Using the ET Watering Index

Check the index once a week, on the first of your assigned watering days. The "weekly watering amount" shown is the amount needed over the next 7 days to replenish your lawn. If you're watering only once weekly, apply the entire amount to your lawn. If you prefer to water twice a week, apply half the watering amount on your first scheduled watering day, and the remainder on your second day.

What do all these numbers mean?

These numbers show various weather data that help determine how much water your lawn really needs. High and low temperatures (shown) are just two of the factors used to calculate the potential evapotranspiration amount, shown here as "ETo." You can think of ETo as the amount of water a plant would need if left constantly exposed to the sun, above the ground. Since most plants are actually in soil and have varying amounts of sunlight throughout the day, we apply stress coeffients for an average St. Augustine lawn in Austin to determine the ET Index.

What is evapotranspiration?

Evapotranspiration is a measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e., evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). The equation used to determine potential ET (ETo) considers a large number of weather factors, including wind, temperature, dew point, humidity and solar radiation.

If we know how much water is lost each day, why don't we water daily?

Allowing your lawn to dry out between waterings actually encourages a deeper root structure -- the grass has to look deeper in the soil for the water it needs. Plants with deeper roots are better able to withstand drought, harder to uproot, and can be more resistant to disease. Plus, the daily ET amounts for Austin are usually very small. Most automatic irrigation systems cannot be adjusted to water less than 1/4" with each operation, so it would be next to impossible to apply only a tenth of an inch daily.

How do I know when I've watered enough?

Set empty tuna or cat food cans at intervals around your lawn and garden. Turn on your sprinkler for 15 minutes. Measure the amount of water in the cans. Now that you know how much water your sprinkler puts out, you can figure out how long to leave it running when it's time to water.

Example: After 15 minutes of running your sprinkler, you have an average of 1/4" in your tuna cans. When the ET Index is 1/2" (and you haven't had any rain), you should run your sprinkler for 30 minutes. For an ET of 1/3", you'd run the sprinkler for about 20 minutes.