Geoscience Reference
In-Depth Information
Irrigated agriculture
One of the most important developments in human society was the shift from a subsistence
way of life based on hunting and gathering food from the wild to one primarily based
on food production derived from cultivated plants and domesticated animals. The links
between early agricultural management and the emergence of urban civilizations in just a
few independent centres around the world have been noted in
Chapter 3
along the alluvial
valleys of the Tigris-Euphrates, the Nile, and the Indus. These links developed from the
high levels of organization needed to manage permanent agricultural fields and systems of
irrigation. Another centre of early crop irrigation using river water was in the Zaña Valley
on the western slopes of the Peruvian Andes, where archeologists have unearthed a system
of small-scale gravity canals that were being used at least 5,400 years ago, and probably
6,500 years ago.
Irrigated agriculture is arguably just as important today as it was to those early civilizations,
and although several sources of fresh water are used to irrigate cropland - including
groundwater, lakes, direct runoff, and various forms of wastewater - rivers remain by far
the most important. The methods of storage (in reservoirs) and distribution (by canal) have
not changed fundamentally since the earliest river irrigation schemes, with the exception
of some contemporary projects' use of pumps to distribute water over greater distances.
Nevertheless, many irrigation canals still harness the force of gravity. Half the world's large
dams (defined as being 15 metres or higher) were built exclusively or primarily for irrig-
ation, and about one-third of the world's irrigated cropland relies on reservoir water. In
several countries, including such populous nations as India and China, more than 50% of
arable land is irrigated by river water supplied from dams.
The knock-on effects of withdrawing water from a river to irrigate crops can be striking.
In some cases, it may induce a complete transformation of river dimensions, pattern, and
shape. One example of such 'river metamorphosis' comes from the western Great Plains
of the USA, where rivers described by European Americans towards the end of the 19th
century as wide, shallow, braided channels with only sparse vegetation along their banks
have since been altered dramatically. The regulation of river flow for irrigated agriculture
resulted in lower seasonal peak flows, higher base flows, and a change in regional water
tables that promoted the establishment of trees along river banks. The combination of these
changes to flow regime and bank resistance resulted in the rivers becoming narrow, sinuous
channels flanked by dense forests within just a few decades.
