It is interesting to contemplate the following progression:
Irrigation → high production farming → cities.
This is the story of Mesopotamia, the first substantial farming culture on earth. Its agricultural productivity supported the population density required for urbanism.
Before the settlements at Sumer, irrigation was developed in the steppe areas of Mesopotamia between the rivers and the Zagros Mountains. But there were limitations to productivity, including the supply of water and the characteristics of the soil. Men, in their crude knowledge of farming, had to rely on natural processes which were unpredictable.
In Sumer, however, there were no limitations. The alluvial plain was rich and fertile, water from the rivers plentiful, and the soil was easy to work because it was free of stone. The Tigris River is 1,100 miles long, flowing from the Armenian Plateau to the Persian Gulf. With four major tributaries, it is subject to significant flooding each year. At Kut, for example, the river rises from four feet to twenty-six feet.
To grow barley, one needed 40-50 days of moist soil, which naturally presented itself when the river began to recede. The Mesopotamians used a scratch plow (Ard) to create furrows in the soil for planting. It was a crude implement, incapable of turning the soil, but turning the soil was unnecessary since the land renewed its nutrients with each seasonal flood.
To harness the river’s power irrigation canals were constructed to hold water and control distribution, further extending the growing season.
Large crop production density supported a high population density which set the stage for development of urban areas. These were not large cities, but they were the first cities, numbering 15-20,000 people. Sumeria created a new dynamic of human interaction, including social stratification, labor differentiation, and sophisticated political systems.