Water is an essential component for plant growth. For most turfgrasses, water comprises 75 to 90 percent of the fresh weight of the plant. Only 1 percent of the water absorbed is used for metabolic activity. Although small, the amount of water used for metabolic activity is crucial to the plant’s survival. For example, turfgrasses would be unable to photosynthesize (produce food from sunlight) without water. Most water is used during transpiration, which is the plant’s primary cooling process. Water, absorbed by the roots and carrying nutrients, is dispersed to all cells for function and growth. The uptake of water is also crucial for maintaining turgor pressure of plant cells, which results in plant rigidity and cell elongation. Turgor pressure is a critical function in turf resiliency that influences its ability to tolerate wear and recover from traffic damage. By considering the factors that contribute to water loss, turfgrass managers can devise effective irrigation plans for specific sites.

Evapotranspiration (ET) is the process by which water is transferred to the atmosphere from plant surfaces. There are two components to this process: evaporation and transpiration. Evaporation is the physical process whereby water is changed from a liquid to a gas. This occurs on water surfaces such as ponds, streams, wet soil, or wet vegetation. Transpiration is a plant process whereby water is evaporated through a series of openings on the leaf surfaces called stomata. For all practical purposes, it is virtually impossible to separate these processes on a turf area. The amount of water lost through evaporation is minimal, meaning the great majority of moisture loss is through transpiration. Evapotranspiration produces a cooling effect because energy is consumed in the process. This process resembles how humans are cooled through sweating.

Several environmental conditions affect the rate at which moisture is lost from the turf surface. The most important of these conditions are radiant energy (sunlight), atmospheric vapor pressure (relative humidity), temperature, wind movement, and available soil moisture. When using best management practices (BMPs), the type of turf is not a significant factor in ET as long as the turf is actively growing and has adequate soil moisture. Minimal ET rates occur under dark, cloudy days with high relative humidity, low temperatures, and little to no wind. Maximum ET rates occur on bright sunny days with low relative humidity, high temperatures, and moderate to high winds.

The concept of reference evapotranspiration has been developed to predict water requirements of plants based on climatic data. Reference evapotranspiration (ET_o) is defined as the water loss from an actively growing clipped turf with an adequate supply of soil moisture. Therefore, ET_o can be useful in predicting the water requirements of irrigated turf.

Advances in turfgrass irrigation management have led to several instruments that can provide a measure of ET. Instruments such as lysimeters or soil moisture sensors can accurately measure water loss through ET, but also can be very laborious or expensive to install. As a result, managers of many turfgrass sites do not have a way to directly measure ET. For these situations, researchers have developed empirical methods to estimate ET_o, from which estimates of ET can be made. These include the Thornthwaite, Blaney-Criddle, Hargreaves-Samani, Penman, and Penman-Monteith methods.

• Thornthwaite method (1948) uses temperature and day length data. In some situations, this method has been less reliable in short-term prediction due to limited inputs.
• The Blaney-Criddle and modified version (1977) use temperature and percentage of daylight hours to predict monthly ET_o. The accuracy of these models is similar to that of Thornthwaite, and they should be used only to provide a rough estimate of ET_o for a month or longer.
• The Hargreaves-Samani method (1985) uses only maximum and minimum temperature data to predict ET_o. It can provide a quick estimate of ET_o for short time periods but should be used only when temperature data is all that is available.
• The Penman equation (1948) predicts daily ET_o based on net solar radiation, temperature, vapor pressure, and wind speed, and thus requires a more extensive data set.
• The FAO 56 Penman-Monteith equation (1998) incorporated a specified vegetative surface resistance of 70 siemens per meter. This value is associated with a clipped grass height equal to 0.12 meter (about 5 inches).
• The ASCE Standardized Penman-Monteith equation (2005) is the same as the FAO-56 Penman-Monteith equation but can be applied to both a grass and alfalfa reference crop. Alfalfa has both a rougher surface and a higher canopy height (0.5 meter compared to the 0.12-meter height of clipped grass), resulting in a lower surface resistance and greater reference evapotranspiration, denoted as ET_r.

