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Measure Your Water Pressure Backwoods Water Method

 

caution
STOP! If you’re using one of those design-it-yourself brochures or websites all bets are off! You need to either forget you ever read them or do not continue. You can NOT try to mix the “almost guessing” methods most of them use with the method in this tutorial. If you are going to use this tutorial (and you should!) you need to use ONLY this tutorial. If not you are going to be in big trouble. Gravity flow is tricky, this is not going to be an off-the-shelf irrigation system.  Use this tutorial exclusively and save yourself a lot of grief and get a professional quality sprinkler system. Please, please, please! Thank you. You are helping save my sanity. Now on with the tutorial.

Tip

There is a Design Data Form that you can print that will make things easier for you. If you would prefer a pdf version see PDF Design Data Form.

What in the World?

This is going to be interesting. The “Backwoods Water Method” is the catch-all category for everything that doesn’t fall into the other two categories (city water or systems that use pumps.) I’ve no idea what kind of water system you have, but it seems that most likely it will be some type of gravity flow system, so here’s some information about gravity flow systems. The principles here apply to just about any type of system, so you should be able to figure out at least a rough idea of your water supply using this information.

cautionDo not use the methods below if you have a pump or if your water comes from a water company pipe!

When dealing with gravity flow systems your water supply is effected by at least three different factors. They are water availability, pipe size, and the elevation of the water supply above your irrigated area (known as “pressure head”). These are really the same factors that determine water supply for all irrigation systems, you will simply measure them using different methods.


Measure the flow.

If your system ever runs dry, or the flow appears to vary from time to time, you will need to take that into consideration when measuring your flow. You really should measure flow at a “worst case” time, that is, at a time when you are experiencing low water availability. This is probably not practical, so you may need to do a bit of guessing and adjust your test results accordingly.

The “Bucket Method”. We’ll start by assuming your water is already being piped to the location of the proposed irrigation system (or someplace close to it.) A typical situation would be a small dam created using sandbags in a stream and a pipe is stuck in under the sandbags. Water collects in the area behind the sandbags and some of it is diverted into the pipe. Normally a piece of nylon or galvanized steel window screen or mesh hardware cloth is placed over the pipe inlet to keep out small fish and twigs. The pipe transports the water to the area you want to irrigate. The Bucket Method of measuring flow is pretty easy, but you may get wet! Simply measure the time in seconds it takes to fill a 1 gallon container from the pipe! Measure the flow at the downhill end of the pipe. If there is a hose on the pipe end, take it off as the hose will restrict the flow. To assure a more accurate measurement turn on the water and allow it to flow freely for a few minutes before you take the measurement. Avoid measuring the flow from a small valve such as a hose bib, as the valve may substantially reduce the flow. Remove the valve and measure the full flow from the open pipe end if possible. Get a one gallon container, and time how long it takes to fill it with water. For the best accuracy measure the flow 3 or 4 times and average the times together. The formula to find GPM is 60 divided by the seconds it takes to fill a one gallon container (60 / seconds = GPM).

pencil

Enter the Maximum GPM (inflow) on your Design Data Form.

Example: The one gallon container fills in 5 seconds.
60 / 5 = 12 GPM.

(60 divided by 5 equals 12 gallons per minute.)

If you have a high flow you may need to use a larger container to get an accurate reading. To determine GPM using a larger container take the container capacity in gallons, divided it by the number of seconds needed to fill container, then multiply times 60. The result is the GPM.

Container size in gallons / Seconds to fill container X 60 = GPM

Example: Using a 5 gallon container it takes 14 seconds to fill the container.

5 / 14 X 60 = 21.4 GPM.

(5 divided by 14, then multiplied times 60, equals 21.4 GPM.)

 

If you have a Storage Tank.

If you do not have a water storage tank skip down to the section titled “Pressure Head”.

Hopefully the tank is on a hill, or a tower or some other elevated location. If not, the tank won’t help, so skip down to the next section.
If you have a storage tank you MUST measure the water inflow to the tank. Do NOT design your system based only on system outflow, which often exceeds inflow. You must measure both inflow and outflow, and design based on whichever is LESS! To start, drain the storage tank. Now shut off the flow out of the tank completely. Time how long it takes to fill the tank. If you don’t know the tank capacity in gallons you will need to find it (The formula is at the bottom of this page). Divide the capacity of the full tank in gallons by the time it takes in minutes to fill the tank. The result is your “Tank Inflow GPM”.

Example: A 200 gallon tank fills in 10 minutes.
200 / 10 = 20 GPM

If you have a storage tank with a capacity over 1000 gallons you may be able to increase your Tank Inflow GPM by “buffering” it. Multiply the number of hours you will be irrigating per day by 60 (you will probably need to guess the number of hours you will be irrigating, so guess low to be safe). Keep in mind that the irrigation hours per day plus the hours it takes to fill the tank may not be greater than 24 hours! Divide the tank capacity by this number to get the buffer GPM. Add this buffer GPM to the old Tank Inflow GPM to get the new higher Tank Inflow GPM. Buffering simply takes into account the fact that as the water is flowing out to the irrigation system, water is also flowing into the tank helping to refill it. So it will not empty out as quickly as it would if there were no water flowing in! I know that was confusing, so look at the example below which will help clarify. Let me worry about why it works (it does), you just do the math!

Example: 2500 gallon tank capacity. You plan to run the irrigation system for 8 hours per day. When totally empty the tank takes less than 12 hours to refill. 8 hours + 12 hours is less than 24 so we can buffer the Tank Inflow GPM. The buffer formula is:

2500 / (8 * 60) = 5 GPM.

