How Long To Water: Point Source Emitters

How long to water your vegetables or landscape using point source emitters requires a different calculation than we used last week for emitterline. You still need to know the evapotranspiration numbers for your area (click here to learn all about evapotranspiration). In the coming weeks check back on the blog because we are going to make ET super easy for you. You also need to know your crop coefficients. Crop coefficients are properties of plants used to predict evapotranspiration (ET). Some plants need a lot more water than others. Knowing your plants specific water need is essential to knowing how long to water. In the next article we will cover crop coefficients and the importance of hydrozoning. Finally, now we will cover converting gallons per hour to inches per hour – also known as precipitation rate or application rate.

Gallons per Hour to Inches per Hour

To determine how many inches per hour your point source irrigation system waters you need to collect a little information. This includes emitter flow rate in gallons per hour. This is available on technical specification sheets of your products. You also need to estimate the amount of area the point source emitter will water. This varies with pressure so you will need to know the pressure of your system as well. Once you collect the information you can use the formula below to calculate the application rate of a point source emitter:

Application or Precip Rate = 231.1 x q
Area of Coverage

q = Point source emitter flow rate in Gallons per Hour

For example if you are using 1 Shrubbler to water one plant and the Shrubbler emits 4 gph at 15 pounds of pressure, the area covered is 12 inches by 12 inches and your calculation would look like this:

Application or precip rate = 231.1 x 4
                                                  12 x 12

Or a total of 6.4 inches per hour.

RunTime Calculation

Using the information above a run time of one hour will result in 6.4 inches of water applied, or .10 inches per minute. If the reported ET for the week was 1 inch and we need to add 1 inch of water to the soil we need to run our controller for 10 minutes on that particular zone.

1 inch by .10 (the inches per min) = 10 minutes to get 1 inch of water.

Remember this is the amount of water per week. We can divide the 10 minutes by the number of days we want to water. If you want to water two times a week it would be 10 divided by 2 for 5 minutes per watering cycle.

There are many additional variables needed to apply the appropriate amount of water to your garden or landscape. This covers just one basic concept. Other factors like soil, sun intensity, root depth, plant type, sprinkler efficiency, slopes and micro-climates (just to name a few key factors) all contribute as well. In future articles we will be discussing all those factors as well as how best to calculate run times for sparse plantings using point source emitters. If you enjoyed this article please consider subscribing to the blog or following me on twitter @H2oTrends.


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2 Responses

    1. There are 231 cubic inches per gallon of water. And if there are 144 sq inches per sq ft we know 1 gallon of water sitting on 1 sq ft area would be 1.604 inches deep (231/144 = 1.604). Or, 1″ of water sitting on 1 sq ft is .623 gallons..

      When converting gallons per hour to inches per hour we need to know inches per gallon – the gallons cancel out and we’re left with inches and hours, and ultimately inches per hour.

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