Hey Hersh
I don't mean to throw in a booger, but the runoff calc is affected by the soils that it has to pass over on the way to the pond. It can get a little complicated. Being anal-retentive about this kinda stuff, I did some of the soil type runoff calc's when I was planning my hole in the ground. Once you catch your rhythm, it's kinda cool... in a bean-counter kinda way. I don't know how detailed you wanna get, but the formulas are contained in the
USDA publication 590 . It starts on PDF page 24 under the category "Estimating Storm Runoff".
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