Maintaining a Water-wise Landscape

The water requirements of a plant are determined by how much water is lost through the combination of evaporation and transpiration, also known as evapotranspiration (ET). The speed at which water evaporates from the soil surface and plants transpire is affected by many factors such as plant type, temperature, wind, humidity, and more. While the calculation of ET is complex, the application is simple and very useful. As temperatures increase, the water requirement of the plant increases. When temperatures are cooler, ET decreases and water is applied less frequently. In short, plant water requirements change as the weather changes. For context, ET is measured in inches and will be used interchangeably with water need throughout this publication. ET data for various statewide locations can be found on the Utah Climate Center Website. An informative video detailing how to use the Climate Center website is available on Utah State University's Center for Water Efficient Landscaping’s YouTube channel.

Water budgets

A water budget is a tool used to estimate or determine the water needs of a landscape. It is useful to access historic ET data from sources like the USU Climate Center to estimate seasonal irrigation needs and develop a tentative irrigation schedule. While the schedule will need to be adjusted based on real-time weather conditions, it is a useful baseline. Similar to ET, there are several factors that impact the water budget:

1. Evapotranspiration (ET). As previously mentioned, the Utah Climate Center has a series of weather stations throughout the state that gather ET data. These weather stations calculate a reference ET (ETo), or water need, of a single plant type, cool-season turfgrass. The calculated water need of this single plant type is a reference point for calculating the water need of other plant types. ET data collected from weather stations is called the reference ET or ETo.
2. Plant type. Plants are commonly categorized by water need, i.e., low, moderate, or high water use, but what does that really mean? When a plant tag or catalog description refers to a plant as a low or high water user, they are comparing the water need of that plant to the water need of the reference plant (cool-season turfgrass), aka reference ET. Reference ET is a tool used to calculate the water needs of other plants. For example, low-water-use plants require 10-30% of the water that the reference plant does. If the historic reference ET or ETo for the month of July is 10 inches, low-water-use plants will need 10-30% of this amount, or 1-3 inches. Figure 2 describes plant water needs compared to reference ET. These are broad categories. When the water need of an individual plant has been studied and a specific water requirement has been determined, it is referred to as a plant factor or crop coefficient. For example, research has determined that the Sedum acre (aka stonecrop) has a plant factor of 0.25, meaning that Sedum needs 25% of the water that the reference crop needs, ETo. If a specific plant factor is not known, it is wise to start in the middle of the recommendation and adjust up or down as needed.
3. Irrigated area. When calculating a water budget in inches of water needed, the size of the area being irrigated is not a factor (water need = ETo X the plant factor). When converting inches to gallons, discussed below, the size of the area being irrigated plays a role. The goal of the irrigator is to wet 50-60% of the plant’s root zone. This is typically accomplished by placing the irrigation system within the drip line of the plant. The drip line is the area beneath the plant canopy. For columnar type plants (tall and slender), this is not an effective method. Rule of thumb: there is a correlation between root spread and plant height. For example, if a tree is 15 feet tall, the roots spread 15 feet wide on either side.

Converting inches to gallons

The reference ET is calculated in inches, which is useful when irrigating with a sprinkler system but not as applicable when using drip irrigation. To convert inches needed to gallons needed, multiply the inches needed by the size of the area that is being irrigated and 0.623, which is a conversion factor that converts inches to gallons. It takes 0.623 gallons of water to apply 1 inch of water to 1 square foot of space.

Gallons Needed = Inches needed X Irrigated area X 0.623

Practical Water Budget Example

Let’s pretend we live in Cedar City, UT and we have a planting area with 3 established low-water-use perennials. Each plant is roughly 18” tall and wide. After accessing the data available on the USU Climate Center website we determine that the average historic reference ET for the month of June is 7”. Here’s what we know:

Water Budget Calculations:

1. Water need in Inches = ETo X the plant factor
2. Gallons Needed = Inches needed X Irrigated area X 0.623

Factors that Impact the Water Budget:

1. Reference ET = 7
2. Plant type = low water use (10-30% of ETo)
3. Irrigated area = 1.75 ft2 (calculate the area of a circle with an 18” diameter)

Plugging in the Numbers

Reminder: low water use plants need 10-30% of the water that the reference crop requires. If you’re not sure what the specific plant factor is, start in the middle and adjust up or down as needed.

Math tip: 10%=.10, 20%=.20, 30%=.30, etc.

Summary

Each of the three low water use perennials in this example will need roughly 1.5 gallons of water during the average June in Cedar City UT. Next let’s determine if we should divide the water requirement of 1.5 gallons into multiple irrigation events over the course of the month and if so, how much water we should apply with each irrigation.

One of the main reasons plants die in the landscape is due to overwatering. It is rare to have a situation where daily watering is necessary, yet it’s a common practice. Once the water need is calculated, determining irrigation frequency can be as simple as digging a small hole to inspect the soil moisture content. The soil surface may look dry, but digging down 2-3 inches may tell a different story. If the top few inches of soil are dry, it’s time to water. If the soil feels moist, wait a couple of days and check again.

