posing a threat to crops. gation may have existed during Neolithic times as well.
One solution to such variation is to store water in a The tools to build such systems were already available, as
reservoir. However, water storage was quite rare in early was the social organization. Only so many effective
world history.The city of Jawa in the Jordanian desert had designs of an irrigation system exist, and they have
a storage dam by about 4000 BCE. Tank irrigation sys- existed around the world during all time periods.
tems that stored water behind a small dam were wide-
spread in southern and southeastern Asia by the first Drainage
millennium BCE. Roman engineers built many small stor- Drainage systems are the reverse of irrigation systems.
age dams of masonry, but these dams may have been Irrigation systems move large amounts of water to the
meant for domestic use rather than for irrigation.An early top of the fields, then break it down into smaller and
dam in the New World was the Purron Dam in Mexico, smaller packages for distribution to fields. Drainage sys-
dated to about 800 BCE.That dam was made of earth and tems collect small amounts of water at many places high
at its maximum was 19 meters tall.The dam was in oper- up in the system, combine these small amounts into
ation for approximately one thousand years and could larger and larger channels, and collect the total at the bot-
store more than 3 million cubic meters of water. Purron tom of the field system.The problem then is where to put
was one of only two storage dams known to have existed the drainage water. If it accumulates it can flood the lower
in the highland area of Mexico. Beginning during the late fields. The drainage water so collected is eventually put
nineteenth century people have built many massive into a large body of water (a river, the ocean), and often
storage dams in most parts of the world. Machinery and people can use gravity to put it there. Gravity works well
modern materials—and a plentiful supply of money— as an energy source, and ditches again are used to chan-
have been central to these efforts. nel the water. Another technology for draining water is