Today, irrigation does not require a hierarchical social structure to be sustainable. However, historians believe that the lack of advanced technology in the past meant that a hierarchical social structure was necessary to sustain large-scale irrigation. The reason for this is that irrigation projects required a great deal of work and that work could only be accomplished if there was a hierarchical, stratified social system.
In ancient times, irrigation projects could only be completed using massive amounts of labor. People had to dig the ditches and canals by hand. They had to build any levees or other infrastructure by hand. All of this took a great deal of effort and a tremendous number of man-hours. Today, the presence of technology greatly reduces the man-hours needed for irrigation projects, but these technologies did not exist in ancient times.
So, why was a hierarchical, stratified social structure needed in order for all this labor to be mobilized? The reason for this is that the people would not have volunteered to go work on these projects. They would also have had a hard time organizing the projects. This meant that they needed to have a strong government that had the power to tell the lower classes what to do.
The farmers and lower-class workers of ancient times would have had to work very hard and very long simply to provide for their families. They would not have been eager to volunteer to take time off and go work on a project that would not help feed their families. Left to their own devices, they would not have worked on the projects. This is where the stratification comes in. In a stratified social structure, the people at the top of the society can order the lower classes around. The lower classes have little choice but to obey. In a stratified society, the government could simply order the peasants to go and work on the irrigation projects. That way, the projects would get done.
Historians believe that massive public works in ancient times only got done through some sort of coercion. People had to be forced to work on these projects. In order to force them, it was necessary to have a stratified social structure so that the rulers had the power to coerce their labor. This is why irrigation projects could only be accomplished in a stratified social system.