Irrigation methods on the Nile

from Wikipedia, the free encyclopedia

The irrigation methods on the Nile have their origins around 5000 years ago, when the Egyptians began not only to cultivate the areas flooded by the Nile , but to systematically use the annual floods to irrigate specially created fields. Their methods remained largely unchanged and outlived conquests of the country by Persians, Greeks, Romans, Arabs and Turks as well as epidemics and famine. Only the rapid increase in population that began in the 19th century and the pursuit of additional sources of income through the cultivation of previously unknown plants on the Nile required more intensive use of the soil and thus other methods as well as the control of the river through weirs and large sewer systems.

Seasonal irrigation in flood basins

The basis of Egyptian agriculture was the annually recurring and predictable flooding of the river plains and the Nile Delta, which has its origin in the heavy rainfalls in the Ethiopian highlands with its more than 4000 m high mountains. These amounts of water largely flow into the Blue Nile and the Atbara and thus into the Nile. At Aswan in southern Egypt, they let the Nile level rise from June until it peaked in August and dropped again in September. The tidal wave, moving slowly downstream, reached the Nile Delta about two weeks later. In Cairo it began in early July, peaked in the 14 days from late September to early October, and then fell again. The lowest water level was reached in May.

The Egyptians divided the agricultural areas into flood basins that could be 500 to 48,000 feddans in size (2 to 200 km²). They were surrounded by dams and equipped with inflow and outflow channels. The basins were flooded once a year at the time of the highest water level of the Nile flood and then closed for about six weeks so that the mud could settle and the soil could be moistened. The remaining water was then drained into neighboring, lower lying basins and into the Nile, which was already falling again. The seeds were sown immediately after the basins had been drained, usually in November or early December. It only took three to four months to harvest; in the subsequent drought, agriculture was hardly possible. This meant that only plants could be grown that fit into this watering and time schedule.

More distant and slightly higher areas could also be flooded via channels with a lower gradient than the Nile. However, the system was sensitive to the fluctuations of the individual floods. If the threshold height was too low, higher pools were not filled sufficiently or not at all, which led to failed harvests and thus to famine; too high a threshold destroyed dams and houses.

With this damming method , the soil was not excessively stressed, and fertility was maintained through the annual input of mud. It was therefore not necessary to leave land fallow . Salinization did not occur; in summer the water table was well below the surface, and any accumulated salts were washed away during the flood.

With the after 1500 BC Shaduff , a kind of draw well, taken over from Mesopotamia in the 3rd century BC , and the sakia , a bucket-wheel driven by oxen, introduced in the Hellenistic period , managed to irrigate fields with water from the groundwater, from the river or from canals even during the drought to create additional cultivation areas. Occasionally the noria , a bucket wheel driven by the water current , probably taken over from Syria, was used. With these devices, however, no large amounts of water could be pumped and therefore no large areas could be cultivated.

With the seasonal basin irrigation, 2 to a maximum of 12 million inhabitants could be fed in ancient Egypt. At the time when Egypt was a Roman province and contributed to the supply of the empire with grain, 10,000 km² of agricultural land should have been cultivated. After late antiquity, the methods and facilities slowly deteriorated, so that the population also declined. By 1800 the country only had about 2.5 million inhabitants.

Year-round canal irrigation

Muhammad Ali Pascha (1805–1848 Viceroy of Egypt) introduced various reforms and endeavored to expand the cultivable area and to generate additional income from the cultivation of cotton . For this purpose he had the Delta Barrages built over the Rosetta and Damietta arms, which, after his death and after initial difficulties, dammed the Nile so far at low water that a new canal system in the Nile Delta could irrigate the fields all year round. For this purpose, main and branch channels distribute the water in summer channels, which only carry water in summer, and in alluvial channels, which are only opened during high tide. Towards the end of the 19th century there were 7,200 km of summer and 4,000 km of alluvial canals in the delta.

With year-round irrigation, two, sometimes even three harvests were possible for the first time. In addition, cotton could be grown for the first time, a plant that does not tolerate drought, but also no moisture in the soil and has a longer growing season.

Scheme of the main irrigation channels

Ismail Pasha (1863–1879 viceroy) had the Ibrāhīmiyya Canal , which began in Asyut and was completed in 1873, built. The canal, initially 320 km long, originally branched off from the Nile without any special weirs and, then as now, is used for year-round irrigation of the fields along its course. Between 1898 and 1903 the Asyut weir and various weirs were built in the canal in order to better control the irrigation that supplies an area of ​​around 2,300 km².

