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14-05-2015, 10:45

Silting and Salinization

The irrigation techniques developed by Mesopotamians were magnificent, allowing them to increase agriculture production, maximizing the land cultivated. However, overexploitation of land and water resources for agriculture affected the environment. Canal branches and flood protection structures built to reduce or avoid flood

Silting and Salinization

Fig. 2.17 Schematic layout of an agricultural complex in middle Euphrates. (Adapted from Buccellati, 1990, p. 163, Fig. 1)

Surges intensified silt deposition. Also, over-irrigation brought salts into the surface soil, becoming infertile. The above problems affected settlement patterns, land-use and sociopolitical control (Walters 1970; Jacobsen and Adams, 1958) and they are reflected in the literature, as mentioned in Tablet II of Atra-Hasis epic: ‘.. .the black fields became white, / the broad plain was choked with salt...’ (Lambert and Millard, 1999).

Given the topographic and climatic features of Mesopotamia, part of the salt is absorbed by colloidal clay particles and part is washed down into the water table, which has a very limited capability of moving them away, facilitating salt accumulation in the water table. According to Jacobsen and Adams (1958), “ancient control of the water table was based only on avoidance of overirrigation and on the practice of weed-fallow in alternate years”.

Existing records establish that three major events associated with salinization have taken place in antiquity in a time span of almost four millennia, from 2400 B. C. to 1200 A. D. (Jacobsen and Adams, 1958). The gravest was the first, which happened in southern Mesopotamia between 2400 and 1700 B. C., and originated from conflict related to the use and control of water in irrigation channels fed by the Euphrates between Umma and Girsu. Both cities had been involved in a long dispute over a fertile border district watered by the Euphrates. Umma, located upstream controlled the water supply to the Lagash region (whose capital was Girsu). At some point of the conflict, one of the Girsu’s rulers decided to build a large canal to bring water from the Tigris in order to be independent of the limited water delivered from the Euphrates. Although the canal could supply enough water to the region without interference from Umma, it increased seepage and flooding in addition to overirrigation, favouring conditions for soil salinization. Jacobsen and Adams (1958) stated that ‘by 1700 B. C. this canal had become large and important enough to be called simply “the Tigris”, and it was supplying a large region west of Girsu that formerly had been watered only by the Euphrates’.

The resulting salinization due to such a large canalization projects, including dams and other related facilities are present until nowadays, covering the complete region.

To handle soil salinization people replaced wheat by more salt tolerant crops. Thus, Jacobsen and Adams (1958) indicate that about 3500 B. C., the same proportion of wheat and barley were cultivated in southern Mesopotamia. Because barley is more salt tolerant, one thousand years later, it accounted for 83% of the crop. About 2100 B. C., barley accounted for more than 98%, and by 1799 B. C., the cultivation of wheat ‘had been abandoned completely in the southern part of the alluvium’. The same decline of barley yield took place. At about 2400 B. C., the average yield was 2537 lt/ha. In 2100 B. C., it had diminished to 1460, and by about 1700 B. C., ‘had shrunk to an average of only 897 lt/ha’.