Anyone know a formula for water harvested by a swale?

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?

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

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

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

^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

The swale will also catch runoff water from upslope. How much that is will depend on your swale spacing, soil porosity, etc.

^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