Why does inline drip irrigation take so long?
Updated: Aug 3
For gardeners who are used to pop-up sprinklers and their run times, it is a surprise when they switch to drip systems how long they need to run their systems. If you are used to run times like 15 minutes for a zone and then switch to a smart irrigation controller with inline drip nozzles, the 60 minute run time seems absurd. But these nozzles are slow. So let's take a look at some of the math (simplified!) that a smart irrigation controller might take into account:
Let's take an example of a Dymondia ground cover (as a lawn substitute) as our example. I am going to make some (typical) assumptions on your drip system and use Sunnyvale,CA as my reference location.
The reference evapotranspiration (ET) rate for Sunnyvale is 6.5 in/hr in July (hottest month for us and this is what is used by algorithms to adjust Weather Based Irrigation Controllers ). This the number used by fescue grasses.
Therefore, weekly ET rate for the hottest month is = 6.5 in/h /31 *7 = 1.48 in /hr
The plant factor (how much of that reference a plant actually needs as compared to a fescue lawn) for Dymondia is 0.3 (range for low water plants is 0.1 to 0.3).
=> Dymondia needs in July = 1.48 in.hr * 0.3 = 0.44 in/week
Assuming a distribution uniformity of 0.9 for drip irrigation and a precipitation rate of 0.5 in/hour for your nozzles, let's see how much we need to irrigate:
=> 0.44 in/hr divided by 0.9
multiplied by 60 min/hr
= 59 mins per week
This is how long the Dymondia zone should run per week after establishment. If you have run-off or pooling issues where the water cannot be absorbed quickly enough (due to soil type, etc), then you need to do a cycle-and-soak run where you divide the need into multiple run times with a wait time in between.
Let's assume it takes 30 minutes for water to pool undesirablely, we need to stop watering at 30 mins and wait.
=> For 59 mins run time, we need 2 cycles, so we would run the system 59/2=~ 30 minutes with at least 30 mins wait in between for soaking.
What happens the next after establishment for low water plants?
You still need 59 minutes per week, but you can run it every other week. So multiply the need by 2 weeks = 59 * 2 = 118 mins for a 2-week cycle.
If runoff happens every 30 minutes, we need to run the irrigation 4 times for 30 minutes each with a 30 minute (or more) wait time between each run.
The smart irrigation controllers sometimes will have cycle-and-soak settings and will do this division automatically for you. And that's it for now! Best wishes and good health for you and your garden.
Additional information and Footnotes:
* There are other factors not discussed here such as distribution uniformity.
* For professionals, please take a look at QWEL training for in-depth examples
* Useful sites: