Ancient irrigation styles depended very much on the physical geography and geology of the area, and the engineering skills available. Four different styles of irrigation were developed very early in agricultural history. All irrigation systems depend on taking water from natural sources and diverting it to artificial channels or ponds where it is applied to crops.
The Nile valley is rainless and extremely fertile. Herodotus wrote more than 2000 years ago, "Egypt is... the gift of the river." Egypt depends on the Nile in a way that no other nation does. 97% of Egyptians live on 2.5% of its area. The prosperity of the Nile valley civilizations has depended throughout recorded history on the efficiency with which the central government has organized the best use of the river water. Crops could be stored after years of abundance, for example, and irrigation schemes could be both built and maintained.
The Nile receives its water from the tropical highlands of Africa. The river receives no tributaries at all for the last 1500 km of its course across the Sahara Desert to the Mediterranean. In Egypt, far from its sources of water, the Nile has no sudden flood-wave crests. The annual flood starts in June as snowmelt and summer rain flow down the river. It rises gently to its peak in late September and early October, then gently subsides by the end of December. The Nile is one of the most predictable rivers in the world, and its "flood" period averages more than a hundred days, rather than being very short-lived like those of other rivers.
At first, Egyptian agriculture along the Nile was based on growing winter crops after the annual floods had subsided. Egyptian irrigation was based on several facts. There was only one water source (the river) which was too powerful to control. Irrigation works therefore had to be passive in construction, and built relatively high along the river bank so that they dealt only with the peak of the flood. The river valley is flat-floored, but narrow and steep-sided, never more than 25 km wide until it reaches the Delta below Cairo. Irrigation schemes could therefore not carry water any great distance away from the river.
The ancient Egyptians built large flat-bottomed basins for growing crops along the river banks, and simple sluices that diverted water into them at the peak of the flood. It was easy in engineering terms, if not in labor, to arrange for good water flow through several basins in succession, controlled by simple gates. Water was allowed to stand in the fields for 40 to 60 days, then was drained off the crops at the right moment in the growing cycle, downstream back into the river. There was always plenty of water, so salts never built up in the soil; and the flow in the canals and ditches was strong enough to avoid silting. (Silt that settled in the basins was beneficial in two ways: it made the floors of the basins evenly flat, and it brought a lot of nutrients with each year's flood.) Ditches and canals were short, and the typical irrigation scheme was very local.
The design of the irrigation system depended critically on knowing in advance the height of the annual flood, and the Egyptians developed a system of "Nilometers" at various points along the valley. Rapid communication and early warning of the height of the flood as it rolled downstream from the south made a great difference to the size of the harvest. Herodotus wrote that the Egyptians "get their harvests with less labor than anyone else in the world."
Early irrigation was rather local and primitive, and food was not stored efficiently, so the early civilizations were vulnerable to long-term fluctuations in the Nile floods. There was no significant attempt at water storage: since all the water came from the Nile, any storage would have meant damming the river, which was far beyond the capability of the ancient Egyptians. Therefore their irrigation system was passive, and early Egyptian civilization depended largely on one winter crop per year. After it was harvested in the spring, the land lay fallow until after the next flood. Only in a few places with very wet soil was there any chance of a second crop, and among these areas were Abydos, Memphis, and Thebes, the great centers of ancient Egyptian civilization. They lay along the river, upstream from the Delta.
The Old Kingdom, the Middle Kingdom, and the New Kingdom were periods in Egyptian history when strong central government flourished in times of prosperity, followed by periods of stagnation in economy and population, often accompanied by social, military, and artistic decline. It's not clear whether strong central government resulted in effective irrigation and good crop production, or whether strong central government broke down after climatic changes resulted in unstable agricultural production.
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