A more calculated approach to determining irrigation frequency is to utilize  ET data and consider the plant classification, depth of the plant’s root system, and soil type. Plants with a higher plant factor, covered above, will require more frequent irrigations when compared to those with a lower plant factor. The soil type determines how much water the soil can hold, which affects the amount of water that can be applied with each irrigation as well as the frequency of irrigation. 

Irrigation depth

Figure 3 provides a target watering depth based on plant type. It will take a larger plant with an extensive root system, like a tree, longer to deplete half of the water from the top 2 feet of soil than it will take a lawn to deplete half of the water from the top 6 inches of soil, which is why the lawn is irrigated more frequently but with less water than trees.

Depletion rate

The goal when watering the landscape is to irrigate once 50% of the available water in the soil has been used by the plant and lost through evaporation. The previously mentioned example of digging a small hole to evaluate soil moisture content is effective but not always practical. As discussed previously, soils covered with a 2- to 4-inch layer of mulch will lose less water through evaporation than those without.

Soil type

Water holding capacity varies by soil texture. Sandy soil types have a low water holding capacity and will need more frequent irrigation than those with higher clay content. The water needs of the plant do not change, but the irrigation frequency will. Figure 4 (Figure 4. Available Water Capacity by Soil Texture. Source: noble.org)  illustrates the water holding capacities for different soil types. For an accurate soil texture classification, soil samples can be mailed to the Analytical Laboratories at Utah State University. 

Practical Example

We previously determined that low-water-use perennials need roughly 1.5 gallons of water during the average June in Cedar City UT. Soil test results determined that we have a silty clay loam soil type that can hold roughly 2” of water for every foot of soil. Here’s what we know:

1. Irrigation Depth. The majority of our perennial plant roots are growing in the top 12” of soil Figure 3.  This is our target irrigation depth.  
2. Soil Type. Our soil type can hold 2” of water in 12” of soil, which is our target irrigation depth.  The soil does not have the ability to hold more than 2” of water within this root zone. 
3. Depletion rate. The goal when watering the landscape is to irrigate once 50% of the available water in the soil has been used by the plant and lost through evaporation.  In this example, our goal is to irrigate once 1” (50% of 2”) has been lost through evapotranspiration.   
4. Using our math skills we can determine that 1 gallon of water will wet our ideal irrigation area of 1.75 ft2 with 1” of water.  

Summary

Our water budget calculations in the previous section determined that we need 1.5 gallons of water during the month of June.  Based on soil type, irrigated area, depth, and depletion rate, we determined that we need to apply 1 gallon of water with each irrigation event. In this situation, applying one gallon of water every 15-20 days would be appropriate. 

The application rate of 1 gallon of water with each irrigation event will not change unless the soil type changes.  The frequency may need to be adjusted based on real-time weather conditions. 

Once the water need and irrigation frequency are determined the final piece of the puzzle is calculating how long to run the irrigation system.  In order to accomplish this, we need to know how quickly our system applies water. 

Precipitation rate

Precipitation rate is the speed that water is applied to the area that is being  irrigated. Drip systems are fairly easy to calculate.  For example, if you have 1, 1 gallon/hr emitter that waters a plant, that plant is receiving 1 gallon of water per hour.  If you have 10, 1 gallon/hr emitters that are watering a plant, that plant is receiving 10 gallons of water per hour.   

Sprinklers systems are more complicated to calculate. USU, in partnership with water districts and municipalities throughout the state, offer free water checks to help.  The Water Check Program sends trained technicians to your home or business to perform a water audit of your irrigation system.  Their goal is to determine the precipitation rate of your system which they then use to calculate how long you should run your system. This information can then be used to program a Smart Irrigation Controller or translated into a runtime and used to program your standard irrigation controller.  The calculated runtime won’t change during the growing season, but the frequency of irrigation will change as ET changes (remember, ET is affected by weather conditions).  Find more information on lawn watering, water check, and statewide resources in the Watering the Landscape: Make it Easier with Evapotranspiration (ET) fact sheet.  

Practical Example

We previously determined that low-water-use perennials need roughly 1.5 gallons of water during the average June in Cedar City, UT. Based on soil type, irrigated area, depth, and depletion rate, we determined that we need to apply 1 gallon of water with each irrigation event. In this situation, applying one gallon of water every 15-20 days would be appropriate, but now we haven’t determined how long to run our system. 

The short answer, it depends. Drip irrigation would be the appropriate method for irrigating the perennials. Run time will be dependent on the size and number of emitters used. For example:

Summary

To tie these concepts together, a general irrigation schedule and water need for perennials is outlined in figure 5. This schedule follows the examples used throughout this publication. It is based on historic ET data and is an appropriate guide for most of the state.  Warmer locations, like St. George, Kanab, and Moab will have a higher water requirement, but could use the guide as a starting point. The amount of water applied with each irrigation will depend on your soil type (a water holding capacity of 2” of water in 1’ of soil was used).  As highlighted in figure 4, different soil types have the ability to hold different amounts of water. Reach out to your local Extension office for help modify the schedule to meet your conditions.