Smart Watering: Adjusting Your Sprinklers for Optimal Soil Moisture

Objective

The goal of this project is to conduct a water audit of your home sprinkler system to see if you can save water while keeping your lawn and garden lush.

Introduction

Watering your lawn and garden can easily account for more water use than anything else around the house. This project will show you how to make sure that your sprinkler system is not wasting water.

The easiest way to measure soil moisture is to use a soil probe (see the Materials & Equipment section, below). You should be able to find one at a hardware store or gardening center near you. With the the soil probe you can take a small "core sample" from your lawn so that you can see the profile of soil moisture at the surface and at the plant roots. It's a lot easier than digging a hole with a shovel or trowel, and it disturbs a much smaller area.

The soil probe is also useful for gathering soil samples for soil nutrient testing. Buy a soil test kit and use it to measure the nutrients in your soil. This way you can buy fertilizers that replace only the nutrients that your soil needs.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• root depth,
• soil moisture.

Bibliography

• Barlow, Dooly, 2006. "Maintaining Sprinkler Systems Help Conserve Water," The Samuel Roberts Noble Foundation [accessed June 23, 2006] http://www.noble.org/Ag/Horticulture/MaintainingSprinklers/Index.htm.
• McCausland, J., 2006. "Smart Summer Watering Tips," Sunset July, 2006: 68.
• City of Greeley, 2006. "Watering Restrictions," Water Conservation, City of Greeley, Colorado [accessed June 23, 2006] http://www.greeleygov.com/cog/pageHome.asp?fkOrgID=41.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• several (5–10) same-size containers for collecting water (empty cans work great),
• instructions for your sprinkler controller,
• shovel, trowel, or soil probe (see picture, below) to test soil moisture at root depth. You should be able to find a soil probe at your local garden center or the gardening department of the hardware store.

  A soil probe (shown above) is a tool for taking a core sample of soil. It is very useful for checking the moisture content of the soil at root depth. You can also use it for collecting soil samples for nutrient testing.

• Optional: monthly water bills with record of usage from your parent (may be available online from your water company),
• camera for before and after photos of lawn and garden areas.

Experimental Procedure

Measuring Soil Moisture at the Roots

You will be measuring soil moisture many times during this project in order to figure out how quickly your lawn dries out, and how deeply water penetrates when you run the sprinklers. Here is a procedure for measuring soil moisture with a soil probe (City of Greeley, 2006).

1. Push the soil probe into the soil as far as it will go. (If it will not go in, the soil is too dry or compacted. You will need to water the soil first.)
2. Twist the soil probe slightly and pull it out of the ground.
3. Feel the soil. If the soil is moist, there is no need to water at this time.
4. Check the root depth. Look at the soil sample to see how far the grass roots extend into the soil. The deeper the roots, the healthier the landscaping. Your sprinklers only need to water enough to penetrate to the root depth. The grass can't absorb water from below this depth, so water penetrating further than this is wasted.
5. Optional: take pictures of the soil cores to show the different moisture conditions on your display board.
6. After you have made your measurements, replace the soil sample in the ground and tamp it gently into place.
7. The pictures below show different moisture conditions you might find (City of Greeley, 2006).

   Wet Below the Roots. Rains can move water deep into the soil well past plant roots. Consistent watering below the root zone only wastes water. Use the soil probe to measure the root and moisture depth. Picture shows severe over-watering, note the muddy soil on top, and very shallow root depth.
   Dry on Top and Wet Below. This is the most common finding. If the soil looks dry on top, from the sun, it is thought that it's time to water. Use the probe to check the moisture in the plant's root zone before turning on the water. This picture shows ample moisture in the root zone.
   Dry on Top and Dry Below. Observe the plants, if they have begun to show signs of stress, watering was needed sooner. However, if there are no signs of stress or wilting, you can see how long to let the soil dry before watering. In this case, the turf is still very green, the water can remain off.

