Water Management on Turfgrasses
Texas Cooperative Extension
Text and images copyright © Richard Duble.
Having witnessed the effects of water shortages in many sections of the
country during the past decade, we can appreciate the value and need for
conservation of water. And since scientists forecast more extensive droughts
throughout the next decade, we must become more conscious of water use.
Only through careful management of our water resources now can we expect
to have adequate water supplies for turfgrass irrigation in the 21st Century.
In some sections of the country, water use for turf irrigation may seem
insignificant, but in other areas it accounts for 50 percent or more of
the consumption of city water supplies during the summer months.
If water conservation does not get your attention, perhaps improved turf quality will. Both of these effects are products of a properly designed and managed irrigation system. Simply installing an irrigation system to provide coverage to a sports field, golf course, or does not constitute design. Climatic conditions, rootzone properties, grass species, turf use, and water source must all be considered. In addition, pipe size, nozzle diameter, operating pressure and spacings, which together determine the rate and uniformity of application of water, must also be considered. Efficient use of water will, in time, become the most important design consideration for an irrigation system.
Climatic ConsiderationsEvaporation Losses. Direct evaporation from sprinklers can account for a 50 percent or greater loss of water in a desert climate, or for only a negligible loss of water in humid climates. Evaporation losses from irrigation increase with solar radiation, temperature, wind movement, and operating pressures, and decrease with relative humidity and nozzle diameter. Losses due to evaporation are higher for single row irrigation systems with little overlap than for multi-row systems with considerable overlap. Low trajectory nozzles also help reduce evaporation losses.
Where climatic conditions favor high losses due to evaporation during summer months, the irrigation system should be designed to operate at a minimum pressure, to have maximum overlap, to use as large a nozzle as practical, to use low trajectory nozzles and to operate during night and early morning hours. Under conditions that favor evaporation, daytime sprinkler irrigation can require 30 percent more water than night irrigation.
Transpiration. Transpiration may be defined as the movement of water vapor through the plant to the atmosphere. Most of the water transpired through the plant moves through openings in the leaves called stomates. In actively growing turfgrasses, water continuity exists from the soil (roots), through the plant, to the leaves where evaporation occurs through the stomates.
The primary benefit of transpiration is the cooling effect resulting from the evaporation process. In the absence of transpirational cooling, leaf temperatures can approach 130°F. In some locations with grasses such as bentgrass, transpirational cooling must be supplemented with syringing in midday to increase evaporative cooling on very hot, summer days.
The amount of water lost through transpiration is a function of the rate of plant growth (transpiration is very low during the dormant season) and several environmental factors - soil moisture, solar radiation, temperature, humidity and wind. Transpiration rates are highest during summer months when soil moisture, solar radiation, temperature and wind speeds are high. Transpiration rates are also higher in arid climates than in humid climates because of the greater water vapor deficit between the leaf and the atmosphere in dry air.
Thus, transpiration losses may be as high as 0.4 inch of water per day in desert climates during summer months; whereas, under similar temperature conditions in humid climates the daily losses may be only 0.20 inch of water.
In addition to transpirational water losses from turf, evaporative losses from the soil also occur. With the exception of plant growth rate, evaporation losses are dependent on the same environmental variables as transpiration - soil moisture, temperature, solar radiation, humidity and wind.
The loss of water from the soil by evaporation and through the plant growing on the soil by transpiration is called evapotranspiration. During the growing season, transpiration accounts for most of the evapotranspiration (ET) losses from established turfgrass sites.
Because of the difficulty of measuring ET rates in the field, great efforts have been made to relate ET to readily available climatic data. Thornthwaite, Penman and Blaney-Criddle all developed rather complex equations to relate ET to climatic data. In an effort to simplify these relationships, turfgrass researchers found that reasonable estimates of ET can be made from pan evaporation data collected by most weather stations. For warm season grasses ET can be estimated by the following relationship:
ET (warm season grasses) = (0.75 x pan evaporation rate)
For cool season grasses the rate is increased to 0.85 times the pan evaporation rate. These estimates are most accurate during the growing season for the respective grasses. Estimated monthly ET rates for Texas locations are shown in Table 1.
