Ideally, you’ll be able to give your grass the exact amount of water that it needs without wasting water in the process. We use software that calculates exactly how much water is needed for a plant to thrive in any given environment. It’s a more complex process than many people imagine. In this article, we’ll explain the basic steps to determine how much water your grass (or any plant) needs.
Step 1: Determine your evapotranspiration rate.
Evapotranspiration is a big word with a pretty simple meaning. Evapotranspiration (ET) is the amount of water lost from the ground, either through evaporation or through plant use. Naturally, there is more water lost during hot months or when factors like wind and humidity increase evaporation.
Finding the right amount to water is critical for plant health
You can choose to get very specific with calculating the exact rate for your property. But it can be time consuming. If you’re doing these calculations yourself, it is easier to look for an average ET rate for your region (note: Banyan Water does calculate ET rates for individual properties, and for each zone on a property). For example, in central Texas, the following rates are reasonable averages:
Winter: 0.35 inches per week
Spring: 1.0 inches per week
Summer: 1.4 inches per week
Fall: 1.0 inches per week
Step 2: Determine how much water your property is getting from rain or irrigation.
Rain is wonderful and delivers a lot more water per hour to your property than most irrigation systems. You can always check local news or weather data to see exactly how much rain you received. Here are some amount references:
Rain Shower: 1” – 2” per hour
Hard rain: 3” – 4” per hour
Flooding Rain: 7” – 15” per hour
Irrigation systems vary quite a bit depending on how they’re set up, the amount of pressure that they receive, and how well they are maintained. Here are some typical irrigation rates for various types of systems:
Drip Irrigation: 0.5” – 0.75” per hour
Spray Nozzles: 1.25” – 1.75” per hour
Traditional Rotors: 0.75” – 1.25” per hour
Determine the amount of water being delivered by your irrigation system by placing tuna cans (or any cans with flat bottoms and straight sides) around your watering area and water for 30 minutes. The depth of water in the cans (x2) will tell you how much water per hour your system delivers.
Step 3: Calculate your plants’ weekly water needs.
Different types of plants need different amounts of water each week. For our calculations, we multiply by the following numbers:
Water-loving plants and grass – water need factor: 1
Drought-tolerant plants and Bermuda grass – water need factor: 0.6
Succulents – water need factor: 0.3
If the area that you are watering gets a lot of sun, increase the water need factor a little, if it’s in the shade, decrease slightly.
Step 4: Put it all together.
Now you’ve calculated how much water you are losing to ET, how much you are adding back within an hour, and how much your plants need. Combining all of this information will give you a pretty good idea of how much you should be watering each week.
(Water need factor ÷ number of watering days per week ÷ irrigation rate)
x 60 = the amount of time to run your irrigation system each time you water
Note that when it rains, this calculation changes and you may want to water significantly less. To determine when to start watering again after rain, check the soil moisture. Dig down 2” and get a little soil to roll between your fingers. If it feels moist, you don’t need to water. Continue to check every day or two until it feels dry; then begin watering again. Of course, if plants are showing any signs of heat or drought stress, start watering again sooner.
ET rates, soil types, and other factors can vary widely by region, so use this calculation as a guide, but pay attention to your landscape health when you’re watering to achieve the right schedules for your property. It is likely that if you have different areas on your property with different conditions and plants. For the most efficient water use, you’ll want to calculate watering amounts for each zone, rather than use one calculation for the entire property.
The following infographic provides a handy visual reference to walk you through these calculations.