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Anyone know a formula for water harvested by a swale?

 
Posts: 1131
Location: Central Wyoming -zone 4
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so im trying to put a numerical value of water harvested by a swale so that i can give a customer something to chew on and realise the value of the ditches im diggin

the swales in question are 12 ft long, 4 inches deep storage capacity at the lowest amount and about 1 foot wide, so how much water would these be harvesting per inch of rain?

does anyone have a formula for this or know where to find one?
 
gardener
Posts: 582
Location: Lower Mainland British Columbia Canada Zone 8a/ Manchester Jamaica
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i do, just about every water harvesting system has a formula. im on the bus atm but ill dig it out of brad langcasters rainwatef harvesting book when i get home. try his book name plus the word formulas maybe you can find it on the net in the meantime
 
steward
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Location: Currently in Lake Stevens, WA. Home in Spokane
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That swale will harvest 7.5 gal. per inch of rain.

The simple formula is divide square feet by 1.6: 12'/1.6=7.5

 
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Location: Arizona low desert
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Forgive a question from someone just learning? Why wouldn't it be 30 gallons?

At 7.5 per cubic foot, 12' holds 90 gallons. Divide by 3 since it's only 4"deep would put it at 30 capacity. Presumably percolation rate, runoff and rainfall amounts change the equation but I haven't learned those details yet.

Thanks for the thread
 
Devon Olsen
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Location: Central Wyoming -zone 4
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hugelkultur dog chicken
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^if the swale is full then i think that 30 gallons is the correct answer, however, one inch of rain would be 7.5 gallons from my calculations... apparantly there was another thread on here and i did some calculation after figuring out an acre inch of rain is 27000 gallons
 
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The swale will also catch runoff water from upslope. How much that is will depend on your swale spacing, soil porosity, etc.
 
Devon Olsen
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Location: Central Wyoming -zone 4
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^thats a good point, and in our soil applicable because the swale isnt likely to collect ANY water unless its coming down fast enough not to sink into our sandy soils immediately
 
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