Ok I am going to throw this out, Remember when they had a trickling of Water out the windmill. It was not much and muddy.. I thought at the time it was because they had to dig to the well. But I think the pipe was already there. They did not dig a well..

So the tunnels under the city could be the natural water ways. They dammed them up some where and diverted the flows. So the water they were getting was not much cause there would be nothing but muddy water left at the bottom. Just a Thought.. after reading a couple posts.. Anyone with a map of the natural springs or underground streams?