In the absence of official pan evaporation data you can collect your own data using a 4-foot diameter, 10-inch deep pan with vertical sides. The pan should be placed on blocks so that it is about 6 inches above soil level. You can measure the loss of water, in inches, by measuring the depth of water in the pan each morning at the same time. If you are just interested in average ET rates, measure the depth of water at 3-day intervals and divide the loss by 3. Also, for warm season grasses multiply the pan evaporation loss by 0.75 to estimate ET rates.
Rootzone PorosityPorosity. Soils consist of solid particles and pore spaces which are filled with either air or water. Pore space may account for 40 to 50 percent of the soil, depending on texture, structure, degree of compaction, and other variables. Individual pore spaces are classified as small pores (capillary pores) or large pores (non-capillary pores). Small pores are generally filled with water and large pores are filled by air. Total pore space and pore size distribution determine most of the physical properties of soils that are important to irrigation practices.
Water moves downward in a soil through the large pore spaces until the flow is interrupted by a significant change in pore size. A barrier such as a compacted soil, gravel layer, or clay pan will impede the downward movement of water. Where these barriers are near the surface of the soil, irrigation rates and schedules must be adjusted to prevent excessive surface run-off or leaching, depending on the nature of the barrier.
Infiltration and Percolation. The rate of movement of water into a soil is called the infiltration rate. A dry soil may have a very high initial infiltration rate, but as the soil pores become filled with water (saturated) the infiltration rate decreases sharply. In a saturated soil the infiltration rate is equal to the rate at which water moves through the soil profile-the percolation rate. The infiltration rate and percolation rate are critical physical properties of the soil that must be considered when designing and operating an irrigation system. Both of these properties determine the rate at which water can be effectively applied to a soil.
Water Retention. The soil serves as a reservoir for water storage. A clay soil may store 2 to 2° inches of available water per foot, whereas a sandy loam soil may hold only 1 to 1° inches per foot. For an irrigation system to be efficient, the water in the rootzone of the soil should be completely recharged by irrigation when 50 to 60 percent of the available water has been depleted. For some turfs this practice may require as little as ° inch of water or as much as 1° inches per application. Few sprinkler irrigation systems are designed to apply more than 1 inch of water per application. Obviously, the more available water the rootzone will hold, the longer the irrigation interval (days between irrigations) can be.
Surface ConditionsTurf development and soil surface conditions can restrict water infiltration rates in the same way that soil profile characteristics restrict percolation rates. A dense thatch layer, surface crusts, or a non-wettable sand can severely reduce water infiltration rates. Unlike soil profile characteristics, these surface conditions can be readily corrected through cultivation, soil amendments or wetting agents. Cultivation (aeration, vertical mowing, and topdressing) provides an effective means of removing and controlling thatch accumulation in turf.
Where thatch accumulation is excessive, significant amounts of irrigation water may be required just to wet the thatch layer. Evaporation losses are considerably higher from thatch than from soil. A heavily thatched turf is usually shallow-rooted which also prevents effective utilization of irrigation water. Where the root system is restricted to the thatch layer, light and frequent applications of water are more efficient than more thorough irrigations.
Soil amendments such as organic matter, calcined clay aggregates (Greens Choice and Turface), gypsum, or lime may be used to alleviate surface compaction and increase infiltration rates. When sodium salts from saline irrigation water disperse the soil particles and seal the soil surface, gypsum may be used to improve surface conditions.
Under some conditions sandy soils may develop non-wettable properties. This characteristic has been attributed to an organic substance produced by soil microorganisms that coats the sand particles and binds them together. Some wetting agents in combination with aeration have been effective in alleviating this condition.
Several types of soil profiles are illustrated in Figure 5-1 (A-E). Figure A represents the ideal situation where soil can be effectively wetted to a depth of 18 to 24 inches. In figure B, the clay pan limits the time that the irrigation system can be operated at one setting, because only the upper 6 inches of soil can be effectively wetted. If water continues to be applied after the top 6 inches are wet, the soil will become saturated and run-off will occur.
In Figure C, the rate of application of water is limited by the percolation rate of the upper 6 inches of soil. Figure D represents a typical USGA golf green profile. In this case, irrigation should be stopped after the top 8 to 10 inches of soil are wet. Continued irrigation after that point would results in excess water losses due to leaching. Figure E illustrates a situation where surface compaction restricts the movement of water into the soil profile.