Checking Out Your Sprinkler System

1. Have one of your parents show you how to use the electronic controller for the sprinkler system, or learn how to use it by reading the instruction manual.
2. Go through the controller's programs and write down the number of minutes that the sprinklers are turned on for each zone, and the number of times per week that each zone's sprinklers are activated. Calculate the total number of minutes per week for each zone. This is your baseline number.
3. Check out each zone for uniformity of water distribution.
   1. Randomly place cans within the zone and run the sprinklers for the zone for 5 or 10 minutes. To prevent tip overs, use a double cup to measure the water. Pin one cup down in the grass with a golf tee, then put a second cup inside it to collect the water. No tip overs!
   2. While the sprinklers are running, check to make sure that each head is operating properly. Check for proper alignment and flow rate, and make sure that there are no leaks.
   3. Measure the depth of water in each can.
   4. Calculate the average depth of water for all of the cans. This will tell you how many centimeters of water your system delivers to the zone within the 5 or 10 minute time period.
   5. Calculate the standard deviation of the water depth for all of the cans.
   6. If any of the cans has significantly more or less water than the others (say, more than 2 standard deviations from the mean) check the sprinkler heads in the area around that can. If there was too little water, the head may be misaligned, or it may have too low a flow rate, or it may be clogged. If there was too much water, the head may have too high a flow rate or a leak.

Use Soil Moisture to Optimize Sprinkler Running Time

1. Make a map of your yard and identify the different microenvironments for soil moisture sampling. For example, shady vs. sunny areas, grassy areas vs. garden areas, high vs. low areas. Be observant and note where soil dries out more or less quickly than other areas. At a minimum, you will want to check the soil in each of the control zones of your sprinkler system.
2. Optional: take "before" pictures of each zone on your map.
3. Use the soil moisture testing procedure above. If your soil probe showed moist soil, there is no need to water at this time. Adjust the watering time on your irrigation system's clock, or better yet, turn off the automatic sprinklers completely until the soil dries out.
4. When you water, keep track of the number of minutes your sprinkler system runs. From your water can calculations (above) you can figure out how many centimeters of water you are delivering with each cycle. Adjust the number of minutes the sprinkler system runs for each zone according to your soil moisture measurements. (Decrease the number of minutes if the soil is too wet; increase the number of minutes if the soil is too dry.)
5. Repeat soil probe sample periodically until you understand the water needs of various areas of your landscaping. At first, you should test before and after each sprinkler run. As you collect more measurements, you should be able to use them to predict how much water each area of your lawn needs. For each zone, make a graph of soil moisture depth vs. number of centimeters of water delivered by the sprinkler system.
6. If it rains during your experiment, you can put out cans to measure the amount of rainfall (or check the local weather report if you're not able to do this). Check with your soil probe to see how far the rainwater penetrated into the soil. If you have enough data from your sprinkler system, you might be able to make a good guess at how much rain fell!
7. Soil probes can also be used to monitor moisture in flower beds, ground cover areas, and other plantings. Remember: these areas generally need about half the water needed by grass.
8. After 1–2 weeks, compare the amount of water you use for your lawn now to the amount at the beginning of the experiment. Are you using more or less?
9. Optional: take "after" pictures of each zone on your map. Are your lawn and garden greener and healthier than before?
   

Variations

• Can Mulch Reduce Garden Water Requirements? Divide a part of your garden into two equal plots, with each plot receiving equal amounts of sun. Cover one plot with five centimeters of organic mulch, such as compost or ground bark. Leave the other plot uncovered. Use the same amount of water for each plot for two or three weeks. At the beginning of the experiment, and at one-week intervals, dig down and check the soil in each plot for moisture content. Which plot holds water better? Which plot shows better plant growth? (McCausland, J., 2006)
• Does your lawn need more water in hotter weather? Keep track of the daily temperature in your lab notebook so you can graph soil moisture vs. temperature. Do you need to run the sprinklers longer to keep soil moisture optimal in hotter weather? Can you figure out how much additional water each zone requires for a 5° increase in daily temperature?

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• Peralez, V., 2005. "Water Conservation: What Can People Do to Conserve Water and Avoid Wet/Dry Spots?" California State Science Fair Abstract [accessed June 23, 2006] http://www.usc.edu/CSSF/History/2005/Projects/S0812.pdf.

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