As we prepare for a likely drought next year, it’s well worth reviewing the great WERI (Univ. of Guam) monograph on how to build a sturdy, simple, inexpensive rainwater catchment system – see Construction of a water tank for Micronesia

And see also, for figuring areas, water needs, etc.
http://www.weriguam.org/reports/item/designing-your-rainwater-catchment-and-storage-system-for-rainwater-catchment-systems-in-saipan.html