Irrigation network is a huge time and labor-consuming enterprise which should be maintained on an annual basis. Canals ought to be dredged against silt which can stuck water flow leading to flooding and crop collapse. However, harvests on the alluvial plain had far exceeded yields in the uplands.
Initially, Neolithic societies in the region practiced rain agriculture, and the majority of population was scattered in the hills of Northern Mesopotamia. The transition of early farmers to the arid and semi-arid zone, where sporadic winter rains are insufficient for crop cultivation, demanded a shift to an opposite principle of irrigation husbandry. This ‘principle’ was disclosed by Ubaidians which allowed them to settle in otherwise inhospitable area of an alluvial plain. These first settlers laid the basis of traditional irrigation starting to monitor water resources. That included harnessing floodwaters, storage of excessive water and delivering it to the fields in due time.
According to ‘The Sumerian King List’ – the first Sumerian historiographic composition and a very arbitrary source of information – this ‘principle’ was revealed by gods to the king of Eridu, the earliest and the largest among Ubaidian sacred neighborhoods. In another composition, ‘The Eridu Genesis’, the first cities before the Flood are glorified for dredging canals and ditches; the success of their irrigation schemes resulted in food surplus. Modern research has acknowledged that the first practical steps in irrigation were taken in the vicinity of Eridu over 7,000 years from now.
When Sumerians mention the secrets of gods, they cite an enormous span of Dreamtime before the Early Dynastic period (2,900 – 2,350 BCE). The time when the power was in the hands of high priests running urban clerical states with the help of aristocratic councils.
Sumerians also believed that the ‘secrets’ of husbandry were disclosed to the human beings by divine revelation rather than acquired through painful experience of generations of farmers. They told a very weird legend about the origins of agriculture. The first irrigation canals had been dug by minor gods who became so exhausted that complained before the divine general assembly about their miserable fate. The assembly authorized the god of wisdom to work out the plan, and the humans were created to relieve the celestial beings of their burden. Since then the man’s role had been to please the gods. Maybe the message was that the high authorities should be trusted in changing the individual’s life for the better. An example of ‘nice’ political brainwashing since in reality the state administration rarely gazed beyond its nose.
The soil in Southern Mesopotamia suffers from dryness (most part of the year) and devastating floods in spring as a result of recent rains and snowmelt in the highlands. Water control needs to overcome such obstacles as drought during the growing season and floods in the course of the harvest season; both calamities can easily ruin crops.
Due to a low difference of heights, the Twin Rivers flow unhurriedly across the alluvial plain splitting into a few branches and forming an extended delta. Throughout the whole span of the Sumerian civilization, the Twin Rivers hadn’t converged into a single waterway (called Shatt-al-Arab) like nowadays.
During the seasonal inundation, they carry fertile silt from the mountains. However, instead of settling on the fields, it builds up on the riverbed or accumulates at the banks. As a result, the rivers are raised above lowlands. Guarded by natural levees, they give out most of sediment to marshlands and swarms on the way to the Gulf. Sometimes the pressure of water is so intense that it breaks the levees and floods the plain. It could result in environmental calamity. Occasionally the entire cities were washed off by huge floods which left 1.5m-layer of clay sandwiched between earlier and later cultural horizons (which prompted the legend of the Deluge to enter the long-range human memory).
Floods serve both as a benefactor and a malefactor. Their sediment fertilizes the soil forming a potential for raising abundant crops. On the downside, when floodwater is trapped in the fields, it cannot be drained due to a flat profile of the land. It can disappear through evaporation or percolation.
Irrigation is basically redeployment of river water for cultivation. The elevation of liquid above the surrounding farmlands prompts the digging of ditches and canals but makes drainage of floodwaters impossible. Some water is absorbed by soil. However, a fast absorption raises the water table to the root zone leading to erosion of soil and water logging. The rest amount of liquid evaporates leaving white fingerprints of salt. Saline soil cannot produce expected yields and makes farmers shift to more salt-tolerated plants.
The large-scale irrigation included the use of feeder canals; initially, dried branches of the Euphrates and later artificial channels. These canals were cut in levee slopes; they allowed to expand cultivation area and promoted population resettlement. Smaller canals brought water to basins feeding the fields. These canals were equipped with sluices and weirs – low dams controlling water flow. At its peak, about 10, 000 square miles (or 2,590,000 ha) were under cultivation. All major cities were eventually linked by a united network of canals faced with burned brick and sealed up with bitumen.
The Achilles’ heal of irrigation agriculture is salinity which is a by-product of flooding. After a few seasons, the soil becomes poisonous and unproductive. To avoid this, traditional agriculture relied on an alternate-year fallowing – a system which allows a field to rest for a year after cultivation.
The fallow year had a direct impact on lowering the water table. The soil would dry up due to lack of moisture; weeds would extract remaining drops of liquid from underground; seasonal rains would wash off deposits of salt; farm animals would graze the weeds fertilizing the field with their manure. The earth would repose dreaming of the next flooding.
Sumerians, who had to deal with a tough problem of population pressure due to continued urbanization, applied sophisticated irrigation schemes that helped them achieve abundance of foodstuffs. This abundance accounts for flourishing of the earliest urban complex society. As a result, the community could sustain a fast-growing number of non-farmers: View full article »