Irrigation is one of the key cultural practices used in turf management, but most turf areas are not irrigated due to the expense of irrigation system installation and the cost of buying or pumping water. As a cultural practice, irrigation should be used to supplement, not substitute for, rainfall—except on very specialized areas or under extreme drought conditions.

Irrigation management must include water conservation practices. Lack of adequate moisture can result in turf stress, dormancy, and even death. Providing the correct amount of water at the appropriate time is important to ensure healthy, high-quality turf, whether for recreational or landscape purposes. To provide the proper amount of moisture at the right time, we must adequately record climatic conditions. Climate records help the turfgrass manager determine if soil moisture reserves are adequate or if irrigation should be scheduled.

Irrigation scheduling based on sound principles is essential because of the many variables involved, including slope, soil types, and rooting depth. Knowledge of the water reserve in the root zone is a key requirement for determining irrigation needs. On many turf areas, most of the root system is often found in the top 4 to 6 inches of soil. On other areas, the depth of rooting can vary from less than 4 inches to 6 to 12 inches, depending on soil properties, how the turf is managed, and time of year. For cool-season grasses such as tall fescue, the peak rooting times are fall and spring. In contrast, the root growth of warm-season grasses, such as bermudagrass and zoysiagrass, often peaks in the summer. With knowledge of soil water storage, daily rainfall, and estimated daily ET, a turf manager can determine when the plant-available soil moisture will be depleted and irrigation will be required. For specialized areas such as golf courses, a weather station located at the maintenance facility can record the information necessary to calculate the daily ET. This rate varies daily based on local climatic conditions of temperature, humidity, solar radiation, and vapor pressure. Weather stations should be sited on well-watered areas to accurately reflect the conditions under which the turfgrass will be irrigated.

Given the imperfect nature of any irrigation system and human judgment, any turf site can be over-watered, correctly watered, or under-watered. Only through careful study and trial and error can the turf manager achieve the most appropriate irrigation timing and soil moisture level, preferably on the drier side. By avoiding a saturated soil profile through irrigation, a turf manager can increase the amount of rainfall that is termed “effective” and, as a result, increase efficiency. The best method of determining whether the proper amount of water has been applied is to determine the depth of water penetration following irrigation by coring with a soil sampler or probe. If water has not penetrated to the desired depth by six to eight hours after irrigation, then the irrigation time should be increased. If water has moved well beyond the desired irrigation depth, then the irrigation time should be decreased.

To avoid runoff, the application rate must not exceed the soil infiltration rate. If necessary, the irrigation system can be cycled (irrigation followed by a period of soaking) to ensure proper infiltration. In addition, one of the primary responsibilities of the irrigation technician will be to monitor the heads frequently to make sure all heads are operating properly and that no head is inadvertently applying water to an area where irrigation is not required.

Irrigation frequency will vary with environmental or climatic factors. The species of turfgrass and its use will also dictate irrigation frequency. Cool- and warm-season grasses will require different amounts of water at different times of the year given their different periods of growth. Checking the depth of the root system with a soil probe can help guide how deeply to wet the soil profile and thereby increase irrigation efficiency.

Water should not be applied at a rate that exceeds the soil’s infiltration capacity. Otherwise runoff may occur, especially from sloped sites, where thatch has accumulated, or on compacted soils. In these situations, it is more effective to apply only a portion of the total water needed and to move a sprinkler or switch to another station to irrigate other areas. After the water has infiltrated and percolated into the soil, apply another portion of the water and repeat the cycle until all water is applied.

A healthy durable turf that withstands minor drought is achieved by irrigating thoroughly but as infrequently as possible. This may encourage deeper rooting. A sure sign that turf will benefit from irrigation is a wilted appearance. One initial symptom of wilting is “footprinting,” where footprints on the turf will not disappear within one hour. This symptom is soon followed by actual wilt, where the leaves of the turf lose an upright erect appearance and take on a grayish or purple-to-blue cast. Usually, only a few areas will appear wilted in the same general location of the turf; these areas serve as good indicator spots when assessing the need to water. Delay watering the entire turf area for another day or so by irrigating only the wilted areas if possible.

