Long after a nuclear war wiped out most of the people on earth, civilization has been knocked of its feet. The San Diegoans, a small tribe from San Diego, were unremarkable to say the least. A small band of 40 some odd nomads who scratch out a living on the coast fishing, and are constantly under attack form bigger tribes, like the Tigres form the far south lands.
But one day, a wise person from the civilized lands, the knowledge giver, who was actually a missionary from a catholic mission, helps them out a bit. He learns their language and befriends them, and, seeing potential in them, gives them survival skills that will help them build into a mighty nation.
My question is: What knowledge of the old towns could he give them that would address the following problems?
They don’t know much about farming, and the plants they do grow aren’t suited to the arid climate.
They have only a few sources of fresh water reserves and those are slowly being taken over by the other tribes in the area.
There aren’t a lot of trees, so building a wall out of wood is impossible. Most metal has been scavenged away, so metal weapons aren’t a possibility. They need some other material