Pond Boss Forums INTRO Questions & Observations Runoff question

I've got a question about runoff. My pond is about an acre, it has about nine acres of drainage coming into it. About half timber, and half alfalfa fields. We've had a very dry second half to the summer. About 1" of rain from August until Nov 1.
Then we have had two nice rains in Nov. about 8" total. Some runoff as it rained hard at times. My pond came up about 14-18".
I've talked to others that have had less rain, but have seen a much more dramatic increase in water level.
I know there must be a million variables. I'm aware of most of them.
My question is, how much "below ground" runoff is normal.
Example. A very slow 1" rain, with no visible runoff, I would expect a 1" increase water level. Am I wrong about this. I know ground saturation is a really big factor. Along with soil types, and vegetation types.
Thanks
Jeff

The less the pond leaks, the more dramatic water level increase you will see from surface runoff.....

Got that. Not a leak issue. More of a run off question. Just wondering what to expect from subsurface runoff into the pond? Maybe this just has too many variables to know.
Thanks anyway

I am kinda a newb to this...but if your pond is a ground water table pond, it's level will fluctuate with the level of ground water no matter how much runoff you get. If it is a runoff pond, then I would not expect what soaks into the ground to change your water level. If it did, then your pond level would drain out when the ground was dry (pond water level would be based on ground water table level).

Of course there are variables.... like if it is spring fed then what soaks into the ground will raise the level of water. Probably too many variables to be able to answer your question without more info.

My small pond is a runoff pond. I don't think it goes up much at all from a long soaking rain...other than the actual rain amount (IE an inch of a long soaking rain=an inch rise in water level). Though the 4+ inches of heavy rain we got recently raised it a couple feet.

Sean

This is not a runoff question, it's a watershed area question. Some ponds saw a bigger rise from less rain because there watershed areas were bigger. A 9 acre shed area for a 1 acre pond is on the small side.

Of course there are many variables, the most important one being it needs to rain, secondly you have to have ample watershed or your pond will only rise minimally without being supplemented.

My watershed area is undersized so I pump water up from a stream below a few times a year when it gets too low.

Thanks guys. I was thinking along these lines. I had read about quite a bit more gain in water level from a given rain, than it was seeing. My take is, it's just impossible to predict. If I completely covered my alfalfa fields with asphalt, I'd see a bigger bump from even the smallest rain. When it's really dry, the ground just soaks up everything. My hope was that maybe there was a given amount that might be seeping downhill to the pond. I think if it does, it's a minuscule amount. Before this last rain there were some pretty large cracks open all over the alfalfa fields, it was so dry. Just not going to get much runoff in that situation.

It's been a dry year here in WV also. One of the benefits of an undersized watershed is that when the monsoons do come, you are not going to have to worry about a breach as much as the guy with too big of watershed area.

I had two alternatives when I built my 1 acre pond with its 10 acre +/- watershed. One was to drill a well and feed it continuously and two was bury 600' of 2" pipe up from the stream below. I settled on the latter for a number of reasons. Have you looked into a well?

The math is simple. If you can figure out what the area is that drains into your pond and the surface area of your pond, you can calculate how much your pond should rise with a given rainfall. If the drainage area is 10x the size of your pond, your pond should rise 10x the inches of rain, plus the rain that actually falls onto your pond. But, unless the sides of your pond are vertical, the area of your pond will increase as it fills. If the ground is saturated, that water will move too, but not necessarily in the same direction as the surface runoff, and very slowly. If your pond has a cut bank on the up slope side and the bed or beds your pond is built in slope into your pond, and there is hydrologic head in those beds, and the beds are sealed on the downslope side of your pond, there might be a slight net contribution to your pond. That is, you might see it continue to rise slightly after the surface runoff stops.

I've thought about a well, but just can't justify the cost. Plus running the electric down to the well, plus the electric cost just running the pump. We had a pretty long dry spell. Three months with hardly a drop. I was down 2.5', it just didn't look that bad. The banks are all fairly steep around the pond, so there wasn't that much additional shoreline exposed, and there was still 12' water depth. It's back up now and looking better.
I guess for right now, I'll just stay on the "hopefully get some rain" plan.
Thx
Jeff

I'm not sure it's always that simple. Not to be argumentative, but a four inch rain in 30 minutes, gives me a lot more water than a six inch rain in 6 hours, and ground saturation, and vegetation effect the outcome also. With a clear difference in what comes out of the woods vs what comes out of the alfalfa fields during a given rain.
I guess I was trying to just understand better how some ponds get a much better bump from a given rain with comparable drainage areas.
There appears to be no majic fix for me. I'll get what I get, and live with it. Ha
Thanks
Jeff

Per Mike Otto, vegetation density and type and type of rain event can make a huge difference in the amount of runoff from watershed. Plus you have soil moisture content...I agree, there's multiple variables which can impact amount of runoff created during precipitation event.

