I bought the Perfect Pond book and I've gotten to the section where it talks about calculating your drainage area and the annual precipitation, etc. It's led me to two questions:

1) In measuring the drainage acres on a topo map, do you measure to the highest counter line or do you measure to the middle of the highest flat? For example if the high point is a gentle sloping ridge doesn't it make sense to measure to the middle of the ridge?

2) There's a formula in the book using annual precipitation to calculate the gallons of water needed in a pond and what would be available for runoff. I live in Iowa, I wonder if average snowfall should be factored in as well as the precipitation? If so, how do you convert inches of snow to an equal equation of inches of rain?

Nominally 10" of snow equals 1" of rain. But official annual precipitation figures have already converted all rain, snow, oobleck, etc. into inches of rain.

Thanks Theo. I need to finish the book and it probably tells me this, but it gets you thinking of these questions. Now that I've done the calculations I have a rough idea of how many gallons of runoff water I'll have, and I have an idea of how many gallons my pond will hold, then what? Does a feasible pond require one year's runoff to roughly equal the pond's water retention volume? That's about how my calculations worked out for my pond site. Once it's full that seems like a lot of water coming in if you figure the pond gets an equal amount of water to it's total volume each and every year.

Here's my stats for anyone who wants to confirm the calculations: pond will be .75-1 acre, 18'=20' at dam so 9'-10' average depth, 6 acres drainage area using contour edges from topo map (10 acres if use midpoint of top countour area - see question 1), 36" annual precipitation.

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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I've found that the amount of runoff is greatly affected by how dry the soil is and how fast the rain falls. A heavy rain that just dumps allot of water will give you allot of runoff. A slow, steady pour might not give you any runoff. I've had days that rained an inch and my pond only went up an inch. I've had other rains of an inch and my pond went up 4 inches.

It also makes a big difference in if the water runoff is crossing grass or going through woods. In my grassy pastures, I get allot of water, but nothing comes out of my wooded areas. The trees, leaves and whatever else is on the ground catches the water and holds it.

Eddie

Hersh,
Another factor to think about is evaporation rates. In Iowa, when you get 36 inches of annual precip over ten acres, some percentage of that will be runoff. Like the guys above say, the kinds of soil and plants determine the percentage of runoff. I've seen land where 100% is runoff, coming off mountains and rock and I've seen it where less than 20% is runoff, because a big percentage of the watershed is sandy and the water percolates downward fast and recharges an aquifer below. You are probably somewhere in the middle. Your NRCS folks can help you with that calculation.
For example, if you are in farmland with good topsoil and heavy clay subsoils, you will receive probably 40-50% of your 36 inches of rain yearly as runoff. If that's the case, you will get 14-18 inches of runoff over 10 acres. That's 140-180 acre inches per year, or roughly 12-15 acre feet yearly. For a one acre pond that averages 6 feet deep, you get at least double the amount of water that you will need. To that end, you need a good spillway to handle flooding rain events. And if you get an average of 40-50 inches of evaporation/sublimation each year, you still have enough runoff to fill your pond. But, you will have fluctuations during part of the year. Your pond will likely drop two feet from time to time. Plan for that.
Good questions. Post again.
FYI, I don't always have time to read through the entire forum, so if you need some clarification about something we wrote in the book, drop me an email at pondboss@texoma.net and I'll respond much more quickly.

I think you can first step to determine the drainage area. Maximum depth is not standardized - is single-minded by each specific condition calculation. We will take into attention the resources used, soil appearances, skills used.

I would suggest seeing if you can get help from your local NRCS office (as Bob suggested). Depending on their workload, they might be able to help you determine the watershed and plan the pond, including sizing the pipe and emergency spillway.

This will not answer your question but maybe will give some more insight.

In my case of a 3 acre pond, my watershed area was not enough (as estimated by my NRCS agent), so he designed a terrace in our nearby farm field that pulled more water into the pond. This put us at what he considered barely enough watershed. I need to actually measure it, but I am guessing about 10 acres watershed for a 3 acre pond with a 42" annual rainfall.

At times during the summer during a hot dry spell we can get down to 18" below full pool. I don't like the pond that low and the lack of flow through causes the water to get pretty yucky (technical pond term). If we have a wet spring or an extra rainy season we can be running over the emergency spillway during a 12" rain in a 48 hour period.

In dry years without large rain events I wish we had more watershed to get some additional flow through the pond to remove excess nutrients (from fish poo from feeding). Then when we have saturated soils and a large rain I'm glad I don't have more. Getting the perfect watershed size for a pond is difficult and it is different for a drought year than it is for a wet year. So a person has to try and hit a happy medium. As Bob stated, your NRCS shoukd have the experience for your area to be of help.

Some areas large rain events are unusual, so will need relatively larger watershed area. Large rain events are somewhat common for us here in SE Ks, as the link to pictures show of our pond.

what our 3 acre pond looks like after a large rain event