Formula For Water Retained By Swale?

Hi. I've sometimes heard permaculture practitioners say that if x inches of water falls on a piece of land, that x gallons of water will seep into the ground through the swale.

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?

Any thoughts?

Thanks.

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. 

figures, numbers, and ideas based on location are all in the air as far as I'm concerned.  The best way to get water to penetrate is with deep deep DEEEP rooted plants, good varieties for me in the NW are red clover, sudan grass, swiss chard, and anything with a decent taproot.    For water retention it's obvious the more organic matter you have in your soil, the more water will be retained, and the more well decomposed mulch you have on your surface, the less with shed and if you have fresh mulch ontop of a mound of gunked mulch, you'll have a nice seal upon your water pocket.  I have tried putting logs under mulch, on soil surfaces.  If you split them up and don't use conifers (which can deture plants and decomposition by what they release) it will act as a sponge after it breaks down more (my trick is putting broken down mulch around the logs then a little fresh chicken shit ontop which will get soaked into the log and surrounding mulch, BUT only in places that arn't next to plants, or won't have plants spread out to that area for a year so it can break down.

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/


Russell

Jonathan_Byron wrote:
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.

Ruso is right, trying to figure out how much water is captured by any given swale is near impossible to be accurate.  You need to take in factors such as humus in the soil, water absorbtion of plant material, and tons of other factors.  To glibly say "X" water falls & "y" captures is misleading to say the least.

While Pakanohida is right in that reality varies from the model... I think Byron gave good advice from an engineering perspective... It is a volume of water estimate driven by your 'model rainfall event'... how big are your rains and how big of a rain are you trying to capture?  THis is more important than annual average.  In a true dryland situation like you have I suspect that concentrating water to fertile sites is the trick for producing human food, thus not just capture but concentration.  You might be able to save much excavation by walking and marking the site during a heavy rain.

Permeability of the soil is the issue.
IE How quickly soil allows water to soak through.
All the comments above about humus, soil type etc alter this factor.
Even dry sand can have no permeability in particular circumstances, knowing this can help you improve things.
In the wild, most hill tops with a forest on top are the major recharge points for ground water.
The area has plenty oir organic matter to hold water so it does not race down the slope and it allows
that water to soak into the ground.
Many juristriditions ban the clearing of hill tops for this reason.