A quick back-of-the-envelope calculation suggests that you would need at least one barrel (~1 meter diameter, assuming 100cm rain per year) per person just to catch enough rain for drinking water, and that is not even accounting for periods of drought and evaporation losses. I don't think they have nearly enough barrels. Maybe they could rig tarps around the barrels to catch more water, but maintaining the tarps (and keeping them from blowing away) would be a chore. They'd probably be better off trying to dig a well and/or building a water tower with a large funnel to catch water. Or maybe they could find some well within driving distance and make regular runs to fill barrels.