Similar question to this one except I want to calculate the urban and rural population needed to support an army.
I want to create a world for a fantasy wargame campaign using some of the elements from questions I have asked here before. The setting is a desert world where water is relatively scarce, but is otherwise similar to earth. There is a large canal network to move the water where it’s needed across many hundreds of miles and the world is populated by humans.
There is a technology level similar to that of ancient Rome 100CE and a civilization based on city states located at the junctions in the canal network. Each city has a fairly large area of influence into surrounding territory. Some are larger some smaller.
In order to construct a vaguely realistic campaign in this world I need to consider the population size and spread.
Is there a basic rule of thumb that I can use to determine population sizes in rural areas, towns and cities? There needs to be enough people so that each city territory can field an army of between 5000 and 20000 without too much trouble.
So in summary what I want to know is for an x thousand man standing army how many people have to live in the city and how many in the rural areas along the canals? Use Roman settlement patterns if I have left anything out.
Out of scope: anything to do with how the canals were built or the backstory (and no magic).
Edit To answer some questions that have been raised:
Do they have slavery yes they do as the Romans did.
Fighting usually occurs in the spring and summer, although there might be exceptions. Some city states have taken control of others to form small nation states. There is considerable tension along many of the canals in contested areas. I’m not sure how wide the canal irrigation zones are but probably 10-20 miles either side of the canals are agricultural land the rest is desert.
There is one harvest per year at a similar time to that which the Romans would have experienced and it is labour intensive.
Conscription would not be used except in emergencies. In such cases any males from the population who might be reasonably fit and capable in a fight (say 15-55 year olds) might be called up for the duration of the emergency.
Map added of the top half of this fictional world in case anyoen is interested