Allowing some subtle wilt stress to develop in a turf will not destroy the turf but may actually encourage deep rooting. Allowing the turf to use 50 percent of its available soil-water prior to irrigation does not make the turf more prone to stress. As drought stress becomes more severe, however, turf becomes more susceptible to traffic injury, insect and disease damage, as well as weed invasion, especially at lower mowing heights. Thus, wilt stress should be minimized for playing surfaces that are mowed at very low heights (such as putting greens) or receive high amounts of traffic from play or vehicles.

The most efficient time of day to water is late evening through early morning (between 10 p.m. and 8 a.m.). Nighttime is generally less windy, cooler, and more humid than daytime, resulting in less evaporation and a more efficient application of water. Contrary to popular belief, irrigating at night does not stimulate disease development and, in fact, prevents extending the period over which turf leaves are wet, a condition that could occur while irrigating during the day.

Some turf areas such as golf greens may have a need for more than one irrigation event per 24-hour period. Accordingly, these sites will need some irrigation during daylight hours. The tendency to water “heavily and infrequently” on these sites will result in an inefficient use of water because these sites typically have rapid drainage. Thus, excess water is readily lost through drainage. Under rapid drainage conditions, site specific watering (that is, hand watering and syringing) is performed during daylight hours because of the need to visually identify areas where the water should be applied. Employees responsible for hand watering and syringing should be thoroughly trained regarding the most effective and efficient techniques for applying water during the day.

Although matching irrigation amounts to a turf’s water needs is the most effective way of conserving water, there are some cultural practices that can help increase the efficiency of turf water use:

• Maintain the soil pH between 5.5 and 6.5.
• Minimize soil compaction through turf cultivation.
• Minimize potential problems from pesticides toxic to the root system, particularly certain preemergence herbicides that may inhibit root elongation.
• Control insect, disease, and nematode pests that feed on the root system.
• Maintain an adequate soil potassium (K) level by conducting a soil test.
• Avoid excessive nitrogen (N) fertilization, especially on cool-season grasses, which forces shoot growth at the expense of root development.
• Maintain as high a cutting height as possible within the confines of the turf use.
• Avoid excessive thatch accumulation, which encourages root development in the thatch or mat layer only.
• Avoid intense mechanical maintenance practices—such as topdressing, vertical cutting, and turf cultivation—during stress periods

A weather station can be a valuable tool to calculate ET_o, and many stations will monitor and record some or all of the following parameters: air temperature, soil temperature, wind speed, wind direction, rainfall, humidity, and solar radiation. These parameters can then be linked with a computer programmed to calculate irrigation requirements. Turf managers can use this information to schedule irrigation system operation to apply the amount of water necessary to replace that lost through ET.

In addition to the weather station being used in irrigation management, the system can record information that can be used in other components of an integrated pest management (IPM) program. For example, information can be used in models for predicting disease development, in calculating degree days for insecticide application, and in determining windows of timing for preemergence herbicide application.

Crop coefficients (K_c) are routinely used in turfgrass irrigation recommendations to better meet the water requirements of the given turfgrass species. This practice is especially important in North Carolina, due to its geographic location in the transition zone of the United States. This zone means that both cool- and warm-season grasses can grow here, resulting in a need for irrigation recommendations for multiple turfgrass species. Crop coefficients (K_c) are the ratio of actual evapotranspiration (ET_a) to ET_o, and can be used to estimate crop (turf) ET (ET_c) by multiplying ET_o by K_c. Water use by cool- and warm-season grasses varies substantially during the year, so researchers have computed K_c values for both cool- and warm-season grasses to better meet seasonal water demands.

The distribution uniformity (DU) of an irrigation system measures the evenness of irrigation across an irrigated turfgrass. A system with a high DU (approximately 0.7) applies irrigation water uniformly and will avoid excessively wet or dry areas across a landscape that may result with an irrigation system with a lower DU (less than 0.5). To determine a system’s DU, an irrigation audit is required. Once a DU is known, it can be used to adjust (increase) irrigation system runtimes to compensate for nonuniformity. For systems with a DU below 0.5, irrigation system adjustments should be made to improve irrigation uniformity.