Jeff, I have a client who just installed a 40 GPM solar well for his pond. I am working on a quote for my ponds, too, and will share what I learn with with forum. Can you imagine free water for life [once the install cost is covered]? Apparently major advances have been made with solar pumps and panels since the last time I researched this topic. Had this been an option for me 9 years ago when I popped my well I would have elected to go that route and save myself a ton on running electricity 1/2 mile to my farm.

Todd, I like your optimism! I have a smaller watershed, also, and when we received multiple 6" rains over the past 12 months, with one over 10", my dam likely would have been severely compromised handling a larger watershed.

That sounds interesting. I do have a very small leak, but if I could just counter the evaporation I'd be happy.
Please let me know how it goes.
Thx

The simple math tells you the amount of water in inches or whatever units you're using, that's available for your pond. It's the maximum amount. To be precise, you have to account for any amount that recharges the aquifer underling the property, the amount that is absorbed by the soil and eventually evaporates, the amount that evaporates from the runoff before it reaches the pond, the amount that evaporates from the pond during the rain event, the amount that is transpired by vegetation, the amount consumed by the fauna and not returned to the watershed, the amount that leaks from the pond (the rate of leakage may be greater when the pond is higher, etc. The main effect of the vegetation is to slow the runoff, not stop it. In most places, it's the land's main protection against erosion. Most often, the toughest part is estimating the area of the watershed. Also, the reported rainfall may not be accurate for your property. Rainfall can vary a lot over a short distance. Your own rain gauge could help in trying to figure out what's going on. The water has to go someplace.

I will relate all relevant details including cost, depth, pipe diameter, and pumps/panels used to build the estimate as soon as I receive it.

Ok, sounds good.
Thx

Jeff, one other factor I haven't seen mentioned much yet is the slope of the watershed. A very gentle slope will result in considerably less runoff unless the ground is soaked to begin with.

My pond has about 8 to 1 ratio of watershed to pond, which is plenty considering east Texas gets around 45 - 50 inches a year and the slopes are pretty darn steep in the hill country.

Great point - slope is an important variable!

As it turns out, there are just too many variables to accurately decide why one pond will come up 8" with a 4" rain, and another a few hundred miles away will come up 4" with a 4" rain. I realize the math side, that you have a given amount of drainage, a given amount of rainfall, and a given pond dimension, and all the rain has to go somewhere.
The bottom line for me is, if I get enough rain, which turns out to be, just the average rainfall, I should be ok. Prolonged periods of drought will drop the pond level every week, and unless I want to introduce a man made water source, I will just sit back and watch what happens.
Seems like a bit of a waste of everyone's time here. Sorry about that. No majic bullet to bring in rain water. I've done all the diversion I can, I just need rain.
Thanks everyone.

Setterguy, Maybe this will help, maybe not. I can semi-accurately predict how much my pond will raise during a rain event. Not exact but usually within an 1" or 2" depending how much rain we get.

First you would need to determine your runoff curve. The curve is based on the composition of the terrain/vegetation. You can calculate this by using Tables 1/2/3/4 on Page 14 of the USDA Pond Construction Guide, probably need to do the weighted curve as explained on the previous page. Then you would need to go to Table 5 on page 18 to get the rainfall curve. Rainfall curve is important as it accounts for ground saturation but only on specific rain events. If the ground is saturated from a 2 inch rain 2 days prior, I would calculate using a 3 or 4 inch rainfall. With those numbers you can then calculate number of gallons that will theoretically flow into your pond after any rain event. Due to the fact that most pond banks slope the number of gallons for every inch will be variable.

I do NOT run these calculations every time. After doing the first few major rains, I determined 43% of the rainfall reached my pond during the spring rain events. It takes a little more guesswork for me during the first part of spring due to the 2 ponds in the upper watershed not flowing out. In late summer and fall this holds true if we get 1.5" or more of rain in a day. I also have a table that tells me how many gallons per 1/2 foot of water in my pond.

It took a while to do this but if you use google maps (or whatever) to look at the terrain and complete measurements, it goes pretty quick. You also only have to calculate it once. The harder part to me is calculating how many gallons you need to come up 1 inch in the pond based on the current depth, thank you AutoCAD!

Hope it helps, first and foremost I hope it makes sense.

It makes sense. Thanks. I have the calculations done by the USDA guy that came out before and during the pond construction. I've got to think a little about ground saturation and getting it factored in. I've got the book, so I'll look up those pages.
Thanks

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