Runoff. Runoff occurs when rootzones are saturated or when precipitation rate exceeds the infiltration rate of the rootzone. Runoff is highest in humid climates and is greater during the cool season than during summer if rainfall is evenly distributed. For a given annual precipitation, total runoff varies greatly across the U.S. For example, a mean annual precipitation of 30 inches is accompanied by runoff in the range of 3 inches in Nebraska, 6 inches in Tennessee, 12 inches in New York and 22 inches in the Rockies. These differences are largely due to seasonal distribution of rainfall. Areas where runoff is greatest receive most of the rainfall in the winter when only limited radiant energy is available for evaporation.
For the range of precipitation normally found in the U.S., a simple equation can be used to estimate runoff:
Runoff [as a % of precipitation (P)] = a P2
where a is a variable based on precipitation distribution and radiant energy. (From Sellers, W.D., Physical Climatology, Univ. of Chicago Press, 1965.)
For a turfgrass site in Texas a averages 0.005 and runoff ranges from 15% to 25% of rainfall. For estimating runoff on an annual basis, I would suggest using 15% on level areas, 20% on areas with about 1% slope and 30% on a 2% slope. A football field, for example, with an 18-inch crown down the center line of the field would have about a 2% slope. Most golf course fairways and lawns have a 1 to 2% slope so that 25% might be a good estimate for runoff on those sites.
Such information is useful when estimating water needs for irrigation. For example, a golf course superintendent in Dallas, Texas could estimate his annual water needs by the following equation:
Water Needs (in/yr) = ET (annual) - [Rainfall - Runoff]
= 60 - [36 - (.25) (36)]
= 60 - [36 - 9]
= 60 - 27
= 33 inches per year
Thus, an average of 33 inches of water would be needed to maintain growth at a maximum level. On a bermudagrass golf course, good playing conditions could be maintained with 60 to 70% of the 33 inches or 20 to 23 inches of effective irrigation.
Species and UseGrasses differ in their water requirements, as some can survive much greater moisture stress than others. The cool season grasses are generally more susceptible to moisture stress than warm season grasses. Buffalograss, for example, can survive long periods of severe moisture stress, whereas bluegrass would be killed by the same conditions. Buffalograss may not look any better than the other grasses during this period, but it would recover when moisture was restored. However, on sports fields and golf courses, mere survival is not satisfactory. All turfgrasses require supplemental irrigation during stress periods to maintain color and growth. During peak water use periods cool season grasses use from 0.3 to 0.35 inch per day; whereas warm season grasses use about 0.25 inch per day.
(cm/month) for July.
Depth of rooting is the most important factor in the drought resistance
of a turf. A shallow-rooted turf is much more susceptible to drought injury
than a deep-rooted turf. Management practices, rootzone properties, and
turf use have a greater affect on the depth of rooting than grass species.
However, grass species and varieties differ significantly in depth of rooting.
Close mowing, overwatering, excessive fertilization, soil compaction, and
thatch accumulation all lead to shallow-rooted turf. Golf course putting
greens are excellent examples of turf that are managed to favor shallow
rooting. The roots of most bentgrass putting greens occupy the surface 2
to 4 inches. Consequently, irrigation schedules must be adjusted to a light
and frequent schedule.
Management practices that promote deep rooting include aeration, thatch control, proper mowing, proper fertilization and infrequent irrigation. A deep-rooted turf uses water more efficiently than a shallow-rooted turf because it can go longer between irrigations (Figure 5-2).
Irrigation PracticesTo maximize efficiency of water use by turfgrasses, irrigation programs should be based on cumulative evapotranspiration losses, soil moisture retention, effective depth of rooting, infiltration rate, and type of turf being irrigated. An irrigation program set up on a calendar basis is much less efficient than one based on the above criteria. Water use on a daily basis can be estimated from pan evaporation measurements, which are available from weather stations throughout the U.S.
Today, computerized irrigation controllers are available that provide irrigation programs based on evaporation data inputted several times an hour and accumulated on a daily basis. Also, automatic adjustments in the program are made daily based on rainfall, temperature and other environmental parameters. These controllers take the "art" out of water management and replace it with "science".
