Irrigation Calculator

Please work through the order of the following steps to calculate the water for irrigation.

Step 1: How green do you want your lawn to be?

Step 2: How much water does your irrigation system put out (in/hr)?

How to determine my preciptitation rate?

Step 3: How many times per week do you run your irrigation system?

Step 4: Is your turf TCWA qualified? 

Step 5: What month are you irrigating?

Step 6: Select a location on the map via clicking on the map or entering a location on the search box on the top right corner. This version only supports locations in the contigious United States (Alaska, Hawaii are excluded.)

Minutes Per Irrigation 

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Supported by Humanistic GIS Lab at University of Washington - Seattle, College of Earth, Ocean, and Atmospheric Sciences at Oregon State University and the division of agriculture, University of Arkansas.

Disclaimer

This calculator is intended only as a watering guide and is based on historical data. For greater accuracy we recommend consulting with your local extension agency. Please make sure your watering practice complies with state and local ordinances. For more information about this calculator contact us at info@tgwca.org.

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What impact does soil have on my water need?

While soil texture doesn’t impact water demand it does affect recommended watering frequency. Sandy soils require more frequent watering with shorter run times. If you’re unsure about your soil type check with your local extension agent.

How do I know if my turf is TWCA Qualified?

All TWCA qualified cultivars carry the TWCA seal. You can learn more about the TWCA Qualified cultivars at https://www.tgwca.org/list-of-qualified-products1.html. If you’re unsure it is best to assume your turf is not TWCA Qualified.

For more information contact us at   info@tgwca.org.

Determining Precipitation Rates of Your Irrigation System

This fairly simple step is crucial to maximizing the accuracy and effectiveness of the Water Star® Irrigation Calculation. 

You have two options for determining the irrigation rate:

Option 1) Direct Measurement
To perform Direct Measurement you’ll need:
  1. Stopwatch - to time the sprinkler runtime

  2. Catchcan(s) -  simple non-tapered containers are commercially available in a number of different retail locations, but any straight sided container will do.

  3. Ruler – for measuring the depth of water in each catchcan.


  4. Pen and notepad – for recording the depth of water in each catchcan.

  5. Calculator- for doing some calculations with the data that you gather.

Very simple, the Direct Measurement approach can be performed with a single catchcan or with multiple catchcans in the same zone at the same time.  Using multiple catchcans in the same hydrozone and averaging out the results gives a better overall view of the precipitation rate. 

These instructions will be given assuming you are using multiple cans in each hydrozone. 

First, place your cans throughout the hydrozone being tested.  **Make sure the sprinklers reach the cans.  Make sure the cans are empty.**

Next, turn on your sprinkler and run it for 10 minutes.  **Make sure you are timing the entire run time.**

Writing down the volumes from each catchcan makes doing the final calculations a breeze.

The calculations follow the equation:

                                         PR= (AW/time)*(60min./1hr) 

                                                                                                     PR = Precipitation Rate
                                                                                                     AW = Applied Water

**Note: If using multiple catchcans the calculation for Applied Water is:**

                                         AW=(AW1  + AW2 +…+ AWn)/n

                                                                                              n = the number of catchcans used in the hydrozone being tested.      

Now, enter the value you’ve calculated into the Precipitation Rate space in the Calculator tool.

Option 2)  Total Area Method

To apply the Total Area method you’ll need:

  1. Measuring tape and two stakes - for measuring the perimeter of the irrigated area

  2. Pen and notepad -  for writing down your calculations before making them with the calculator

  3. Manufacturer Performance Charts – this will give the application rate of each individual sprinkler head in the hydrozone being tested


Use the measuring tape and stakes to measure the edges of the hydrozone being inspected. 
Use these measurements to calculate the area of the hydrozone and write it in your notes as A=(your calculated area)sqft. 

After measuring the edges of your hydrozone, count the number of sprinklers in the hydrozone.  Write this number in your notes as “n=(the number of sprinkler heads in your area of interest)

Next, use the Manufacturer Performance Charts to reference the application rate of each sprinkler head.   Multiply this number times “n”, the number of sprinkler heads you counted in your area of interest.  Write this value down as GPM= (n x gallons per minute per sprinkler head)

Finally, use this equation:

                                            PR = (96.25*GPM)/A
                                                                                                   PR = Precipitation Rate

                                                                                                   96.25 = The constant that converts gallons per minute(GPM)
                                                                                                                   to inches per hour.

                                                                                                   GPM = the number of sprinklerheads x the application rate                                                                                                                  (gal/min).

                                                                                                   A = the area of your hydrozone

Now, enter the value you’ve calculated into the Precipitation Rate space in the Calculator tool.