So if the original Tank Inflow GPM was 4 GPM, the new buffered Tank Inflow GPM will now be 9 GPM (4+5=9).

pencil

Enter the “Tank Inflow GPM” on your Design Data Form.

 

 

How to measure flow in gravity flow systems.

Simple Gravity Flow Water System using a Tank.

 


Pressure Head:

Now we need to measure the amount of “pressure head”. There are two methods, you can use either.

Method #1. Pressure head is based on elevation or, in this case, the height of the tank above the highest area to be irrigated. If you don’t have a tank it is the height from the point where the water enters the pipe. This height is the elevation difference, not the distance away. In other words, if you imagine a level line extending from the tank over your yard, it is the height that line would be above your yard. Take a look at the drawing above. The water pressure in PSI can be determined by multiplying 0.433 times the height (feet) of the tank above the yard. It’s that simple, don’t try to make it harder! It doesn’t matter if the tank is on the top of a cliff adjacent to the yard, or if the tank is a mile away on a hill. As long as the elevation is the same, the pressure will be the same! It’s one of those abstract hydraulic principles I told you about that are hard to understand. (O.K., no doubt some hot shot out there wants to argue with me. So here’s an exception. If the tank was far enough away, much farther than would ever apply here, the pressure COULD vary due to changes in the gravitational pull of the earth and moon. Wow, isn’t that “cosmic”!)

Example #1: Tank is 100′ away on a hill behind the yard. Tank elevation is 70′ higher than the yard.

70 * 0.433 = 30 PSI.

Example #2: Tank is 1000′ away on the side of a mountain. Tank elevation is 70′ higher than the yard.

70 * 0.433 = 30 PSI. Pressure is STILL 30 PSI!

Method #2. An alternate method of measuring pressure is to install a pressure gauge (you can buy them at most plumbing stores) on the water pipe at the pipe outlet or the point you plan to tap into it for the irrigation system. This is probably the easiest method for most people. The water must not be running (turn off all faucets) when you take the measurement! (That’s why it’s called “static” pressure.) Read the PSI from the gauge. Warning: your water system may already have a pressure gauge installed on it. Inexpensive gauges (an expensive top-quality gauge will say something similar to “liquid filled” on the dial) tend to loose their accuracy after a year or two of use, so you may not want to rely on an old gauge.

pencil

Enter the pressure you calculated or measured in the space labeled “Design Pressure”on the Design Data Form.

cautionPossible bad news: If you have less than 25 PSI you just don’t have enough pressure for a standard automatic irrigation system to work well (the automatic valves need a higher pressure to work). You will need to use a manual control system or add a pump.  But before you give up, see my article about automation of a rain barrel irrigation system for some other possible options using automatic valves made for heating systems.


Initial Design Flow:

If you don’t have a storage tank the Initial Design Flow will be the same as the Maximum GPM you measured using the “Bucket Method”. Pretty simple! If you Do have a storage tank the Initial Design Flow will be the lower of your “Maximum GPM” or your “Tank Inflow GPM”. So if your Maximum GPM was measured at 20 GPM, and your Tank Inflow GPM was 18 GPM, your Initial Design Flow will be 18 GPM because it is lower than 20 GPM. Use the lower number! Still pretty simple!

pencil

Enter the “Initial Design Flow” on your Design Data Form.

caution

This is really important! Later in step #3 of the tutorial you will determine the friction loss in your mainline. With a gravity flow system your mainline includes the pipe that brings the water to your yard from the water source! If you have a pipe that goes from the water source to a tank, you do not need to include the pipe that goes to the tank. But the pipe that goes from the tank to the yard is part of the mainline. So when you calculate the mainline friction loss you will need to calculate it for both the pipe going from the water source (or tank) to where your sprinkler system taps in to the pipe, and also the pipe from the sprinkler system tap to the valves. You then add the friction loss for both pipes together to get the “mainline friction loss”. You might want to write this down on the Design Data Form so you don’t forget!

Example: Joe Backwoods has a pipe (pipe #1) that runs from the creek way up the canyon to a storage tank on the hill above his house. From the storage tank, a second pipe (pipe #2) takes water down the hill to his house. Next to the house Joe plans to tap a new pipe (pipe #3) into the house supply pipe to take water to his new sprinkler system valves. Pipes #2 and pipe #3 are considered part of the irrigation system mainline. Joe will need to calculate the friction loss in both those pipes and add it together to find out his “mainline friction loss”. Joe isn’t worried about how to calculate the friction loss, because Joe knows that he will learn how to do it in Step #3 of the tutorial!


Do you have enough water available?

You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters). So if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs.

If you don’t have enough water you will either need to find a larger water supply, or reduce the amount of area watered. Another option is to plant shrubs and use drip irrigation on them. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.

why

There are only so many hours in the day to water. The amount of water needed varies with the climate, these values are typical for hot summer areas where most sprinkler systems are installed (daily high temperatures over 90 degrees F., 32 degrees C.) These values assume you would water as much as 10 hours per day. Water more time and you can have more area irrigated.


Related things…

Filtration:

You should probably consider installing a filter of some type on your system if the water is from a river, stream, pond, or lake. There’s a pretty good chance that all kinds of crud are in the water such as algae, sand, mineral deposits, fish, snails and clams. (No kidding!) All of these things can damage your irrigation system. For more information on filters see the irrigation filtration tutorial.

Storage Tank Capacity:

To find the capacity of an upright, round tank (measurements in inches):
radius * radius * depth * 0.01359 = gallons

example: 48 inch diameter X 48 inch high tank = 376 gallons
(24 * 24 * 48 * 0.01359 = 376)


This article is part of the Sprinkler Design Tutorial Series
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