Anyone know what kind of formula they use to determine that? I'm trying to come up with a figure for a Sonora desert area that averages 11 inches or rainfall a year. How many gallons would a swale allow the ground to retain?
All in All how it works is as a rain drop lands, each level of organic surface from the top gets dibs first, if the water sheds or has surplus after being soaked in by the mulch, it goes down to the soil, and so on.
I've grown orchards in places where organic matter leaches away from the soil, in and area that does not have summer rain and I could not get irrigation. the one thing i really noticed when trying to build soil to ultimately retain water was: the example I have is Jerusalem Artichokes, which is highly drought tolerant being apart of the sunflower family would LIVE in that soil IIIIFFF I would plant in a handful of compost. After harvest though I found that if the soil were dirturbed and mixed from being harvested and one seed put back, there wouldn't be enough moisture in the following season to keep that single plant alive. My theory is that while undisturbed, the plant would grab and hold onto what it could, and in the chaos that is me, whatever was left from what I put there that the Sunchoke was holding onto would not be enough for a new plant to hold grab and sustain itself from.
So i like to rant, and you just got a dose. But i hope it helps/
One acre-inch of water is just over 27,000 gallons. If a swale has a catchment area of 1/10th acre, and there is no overflow, it would make sure that with a one inch rain event, none of the 2715 gallons that fell on the catchment would run off elsewhere. Some of that would probably infiltrate into the soil anyway, depending on the rate of rainfall and soil texture ... slow soakers on sandy areas don't result in much runoff, but a heavy rain over desert pavement might lead to lots of runoff and cause flash flooding.
Thanks, that helps.