At the end of the 19th century, 16,290 km² were irrigated all year round in Egypt, of which 13,913 km² in Lower Egypt and 2,377 km² in Upper Egypt, a significant increase compared to the time of the flood basins.

Under British leadership, the renovation and expansion of the irrigation system continued rapidly. The Zifta weir (1903) in the Damietta arm and the Esna weir ( 1906) were built, with the Naga Hammadi weir (1930) the last areas below Aswan were connected to irrigation canals.

The purpose of these weirs was to ensure a year-round and, depending on the circumstances, even water supply through the canal systems. With them the Nile could be blocked in order to direct the water into the canals. However, their storage volume was very small, which is why the large number of gates in the dams had to be opened to allow the top of the tide to pass. Conversely, no large water supplies could be created to compensate for dry periods. The Nile mud was of secondary importance in this system; on the contrary, it was sometimes a hindrance when it clogged the canals and had to be laboriously cleared.

Incidentally, the Sannar Dam was built in Sudan as early as 1925 to supply the canals of the Gezira project , in whose reservoir a large part of the sediments that formed the Nile mud in Egypt settled.

Water storage

The Aswan Dam was built between 1899 and 1902, and it was raised twice in 1912 and 1933. It was the first dam wall on the Nile with a significant storage volume. Their sole purpose was to regulate the tide in order to achieve a more even flow of water downstream in the low water period; no irrigation canals were connected to them.

The Jebel Aulia Dam on the White Nile above Khartoum , completed in 1937, was built to hold back water during the flood in the Blue Nile, to use the dammed water to compensate for the periods of low water flow in the Blue Nile and thus to contribute to an even flow of water on the lower reaches of the Nile .

Large-scale, large-volume storage

With the amount of water dammed up by the old Aswan dam and the Jebel Aulia dam, the individual low water periods of the respective year could be compensated for, but they were by no means sufficient to bridge a whole year with particularly low water levels.

As early as 1920 there were ideas to create the required storage volume with Lake Tana and reservoirs in Sudan. An elaborated plan to bridge a century minimum was presented in 1946 by H. E. Hurst , a recognized Nile expert and senior British official in the Egyptian Ministry of Construction. After that, not only Lake Tana but also Lake Victoria and Lake Albert were to be dammed and the Sudd made permeable through the Jonglei Canal . However, the plan was rejected by the affected countries, only the Owen Falls Dam at the exit of Lake Victoria was built. In the meantime, Gamal Abdel Nasser had taken power in Egypt . Instead of many projects in other countries, he preferred a large dam under the sole control of Egypt.

This led to the construction of the Aswan High Dam (Aswan High Dam), with which enough water can be dammed up to compensate for a minimum water flow that occurs statistically only once in a hundred years, without Egypt being dependent on the benevolence of other states upstream. In addition, it could be built in a shorter period of time than would have been necessary for the Hurst Plan, and it produces large amounts of electricity that are urgently needed for the country's development.

The flood of the Nile, which had influenced the welfare of Egypt for thousands of years, ended in Lake Nasser (after a large part of the Nile mud had previously settled in the Sannar Dam and the Roseires Dam ). Since then, the Aswan Dam has been able to meet the water requirements of agriculture, power stations and shipping (for tourists) on a daily basis.

However, this also has its limits. Periods of high and low water in the Nile often extend over two or three years, as happened in the late 1980s. In 1988, the water level in Lake Nasser had fallen so much that rice cultivation had to be reduced, only less than two thirds of the electricity generation capacity could be used and large tourist ships no longer had enough water under their keel. Another year with little water flow would have caused serious problems for Egypt: electricity generation at the Aswan Dam would have had to cease completely and irrigation would have had to be significantly reduced.

Expansion of the sewer system

Since the construction of the Delta Barrages, the canal systems have been expanded, expanded and rebuilt together with the construction of the various weirs. The water flow was controlled with adjustable weirs, often of considerable size. Numerous canals were also used as waterways for cargo shipping. In addition to irrigation through canals, a large number of pumps were also used, some in large pumping stations, which were initially driven by steam engines and later by diesel engines. This is how a complex system of main and side, distribution, drainage and runoff channels was created below Aswan, which has completely displaced the original flood basins.

See also

swell