Irrigation

Rain-fed agriculture, the converse of irrigated agriculture, has traditionally meant cultivation of plants without adding additional water beyond that which falls naturally on them from the sky or flows to them as surface water. Obviously, there is a spectrum of human intervention between minor flood control management and massive irrigation works, as well as a spectrum in scale between typical surface flows and the traditional floods of major river systems. A third spectrum relevant to understanding the role of water in agriculture involves the energy needed for supplying the water to the fields, which ranges from low in some gravity-fed systems to high in systems requiring additional energy inputs for raising water to the level of fields well above the water table.

Source: © iStockphoto/David Freund.

It is possible that the first crops cultivated by irrigation were small flood control systems on alluvial fans in arid regions, such as Jericho, circa 9000 BC, but it is just as likely that small flood control management works were built along river systems having a natural flood, and it is unlikely we will ever know when or where the earliest such systems were built, since alluvial soils in flood plains are rearranged annually and the likelihood of the earliest such works leaving a trace down to the present are slim. Significant irrigation works in the Middle East date at least to 5000 BC in the Nile, and an early major irrigation project was completed there around 3100 BC during the 1st dynasty.

If we leave unsolved the question of who was first to irrigate, we can concentrate on the broader ramifications of irrigation technology. Modern measuring systems tempt researchers to believe that irrigation is required where less than a particular amount (e.g., 250 mm) of rainfall occurs. Premodern measuring systems were generally more sophisticated and allowed for temperature, orientation to the sun, soil characteristics, temporal distribution of rainfall, and evapo-transpiration by evaluating rain-fed fields not in terms of area and precipitation, but in more direct terms of the optimal amount of seed they would support. Even small gravity-fed watershed-harnessing systems can concentrate rainfall from a larger area onto a smaller one, and nature does a fair job of this in many circumstances. Thus, precipitation requirements for annuals like grains (trees can survive on the water table if it is high enough) make sense only as [Page 1333]guidelines, and the ethnographic literature is full of the expected exceptions (e.g., the Hunza, in Northern Pakistan, who require only 180 mm for rain-fed agriculture).

Karl Wittfogel developed the idea that irrigation works were both the products of centralized states and, via the control possible by monopolizing the water supply, the basis for the development of a particular (oriental) version of state despotism. This theory was taken up by many scholars but is now not seen as fitting well many of the very areas it was thought to explain best, such as Bali, where a strong case has been made that vast irrigation systems were constructed and regulated without any significant role for the state. If it does not have the broad relevance claimed for it by Wittfogel, some scholars, such as Sidky, claim it has explanatory value for 18th-century irrigation among the Hunza. Stanish suggests that part of the argument may be valid if reworked in terms of linking political complexity to labor intensification and declining returns to labor in areas largely under local control.

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