Water from the Sky

tank

water from heaven, literally---rain. not a lot yet, but a beginning. that's about 5 inches of water in a tank 60 inches in diameter. so let's see----area equals pi times radius squared. okay that's about 2830 square inches. times 5 to get volume is 14150 cubic inches. divided by 12 cubed gives about 8.2 cubic feet. 1 cubic foot equals 7.48 gallons. so-----from one day of medium light rain we got a bit more than 60 gallons.

the rain gutters on our house all feed into two underground collection pipes that discharge on the downhill side of the house. so far only one, serving about one third of the roof area, is connected to the sump system that pumps water into the holding tank. we would have had, from yesterday's rainfall, about 180 gallons if the other pipe had been connected to the sump. all we need is about fifty feet of the corrugated drain pipe to finish the collection system.

the tank we have, a recycled hot tub, holds about 41 inches of water. using the calculations above that means it will hold about 500 gallons. okay. it rains and we don't need to water the garden. now the rain stops, the sun comes out, and we need to irrigate. the bottom of the tank is about 5 feet above the garden. one foot of head, in this case 5 feet at the bottom and 8 feet from the top of the tank, yields 0.433 psi (pounds per square inch). so the maximum gravity pressure from a full tank, which will of course drop as the water level drops, is almost 3.5 psi. that would suffice for the extravagantly wasteful flood irrigation, but not for efficient drip irrigation. we will therefore need another pump to bring the pressure up to 15-20 psi.

the picture below shows the sump collector with a submersible pump and float switch. when the water level gets almost to the overflow (white pipe) the float switch turns on the pump which pushes water into the holding tank (see top picture). when the water level drops to near the bottom the float switch turns off the pump. the big black pipe brings water from about a third of the roof. the big hole in the side of the collector is where the other big black pipe will go, bringing water from the rest of the roof. the small black pipe goes from the pump to the tank. yes, the collector sump is a plastic garbage can. the top is notched for the outlet pipe and keeps out dirt and critters.





collector