GPM of well water for irrigation?

[bebster]

I'm in the process of installing a new well for my barn, but also want it to serve for irrigation purposes for the vegetable and fruit plants and trees. The well contractor suggested a larger pump to serve thses needs, but needs to know the Gallons Per Minute of capacity. Here's a photo of the well casing -- the y-valve for the irrigation hasn't been installed yet.

I plan to have about 1 full acre planted, and was hoping that someone might have a rule of thumb for how much irrigation capacity would be needed for an average planted acre.

If I undersize the pump, it will cycle too frequently. If I oversize it I'll spend more than I need to.

 

[Egon]

At what rate does the well produce. This may be a determining factor on pump size.

Egon

 

[J_J]

What determines the gpm, is the pump, depth of the water, distance that the pump has to pump, size of the pipe.

In general, I would get the largest pump you can afford, a 1 1/2 hp, or 2 hp. would satisfied most of your needs.

A small pump will run forever trying to pump the water you need.

A pump will run until it builds up pressure in the storage tank.
If you are not uning water, the pump will fill up the tank and shut off.

In an irrigation system, you want the pump to run as long as the sprinkler system is on.

In order to irrigate one acre, that will be determinded by the gpm, size of your under ground pipe, and the size ad number of heads, a good pump.

I believe, that one acre is about 43,000 square feet. Each rotating head will cover a radius of about 35 to 40 ft. Then you need to overlap the coverage.

Best thing to do , is to go to Home Depot , get one of those Rainbirds, or Toro planning bookets, fill it out and send it in, and they will figure it out for you . or if you have a cad program on your computer, you can do it.

In a simple system, you turn on a valve with so many heads and water until you are satisfied.

A timer system, requires electric valves and some kind of timer, electro-mechanical, or electronic.

A cheap 6 station electronic is only about $36.00

In my area, they would charge about $2400.00 to irrigate one acre.

You can save about 1/3 by doing everthing yourself.

Good luck with your system.

 

[tx_engineer]

JJ has provided some good information, and the Home Depot/Rainbird solutions can give you a nice starting place (those landscape folks can pencil whip irrigation solutions out in their sleep). However, if you want to get into the nuts and bolts of the design, there are several questions that you must ask yourself and answer, as there aren't any hard and fast "general rules of thumb."

First ask yourself at what rate you want to irrigate. This is a multi-part question; how much and how often. Typically, in designing for irrigation systems, we assume that all landscaped areas should receive a minimum of 1" of water a week (equating to rainfall). For a one acre area, this equates to 27,150 gallons. Now if you plan to apply irrigation daily on the entire one acre over a four hour period to meet this demand, this would mean you would require a system that is capable of supplying a little over 16 gallons per minute (27,150 / 7 days in application period / 4 hours of application / 60 minutes per hour). If you wanted to irrigate only three days a week for the same four hour period, then you would need a 38 gpm delivery rate (pretty big rate - requiring a minimum 1-1/2 main line maxed out). I think you can get the picture there. Talk to your well guy about the well you just had drilled. He should be able to give you an idea as to how long you could have your well pump run before you draw the well down too far and get into any subsidence problems. You will use that information to tweak your application times. (I'm accustomed to dealing with large diameter wells that have 1,200 gpm capacities, and 4 hours is a good pump cycle time on wells of that magnitude - if you kick on a pump that big, you want to keep it going for a bit).

The next aspect, selecting a pump, relates to the physical makeup of the system. What pressure do you want to achieve? Through how much line must the water travel? What appurtenances will cause the system to lose pressure? etc. In engineering terms, this is selecting the "head" of the pump - or it's ability to apply pressure to the system. Think of this in terms of how much strength does a pump have to push water up the hill. For this part, you will need to tell your contractor or pump supplier what your pressure requirements are, and what kind of system you plan to pump through, and they will be able to select a pump for you based on the pumps rating (i.e. 30 gpm at 100 feet of head; where 1 foot of head is equal to 0.433 psi). The HP of the pump doesn't tell you what it can deliver. The HP is a measure of the force required to turn the shaft/impeller at the rate required to supply the flow rate and pressure you desire. Once the pump is selected for flow rate and pressure, you know you have a good fit for your demands.

I hope this helps and that this didn't get too technical for you, but what can I say... I'm an engineer - it's required of me /forums/images/graemlins/smile.gif

 

[Slamfire]

Did I miss somewhere the gpm that the well can produce without going dry? It wouldn't do much good to pump 16 gpm if the well only fills at the rate of 10 gpm.

 

[bebster]

The well taps directly into a big aquafer and it won't run dry for anything I could imaging. If so, everyone in town would run out of water! For all practical purposes, it's a 1,000,000+ gallon tank that gets filled everytime it rains. Thanks for all the great insights so far.

 

[Junkman]

I don't know how things are for water supply at the Cape, but where I am in CT, we talk about 10 GPM equaling the amount of water that comes back into the well casing as you draw the water down. My well produces 2.5 GPM, but having 350' of depth, 4" in diameter give me a good reserve. I have no idea how large the aquifer is, but I do know that in theory, I could pump my well dry if I had a pump that could remove the water from the well faster than it can be replaced. The well installers should have told you the GPM that your well produces. If it is a high number, then you don't have to worry about a water shortage. In a drought season, we have to consider water usage, because the aquifer might not have enough to replenish the well at the same rate.

 

[Currmudgeon]

The issue is not pumping the aquifer dry. It is the rate at which your well can suck water out of the aquifer. This depends on a number of items, but the biggest two are the transmissivity of the aquifer (How easily you can drive water through the sand) and the length of your screen in the aquifer.

Any time you pump a well, you draw down the surface of the aquifer, even if only a little. It's part of the "Water flows down hill" thing. The harder you pump, the deeper the "cone of depression." If the cone of depression gets below your pump, it doesn't matter how many billion gallons of water there are in the county, you ain't gettin any more of it.

If you have a gravel aquifer and a well that penetrates deeply into the aquifer, then you can probably pump hell out of it and have no problems. This is one reason the well is usually extended considerably below the water table and the pump is set deep.

If you have a tight aquifer you may be able to suck your well dry. Then you simply have to wait until if recovers. In that case, you may be better off with a smaller pump and a bigger tank.

Well drillers determine the numbers by attaching a big pump to the well and measuring the distance the water draws down.

 

[EastTexFrank]

Don't you luv it when injenears talk injenearing stuff.

You've got some pretty good explanations there and I hope the information helps. Basically, you start at the end and work backwards. Figure out the number of heads and gpm of the nozzles you need for coverage and see if the well and pump can deliver that amount with a contingency built in for pressure losses in the lines. If it can't, you have to reduce the number of heads working at one time by installing control boxes and increasing the number of stations or using bigger storage tanks.

What I did was to sit down with the well drilling company, the irrigation company and the electrician running the power line and over a cup of coffee we came up with a design that's worked well for the past 7 years. With hindsight, the only thing that I might have changed was to use a single phase pump instead of the 3-phase that is installed. When we lose power at the house, we also lose co-op water, their original tower wasn't that much higher that our house, and without a large 3-phase generator or a converter I can't use the water well as back-up. That hasn't been a problem though since they built the new tower on a higher location.

The easiest way is to let Rainbird do the work for you.

 

[B7510]

Maybe this is another one of those stoopid quistions I frequently ask, but have you looked into drip irrigation?

It uses a fraction of the water (and power) that is used in conventional irrigation. Here are a couple of links if you are interested.

http://www.rainbird.com/ag/products/drip/index.htm

http://www.plumbingsupply.com/drip.html