Pond Boss Forums POND CONSTRUCTION Building a dam Calculating watershed

How do I determine how many acres of land that my watershed comes from?

I use the Google Planimeter.

thanks I will check that out...
Also my pond site has 3 or 4 springs flowing through the area...

How do I calculate the amount of water from the springs?

I'm pretty sure I will have too much water as the springs never run dry so I will need to have some sort of spillway pipe but I need to know how big.

On the spings take a width and average depth measurement. Then get a speed measurement by placing an object in the water and seeing how far it moves in 1 second. In my Fisheries Techniques class they said that you can use an orange. That will give you how many cubic feet per second that is coming out of the spring.

Use this Pipe flow calculator. It will help you determine the flow rate on different size pipes.

Thanks again for the info I'll give the orange trick a try.

What do I measure with the google planimeter? I know the amount of acres surrounding my land but obviously I don't get watershed from all of it right?

My pond site is a low spot with a high watershed and its right at the water level. I've done a couple of test holes about 5 ft deep and they filled right up and have not went down. The property behind me is all higher ground then mine and that is 400 acres.

Any help figuring out how to calculate the watershed acres would greatly be appreciated

Here is a picture of my property and the watershed outlined.


On a topo map the small end of the "V" points upstream. First place a point where the dam will be, then place more points all the way around. For example, on my picture if you go right from point C you will be heading uphill.

aaron, if you can post a topo of yer pond site and surrounding area, folks here can help you. you need to be able to read the topographic contour lines (hills, valleys, divides) yer watershed will be the entire area sloping towards yer pondsite.

if yer springs are relatively small, a quick and dirty estimate of flow volume can be obtained w/ a 5-gallon bucket....funnel the spring water to a deep spot (maybe have to dig a little) stick the bucket in and time how fast it fills..convert to gallons per minute.

you can use google planimeter it will give you the size of teh area you out line

A good trick to measure flow volume is with a piece of bare copper wire or soft baling wire.

Take the wire and bend it to fit the bottom of the stream, following most of the contours except definite holes. Once across, then run the wire back across on the surface of the water. Maybe a better description would be to "wrap the wire around the water". Once that's done, you can cut the wire, and make a square or rectangle out of it, and multiply the length x width. Now you can use the orange trick to easily calculate the flow volume.

A big part of determining your watershed is observation, because maps don't tell the whole story. If you look around after a rain, you can see which land is draining in which direction, and get a rough idea. It's usually a very rough calculation no matter how you do it.

Thanks for all the info all...I would have measured the springs already but a cold front came through and it has been 20 blow zero (wind chill) for the last few days.

When I used the google planimeter I couldnt figure out exactly where my pond site is...I new I was close but obviously I need exact right? Should I order a topo for my property and then find it with gps?

Bobad, I am not sure that wire bending trick is very accurate.
If the stream were broad but very shallow you would overestimate
the cross sectional area a lot if you deformed the wire into a square or a squarish rectangle. For example, take a skinny rectangle (1/2 ft x 5 1/2 ft) with a circumference of 12 ft and an area of 2 3/4 sq ft. You can deform this 12 ft "wire" loop into a 2ft x 4ft rectangle with an area of 8 sq ft. The skinnier the rectangle, the smaller the area, given the same circumference.

LRunkle's right, the most accurate way to do it is to take a rag tape, stretch it tight and level across your creek and use a survey rod or yard stick and take a depth at each increment, usually 1'. Figure out how high your tape is above the average water level and subtract it from each of your depth readings.

You can then plot it in cad, or on graph paper to be highly accurate. Even easier is to just add all the depths and divide by the count and multiply by the width of the creek to get an average end area of that crossing.

Erich