Irrigation systems should be designed to meet the water requirments of turf during the months of maximum use. For example, locations that have a net evaporation loss of 15 cm (6 inches) during the month of July should have the capability of applying about 4 cm (1-1/2 inches) of water per week. Whether the 1° inches of water is applied in two or more applications will depend on turf use, soil moisture retention, infiltration rate and depth of rooting. If the turf is deep-rooted and in a soil capable of holding 1-1/2 inches of water in the effective rootzone, the entire amount could be applied in a single irrigation. Or, where the infiltration rate restricts the rate of water movement into the rootzone, the water could be applied in a series of intermittent irrigations. If the turf is shallow-rooted or if the soil will hold only 1 inch of water in the rootzone, the water should be applied in two or more irrigations.
On an annual basis, warm season grasses will use 40 to 60 inches of water per year, depending on the availability of water. A well watered bermudagrass fairway will use about 60 inches of water per year, or 1.6 million gallons per acre. The same fairway could be maintained in equally good condition with about 40 inches of water, a 33 percent savings in water alone. In addition, energy needed to pump the water, wear on the equipment and fertilizer losses are also significantly reduced.
Under intensive maintenance such as sports fields and golf course fairways rainfall meets about half of the water needs in southern states; irrigation must provide the remainder. Thus, irrigation must provide 20 to 30 inches of water per year in the South and 40 to 50 inches in the West, for warm season turfgrasses. These numbers equal 0.5 to 1.5 million gallons of water per acre or 12 to 36 thousand gallons per 1,000 sq. ft. of turf. Obviously, these quantities of water represent significant dollars.
By promoting deep rooting through thatch control, aeration, moderate fertilization and infrequent irrigation, significant quantities of water can be saved. Further savings can be achieved by planting drought resistant grass varieties.
Practical Water ManagementThe objective of a turfgrass manager is to provide as fine a lawn or playing surface as desired with a minimum use of labor and resources such as water. Although much of the previous discussion concerned the maximum, or potential, use of water by turfgrass; in practice, grasses can only use the water that is available to them. Thus, where precipitation is below the potential water use rate, the actual water use equals the effective precipitation (Rainfall + Irrigation - Runoff). In situations where the rootzone is very shallow, leaching losses must also be considered. Otherwise, for simplicity, we can disregard leaching.
For example, in a non-irrigated gently sloping site in central Texas where rainfall for 1992 was 30 inches, the water use rate can be calculated as follows:
Water Use Rate = Rainfall - Runoff
(inches per year)
= 30 - [.005(30)2]
= 30 - 4.5
Thus, even though the potential water use rate for central Texas is about 60 inches the actual water use rate for 1992 was only about 25 inches. The question the turf manager must decide is, "was the quality of turf adequate for that site?" If not, how much additional water is necessary?
From observations in Texas we know that bermudagrass will survive with about 20 inches of water per year. From research we know that bermudagrass can be kept green during the growing season with only 50% of the potential water use rate, or about 30 inches per year, if the applications of water are timely. For a lawn, 30 inches of water may be all that is needed. But, for a sports field or golf course where growth is needed for recovery, more than 30 inches might be needed.
Therefore, if your objective is water conservation, 20 inches is needed for survival, 30 inches for acceptable color and about 40 inches for adequate color and growth. Those values are for common bermudagrass. Hybrid bermudagrass such as Tifway and Tifgreen require slightly more water for the same level of maintenance.
Buffalograss is similar to common bermudagrass, but buffalograss will survive with only about 15 inches of effective rainfall. However, water requirements for maintenance of color and growth are about the same as for common bermudagrass.
For St. Augustine grass those same parameters might be 30 inches for survival, 40 inches for color and 45 inches for color and growth. Zoysiagrasses are similar to St. Augustine grass.
Tall fescue, ryegrass and bluegrass have the highest water requirements for Texas. Although these grasses can survive with only 30 to 35 inches of water, they require in excess of 50 inches to maintain acceptable color and growth.
