Rainwater Collection
Neat detail by a Bulverde, TX man (Bulverde being a bit north of San Antonio and south of Wimberley & Austin) about a rainwater collection system. Click on "Rainwater Collection" on his website. He made a schematic of the system and has charts detailing the reserve of his system versus the Central Texas rainfall that he has experienced for the past several years.
The purpose of the "roof washer" wasn't obvious to me, but I found a description in this Harvested Rainwater documentation:
A primary strategy is to reject the first wash of water over the roof. The first rainfall will clean away any contaminants and is achieved by using a "roof washer."
The main function of the roof washer is to isolate and reject the first water that has fallen on the roof after rain has begun and then direct the rest of the water to the cistern. Ten gallons of rainfall per thousand square feet of roof area is considered an acceptable amount for washing. Roof washers are commercially available and afford reliability, durability, and minimal maintenance to this function.
The Texas Water Development Board also has information . And the Lady Bird Johnson Wildflower Center has a expansive and beautiful rainwater collection system (of which I'm an enthusiastic fan, as some who've visited the center with me have witnessed).