Why do we need to know?

How many gallons of water per hour does my water system deliver at the faucet? That's an important question when you are planning a drip irrigation circuit. If you have 100 gallons per hour available and your drip circuit uses 180 gallons per hour, each plant will receive less water than you plan.

In fact, some of the plants at the end of the circuit may be receiving close to no water at all due to friction losses in the tubing. It's important to match your drip irrigation circuit demand to your supply.

Let's test it

It's easy. All we need to do is find a container, and time how long it takes to fill it. Let's say that you have a 2-gallon bucket. First make sure that there are no large appliances in operation in the house, like a washer or dishwasher.

Then open the outdoor faucet all the way and time how many seconds it takes to fill the bucket. If it takes 20 seconds to fill the bucket, that works out to three buckets each minute or 6 gallons per minute. Multiply that times 60 minutes per hour and you come out with 360 gallons per hour.

Planning the circuit

That is enough water supply for 360 drippers rated at one gallon per hour or 180 drippers rated at two gallons per hour. At least theoretically. It's best to err on the side of caution when figuring how many drippers or sprayers you can have on a circuit. There is friction loss in the tubing to consider.

We will lose pressure in our 1/2" mainline tubing due to friction losses. That will limit the number of drippers on each circuit no matter what our faucet can deliver. Calculate a maximum combined total of 180 gallons per hour from all the drippers in one circuit, no matter how many gallons of water per hour your source provides. So you could have 90 drippers at two gallons per hour each or 180 drippers at one gallon per hour each.

If you use dripline instead of individual drippers, you could have 360 feet of combined dripline length if the drippers are spaced 12" apart and delivering one-half gallon per hour each. Feel free to experiment. Add drippers or dripline until you can see that the flow rate differs from the first to last dripper or that the flow rate is simply too low.

Keep sprinklers, misters and bubblers on a separate circuit unless the circuit is very short. They all use high volumes of water compared to drippers and dripline. If you are using 1/4" tubing as a mainline, keep to a maximum length of 100' including all branches.

Second circuit

What if we add up all the drippers or dripline or sprayers we need and it exceeds our supply or exceeds what our tubing can deliver? No problem. All you need to do is make two circuits instead of one. Then operate each circuit at a different time of day so that they do not compete for water. You can have one circuit for each faucet, each with its own timer. Or you can set up two circuits on one faucet.

A word of caution. Please hang only one standard timer unit on each outdoor faucet. Two or more timers may be too much weight and may damage the faucet or the pipes in the wall that the faucet is attached to. If you wish to have more than one timer on a faucet, you can attach a length of garden hose to a faucet manifold and then attach the second timer to the end of the garden hose. That is the subject of another How-To article on standpipes.