To maintain turfgrasses with the amount of water indicated in the above discussions, turf managers must apply water effectively. One method of effective water management is to recharge the rootzone at intervals that allow the grass to show slight moisture stress-wilting and discoloration. For example, bermudagrass growing in deep sand might be irrigated at 5 to 7 day intervals with 1 inch of water. The same grass in the Texas Hill Country, where rootzones are typically shallow, may require 0.5 inch of water at 2 to 3 day intervals. Extending the interval between irrigations to the point of showing moisture stress promotes deep rooting of turfgrasses. However, the entire rootzone must be recharged when water is applied.
Sloping sites and sites with very low infiltration rates must be irrigated intermittently to reduce runoff. For example, the site may require 0.75 inch of water to recharge the rootzone, but the infiltration rate may be only 0.25 inch of water per hour. Putting out all the water needed in one irrigation cycle would result in significant runoff. However, by applying only 0.25 inch of water per cycle and repeating the cycle at 1 to 2 hour intervals, runoff can be significantly reduced.
Through conscientious water management homeowners and professional turfgrass managers can conserve water resources and still provide attractive lawns and sports fields.
Watering Golf GreensA golf green presents special problems in regard to irrigation, since a dry soil surface is quite hard and will not hold a golf shot. As a result, a golf green receives between 1/5 and 1/4 inch of water per application and, during summer months, may be irrigated daily. Such a practice promotes shallow rooting, weed germination, disease development and soil compaction. A better solution to the hard surface problem would be to modify and cultivate the soil so that it will hold a golf shot even when moderately dry.
Moisture-indicating instruments called tensiometers may be used to measure the moisture status of the soil and indicate when irrigation is required. They consist of a porous cup, a vacuum gauge and a water-filled connecting tube between the cup and the gauge. When the cup is placed in the rootzone of the soil, water is free to move through the porous wall and come to equilibrium with the soil water. As the soil dries, water moves from the cup and causes a vacuum to be indicated on the gauge; thus, the drier the soil, the higher the gauge reading. When irrigation water is applied or rainfall occurs, water returns through the porous cup and releases the vacuum, which lowers the gauge reading.
By placing the tensiometers at several depths and observing daily or weekly, it is possible to estimate how often irrigation is needed and the depth to which water should penetrate to recharge the rootzone.
Moisture readings with these instruments represent only a small area of soil that surrounds the cup, therefore, sufficient locations over the area should be established so that a representative measurement of soil moisture can be obtained.
Irrigation schedules based on tensiometer readings that indicate moisture stress are much more efficient in terms of water use than schedules established on a calendar basis.
Reductions in water use brought about by monitoring the water needs of bermudagrass fairways using flow meters and tensiometers. The quality of turf was improved each year and approximately 25 million gallons of water was saved on 30 acres of fairways.
Watering schedules depend on grass species, soil type, slope, site use and
other turf management practices.
Grass Species (Drought Resistance)
Grasses with poor drought resistance may need 3 or 4 irrigations per week during summer months; whereas, those with excellent drought resistance may need only one irrigation per week.
Between 1/3 and 1 inch of water should be applied per irrigation depending on turf use, soil type, depth of rootzone and slope. Sports fields and golf course fairways require 0.5 inch or less per irrigation when the site is being used on a regular basis. Lawns should receive 0.5 inch or more per irrigation. Sandy soils require lighter, more frequent irrigations than loam or clay loam soils. And, lawns with a shallow rootzone (4 inches or less) require lighter, more frequent irrigations than lawns with deeper rootzones. Likewise, sloping sites require lighter, more frequent irrigations than level sites.
Total Costs for IrrigationExample:
Total Irrigation Costs = Water + Labor + Equipment + Operating
For a 10,000 sq. ft. residential lawn with a 30 inch water deficit per year the total irrigation costs might include:
$295 (see above calculation)
50 hours (to monitor, operate and repair the system) at $8.50 per hour equals $425
$2,500 for system installation depreciated over 20 years or $125 per year
$35 for repairs plus $75 sewage charge for residential water use
Total Irrigation Costs
$295 + $425 + $125 + $35 + $75 = $955
If the homeowner provided the labor, he could deduct the $425 labor costs. Nevertheless, the example shows that water costs are only a portion of the total irrigation costs to a homeowner or client.