If you’re planning to use a booster pump jump down to the heading Booster Pumps.Everyone else just continue on…

The following is oriented toward wells. If you don’t pump out of a well don’t panic, just substitute river, lake, pond, spring, mud-puddle, or whatever for “well “in the following procedures. “Top of well “would be the high water level of the river, lake… etc. Ok, that was easy enough, right? Let’s move on…

First you will need to find out the “Dynamic Water Depth “of the water in your well. Dynamic Water Depth is the depth of the water below the top of the well, in feet, when the pump is running. OK, I know what you’re thinking- “Jess, you idiot, if I don’t have a pump yet, how am I supposed to know what the water level is when the pump (which I don’t have yet!!) is running? Grrrr!!!” Well, of course you’re right, but as you probably guessed by now, there is a solution. When a well company drills a new well they insert a temporary pump to “break in “and test the well. They refer to this as “developing “the well. As part of this process they also measure the Dynamic Water Depth of your new well at various pumping rates. Your pump company should have a record of this information which they can give you. One warning- you really should have the test repeated if the well is more than 5 years old. Water levels often drop over time. If you can’t find the dynamic water depth and are too cheap to have it tested, you can use the well depth in place of the dynamic water depth. Your pump will likely waste some energy if you use the well depth, this is because the pump will probably be somewhat oversized. You may also find you have problems with the pump cycling on and off if you use a pressure switch to control it (pressure switches will be described later.) There are cures ($$$) for the pump cycling problem. They include special cycle-stopping valves installed at the pump outlet or using the pump start circuit on the irrigation controller to override the pressure switch and lock the pump on.

If you’re not going to be pumping from a well (ie; you are using water from a pond or stream) just use the lowest “dry year “water level of your water supply in place of the Dynamic Water Depth.

Note that the term “draw-down “is often erroneously used in place of Dynamic Water Depth. I often do this myself. So be sure to clarify when talking to your pump company. When the pump is running, the water level in the well drops below the water table. It may drop a few inches or more than 100 feet depending on the type of soil (or rock) the well is drilled into. Often the water level in wells drilled into rock will drop well over 100 feet when the pump is running, as the water can’t easily move into the well from the surrounding rock. At any rate, the real definition of “draw-down “is the distance the water drops in the well when the pump is running. But keep in mind that many people interchange the terms draw-down and Dynamic Water Depth. See the diagram below.

Now you need to figure out the “Elevation Difference “between the top of your well and the highest point in the area to be irrigated. That is, how much higher (or lower) is the highest point in the irrigated area than the top of the well. This may be a negative number if the well is higher than the irrigated area. See the drawing below.

Irrigation System operating pressure. There is one additional ingredient you need to add, which is the pressure to operate the irrigation system. This pressure will be calculated as part of the irrigation design process and if you have a design already it should be noted on the irrigation design. If not, ask the designer what it is, he/she should know. (Note that the emphasis here is on should, as opposed to does. Way too many designers don’t have a clue about pressures, which is a big sign that you better take a long, hard look at that design!). If you have an existing irrigation system that you want to add a new pump to, then you can try measuring the water pressure with a gauge at the point where you plan to tap the new pump into the system. This is one of those unusual cases where you want to measure the dynamic pressure, not the static pressure. So when you measure the pressure make sure that one of the irrigation system valves turned on and the sprinklers are running (run the largest circuit, the one with the most sprinklers).

Chances are you don’t have an irrigation system yet, or even a design. In this case you will need to make an “educated guess”. The following table will help you with your guess:

Minimum Pressures for Irrigation Systems

Drip Irrigation            =          70 feet head (30 PSI)

Spray Type Sprinkler Heads          =          93 feet head (40 PSI)

Rotor Type Sprinkler Heads           =          104 feet head (45 PSI)

Remember, the values above are estimates. Dependent on your actual design you may need more or less pressure. You should design your irrigation system and adjust these values for the actual design before purchasing a pump! Spray sprinklers feature a steady fan shaped pattern of water. Rotors type sprinklers are used for larger areas and feature streams of water that rotate around the sprinkler. See theSprinkler Irrigation Tutorial for more information.

To finish up your pressure requirement calculations you simply add the values of the Dynamic Water Depth, elevation head, and operating pressure head together to get the total head required. Remember that all the values should be in feet of head, not PSI!

Example: You measure a Dynamic Water Depth of 25 feet in your well. The irrigation system is 10 feet higher than the top of the well. You’re going to use rotor type sprinkler heads so you select an operating pressure of 104 feet head. Your total head required would be 25 + 10 + 104 = 139 feet of head!

Source: http://www.irrigationtutorials.com

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