You should go to ...

You should go to www.wasrag.org, the Rotary International RAG for WASH that has developed written guidelines for the initial consideration of how to develop a water supply program under various conditions.  Rain Water Harvesting means you capture the water from the roof or other above ground collection system before it hits the ground and is contaminated, lost to flowing away or other issues.  What you are talking about is capturing, diverting water in a natural stream or watershed and storing it in a storage facility before treatment , delivery and use.   You need to read available guidelines on how to plan, design, construct and operate a community water system before you start.  You are a long way from the end objective a constructing a home or community water system.  Don't give up---just do more homework before you ask professional questions.