The proposed irrigation recommendations were derived for a well-watered turfgrass through simulating a soil-water balance (Equation 1) that included water contributed from rainfall. Potentially, irrigation amounts and runtimes can be reduced in what is known as “deficit irrigation.” Deficit irrigation is a great way to save water as irrigation is only applied to maintain turfgrass functionality when aesthetics are not a primary concern. Depending on the level of acceptable turfgrass quality, varying levels of deficit irrigation can be used to minimize landscape irrigation.

The following tables provide estimates of turfgrass ET as well as guidelines for monthly irrigation requirements of both cool- and warm-season turfgrasses across North Carolina. Predicted turfgrass water use was calculated using the most recent derivation of the Standardized ASCE (American Society of Civil Engineers) Penman-Monteith reference evapotranspiration equation (Allen et al., 2005) and long-term weather averages. Reference ET (ET_o) measures were multiplied by a monthly crop coefficient (K_c) to obtain turf water use for the given turfgrass species. To predict irrigation amounts, a soil-water balance simulation was conducted:

Equation 1. SW_t = SW_t-1 - ETc_t-1 + R_t-1 + I_t-1 - D_t-1 - Roff_t-1

where SW_t is soil water on day t, SW_t-1 is soil water on day t-1, ETc_t-1 is crop evapotranspiration, R_t-1 is rainfall, I_t-1 is net irrigation, D_t-1 is drainage, and Roff_t-1 is runoff.

For each location (Figure 1), the most common soils in the geographic region were used to estimate levels of soil moisture. By combining daily rainfall measures, estimates of ET_o times K_c, and soil parameters, we could determine irrigation amounts. Depending on the location and the amount of data available, irrigation recommendations were based upon at least 12 years and at most 30 years of historical climatic data. While the irrigation recommendations at all locations were based on extensive historical data, it should be noted that these data were averaged across several years, meaning that these values can fluctuate given the climatic conditions experienced in any given year. For example, climactic conditions in both Fayetteville and Kinston result in plant water use estimates exceeding rainfall during the month of March, suggesting the need for irrigation. In most years this may not be the case as turf water use is minimal for both cool- and warm-season grasses. As a result, these values should be used as a guide to schedule irrigation and should be adjusted when needed.

Figure 1. Locations where irrigation simulations were conducted.

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Figure 1. Locations where irrigation simulations were conducted.

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To most effectively use this information, a turfgrass manager must be able to set the irrigation controller to deliver the desired amount of water. Failure to apply the correct irrigation runtimes will lead to decreased efficiency and potential problems such as wasting of water or reduced plant health. Prior to setting run times, the turf manager should make a thorough audit of the irrigation system to identify any system deficiencies and to determine a precipitation rate. The precipitation rate is simply the rate in which the sprinkler heads will deliver water to the turfgrass. It is a depth of water applied per unit time (usually reported as inches per hour). Once the audit is complete and a precipitation rate is known, one can then set an irrigation schedule. The following example explains how to set irrigation runtimes:

Location: Raleigh area

Monthly Water Requirement: July (1.76 inches water)

Turfgrass: Bermudagrass (warm-season)

Precipitation Rate: 1 inch of water per hour

Irrigation Frequency: Three days per week (12 days of irrigation per month)

(Monthly Water Requirement ÷ Irrigation events) = Water Applied per Irrigation Event

1.76 inches ÷ 12 = 0.15 inch

[(Water Applied per Event (inches)) ÷ (Precipitation Rate (inch hour))] = Irrigation Runtime (hour)

0.15 inch ÷ 1 inch = 0.15 hour

Irrigation Runtime (hour) x (60 minutes ÷ 1 hour) = Irrigation Runtime (minutes)

0.15 hour x (60 minutes ÷ 1 hour) = 9 minutes runtime

In summary, the irrigation controller should be set to run for 9 minutes, three days per week to deliver the required amount of water.

*NOTE: Distribution uniformity was not accounted for in runtime calculations. Runtimes may need to be increased to adjust for DU.*

Allen R.G., Walter I. A., Elliot R. L., and Howell T. A. (2005). The ASCE standardized reference evapotranspiration equation. American Society of Civil Engineers, Reston, VA. Accessed 31 January 2018.