Irrigation Management in Summer: timing, amount,syringing, and water quality
Micah Woods, Ph.D.*ABSTRACT
Creeping bentgrass (Agrostis stolonifera) grows bestwhen the average temperature is about 20°C. During the summer in most parts of Japan, the average temperature is more than 25°C. In such hightemperatures, bentgrass greens must be suppliedwith just the right amount of water, at the righttime. Any prolonged deficit or excess of water inthe rootzone is likely to result in severe damage
to the grass. Turfgrass quality is improved, andbentgrass root systems tend to be more extensive,
when the soil moisture content is maintained at alow level. The timing of irrigation is less important
than is the amount of water applied. Wilting grasshas a very high surface temperature, but transpiring grass has a surface temperature similar to theair temperature. Syringing greens does not havea meaningful impact on surface temperatures. In
addition to the quantity of water applied, it is important to know what is in the water. Specifically,
the salinity and the sodium adsorption ratio (SAR)
should be known, so that any potential problems
can be managed before the grass is damaged.

Iportant
rrigation management is one of the most imgreenkeeping jobs. In the hot summer

Figure 1: Summer irrigation of the 9th green atGolden Cross CC, Chiba-ken
wilting point, there is still some water in the soil,
but it is held tightly in the small soil pores and is
not enough to meet the grass demand for water. In
most situations, we try to maintain the soil moisture somewhere above the wilting point, but at or
below field capacity.
On putting greens, the roots, and consequently
most of the water uptake, tend to be at a depth ofabout 10 cm. I usually consider the rootzone to be
the top 10 cm of the soil. The amount of water inthe soil is often expressed in terms of volumetricwater content (VWC) and given as a percentage.
In 100 L of soil, for example, there may be 20 L ofwater. The VWC in that case is 20%. As anotherexample, if we took completely dry soil with a volume of 500 cm3 , and then we added 100 cm3 of water, the VWC of that soil would also be 20%.
The amount of water used by the grass throughtranspiration, combined with the amount of water
evaporated from the surface, is called evapotranspiration and is often abbreviated as ET. This is expressed in mm. The ET is essentially the consumptive water use of an area. It is useful to rememberthat 1 mm of water spread over 1 m2 has a volumeof 1 L.

weather, bentgrass supplied with too little water
will fail rapidly. Bentgrass supplied with too muchwater will fail rapidly also. Ensuring that the grass
is supplied with just the right amount of water is acrucial component of summertime greenkeeping.
We should always remember that grass gets its
water from the soil. So when we are irrigating (Figure 1), we are really managing the amount of water in the soil. If we succeed in maintaining theoptimum amount of water in the soil, the grass
will respond well. The maximum amount of water the soil can hold is called field capacity. This isthe amount of water in the soil shortly after a saturating rain or irrigation, when excess water has Irrigation Timing and Amountdrained by gravity.
There is also a wilting point, the level of soil Let’s consider a few ways we might apply irrigamoisture at which the grass will wilt. Even at the tion. Imagine that we have a putting green with afield capacity of 27% and a wilting point of 10%.
* This document is based on a handout (http://www.files.
The soil will never hold more than 27% water, andasianturfgrass.com/201306_summer_irrigation_jp.pdf)
prepared for the 2nd KGM Private Seminar, held 27 June 2013 we want to always keep the soil at more than 10%
at Takarazuka Golf Club, Hyogo prefecture, Japan.
water. In Figure 2, we see what the VWC would be1

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20frequent
infrequent

100

Soil Volumetric Water Content (%)

Soil Volumetric Water Content (%)

20

frequent
infrequent1004−Aug

08−Aug12−Aug004−Aug

08−Aug12−AugFigure 2: Calculated rootzone (10 cm depth) volumetric water content (VWC) for the first 14 days ofAugust, assuming there is no rain, the daily evapotranspiration (ET) is 4 mm, the VWC on 1 Augustis at the field capacity (FC) of 27%, the frequentirrigation regime applies just enough irrigation toprevent the VWC from dropping below 18% on thenext day, and the infrequent irrigation regime applies enough water to return the soil to FC when
the VWC is predicted to drop below 11% on thenext day.

