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9-03-2015, 05:42

Irrigation and Soil Salinization

Soil salinization involves the accumulation of salts in the soil, which impedes the development of crops and most plants. Soil salinization is a problem particular, but not exclusive, to arid and semi-arid lands. Although a natural process, soil salinization occurs through human intervention as a result of poor planning in the management of irrigated lands. The problem starts when excess irrigation water produces waterlogging, which under conditions of high evaporation rates results in precipitation of salts near the surface of the soil (Artzy and Hillel 1988). For these reasons, the problem of salinization is viewed here in the context of the main irrigation systems in the Near East.

The worst cases of soil salinization in the Ancient Near East occurred in large-scale irrigation systems, especially in the Mesopotamian lowlands, where irrigation was difficult and challenging. One of the major problems was the late spring and early summer floods produced by melting snow and rain in the mountains. Besides creating destruction of fields, these floods brought excess water at the time when it was not needed. On the other hand, the flow of water was relatively low in the late fall and early winter when water was desperately needed. Another problem was that fields lay lower in relation to the main river channels. This situation produced overflowing of water into the fields and made draining the excess water difficult. Consequently, evaporation of the stagnated waters prompted the accumulation of salts in the upper soil horizons. Today, modern technology partially solves the problem through a system of deep drainage to lower and hold down the water table and with the use of chemical amendments to restore soil texture (Artzy and Hillel 1988). However, this technology did not exist in ancient times.

In ancient times, the major breakthrough in partially solving the problem of salinization in Mesopotamia occurred during the Sassanian Period (226-637 ce) when irrigation along the flood plains of the Diyala River, a tributary of the Tigris, was devised (Adams 1981). This system included the Naharawan canal, which was 3,000 km or 1,860 miles long and 50 m or 54 yards wide, and designed to drain the excess water into the Tigris.

In the Nile Basin the situation was far different, for the timing of the floods coincided with the growing of crops. In addition, the fields lay far above the main channel. The adaptation of agriculture to this scheme was much simpler. Floods occurred yearly before the planting season. The flood plain was divided into different basins, which were filled with flood waters, bringing to the fields the nutrients necessary for each agricultural cycle. Once the waters receded to the main channel, the excess water in the fields was drained and the water table lowered. The main problem in the ancient Nile Valley was when floods failed or when they occurred offseason (Butzer 1976). The only areas of Egypt with major salinization problems were the Fayum Depression and the Delta. In the former, the problem lay in the fact that the flow of water diverted from the Nile through the Fayum Canal ended in a closed basin, where excess water had no way to escape (Hamdan 1961). In the lower part of the Delta the water table was often high, especially in areas where the underground was contaminated with salty water from the sea (Hamdan 1961; Stanley and Warne 1993).

Several strategies to cope with the problem of soil salinization have been implemented by farmers on a local scale. One of the common practices in Mesopotamia was to plant the deep-rooted shoq (Prosperina stephanis) and agul (Alhagi maurorum) which absorbed capillary water, thus creating deep-lying dry zones that hampered the rise of salinity (Jacobsen and Adams 1958). Among other strategies, selection of well-drained soils for irrigation proved to be useful, but only worked in small-scale irrigation systems. One of these cases is the irrigation around the Dakhla Oasis in the Western Desert of Egypt, where water was lifted using the saqiya (animal-powered water wheels) into canals and then into raised plots built on sandy deposits (Brookes 1990). The high porosity of these sandy deposits inhibited salinization by letting the excess water drain freely and minimizing waterlogging, the main factor leading to salinization. This strategy could not have been applied to the Mesopotamian lands, where silts and clays in the soils would impede draining of excess water.

The degradation of soils by salinization means that fertile lands turn into a salty desert, forcing farmers to abandon their fields. Under such circumstances, farmers abandoned their lands to become pastoral nomads; this move meant a more secure procurement of living and probably more flexibility in terms of taxation (Butzer 1995). Under this scenario the supply of food to the city, especially grain, failed, thus having an impact on the entire structure of the state. Salinization has been linked to decline of Mesopotamian civilization (Adams and Nissen 1972), although scholars argue about this issue, especially when dealing with the participation of the state in controlling irrigation systems (Wagstaff 1985).

Soil salinization was not a major problem in the small-scale irrigation systems of the Near East, because it was easier to control and in general was implemented in areas with better drainage. Overall, small-scale irrigation systems were more sustainable and ecologically better suited than the large-scale systems. There are several types of small-scale irrigation systems, of which flood irrigation is the simplest and presumably the earliest (Sherratt 1980). Operation is simple, since the main objective is to build cross-channel dams intended to redirect flood waters produced by sporadic rains and to maintain moisture in the soil. In antiquity, these systems were extensively practiced in the driest parts of the Near East, such as the Negev (Evenari, Shanan, and Tadmor 1982) and the Libyan Valleys (Gilbertson et al. 1994).

There were other small-scale systems that were technologically more complex. They included canal irrigation tapping the waters of smaller permanent streams, usually draining from the mountains into the drier lowlands, as was the case of the Al-Ghuta system at Damascus, which to this day taps waters from the Barada River.

Some of the small-scale irrigation systems were known for their technological sophistication, such as the qanat system, which consisted of gently sloping tunnels cut through river-laid material and bedrock (usually limestone) to transmit water from beneath the water table to the ground surface. Once on the surface, the water was distributed by canals. The qanat system was highly efficient since it reduced loss of water by evaporation and consequently avoided salinization (Beaumont 1971).