Figure 3: Calculated rootzone (10 cm depth) volumetric water content (VWC) for the first 14 days ofAugust, assuming there is no rain, the daily evapotranspiration (ET) is 4 mm, the VWC on 1 Augustis at the field capacity (FC) of 27%, the frequentirrigation regime applies just enough irrigation toprevent the VWC from dropping below 14% on thenext day, and the infrequent irrigation regime applies enough water to return the soil to FC when
the VWC is predicted to drop below 11% on thenext day.

for the first 14 days of August, assuming no rain
and a daily ET of 4 mm. For the infrequent regime(orange line), irrigation is only applied to keep thesoil moisture from dropping below 11%. For thefrequent regime, irrigation is applied daily, but justenough to keep the VWC, on the next day, fromdropping below 18%.
In the two irrigation regimes shown in Figure 2,
the average VWC over the first 14 days of August
are 21.9% for the frequent irrigation regime and19.2% for the infrequent regime. In this case, theaverage soil moisture content is maintained at ahigher level by frequent irrigation than by infrequent. But one can actually maintain a lower VWC
by irrigating frequently than by irrigating infrequently. This counterintuitive result is shown inFigure 3.
This case keeps the infrequent irrigation regime
the same, applying irrigation to return the soil tofield capacity only to prevent the VWC from dropping below 11% on the next day. But the frequentirrigation regime shown in Figure 3 now is managed to apply just enough water to keep the VWCfrom dropping below 14% on the next day.
Now the average VWC for the first 14 days ofAugust using the infrequent regime is the same,
19.2%, but the frequent (almost daily) irrigation
regime has an average of 17.6%. Some advantagesto the frequent irrigation regime as shown here include:• VWC never drops as low as it does in the infrequent regime, thus minimizing the chanceof drought stress or the initiation of localized
dry spots
Experiments to evaluate the effect of irrigation
frequency have usually used an approach similar
to that shown in Figure 2, namely, the soil in thefrequent irrigation regime is kept very close to field
capacity. In experiments in which the treatments
are applied in this way (Jordan et al., 2003; Fu andDernoeden, 2009a,b), the turf quality is better, andthe roots are more extensive at the end of summer,
when turf is irrigated infrequently. However, thoseexperiments also maintain average VWC higher inthe frequent irrigation treatments than in the infrequent treatments. In fact, Fu and Dernoeden(2009b) applied twice as much water to the frequently irrigated plots as they did to the infrequently irrigated plots.
A study of these experiments leads me to theconclusion that maintaining VWC at a lower level
will give the best chance for improved turf quality and more extensive roots. As a practical matter,the best approach will usually be a combination of
infrequent irrigation events interspersed with frequent irrigation events (Johnson, 2003). For turfgrass managers, I suggest focusing not so muchon whether irrigation is applied frequently, or infrequently, but rather on applying just the rightamount of water for a particular situation, and on
keeping the VWC as low as possible.

• Lower VWC, on average, which can produce
firmer surfaces and fewer ballmarks
Advice on the use of soil moisture me-ters• More air in the soil
• More consistent playing conditions
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One can estimate how much water is in the soil,or one can measure it. Soil moisture meters al-WWW. ASIANTURFGRASS . COM

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Figure 4: The surface temperature of 4 surfaces at 14:00 on 3 May 2013 in Bangkok when the air temperature was 38°C: a) concrete, 53.6°C; b) wilting Zoysia matrella, 48.8°C; c) transpiring Axonopus compressus,39.2°C; d) transpiring Zoysia matrella, 36.2°C.
low turfgrass managers to make a precise measurement of how much plant available water is in
the soil and how much water will need to be applied at the next irrigation event. Every location,
because of different soil type, grass type, climate,management practices, season, desired turfgrass
performance, and innumerable other factors will
have a different range of VWC and optimum VWC
at any time.
However, there are a few things that are consistent across all of those variables. Here are somethings to remember.
1. After a saturating rain, when the soil is sure to
be at its maximum VWC, but after gravity has
pulled any excess water from the soil, use themoisture meter to measure VWC. In practicalterms, this is equivalent to field capacity for aparticular soil.
2. When the soil becomes so dry that the grassstarts to wilt,1 measure the VWC just as thegrass starts to wilt. In practical terms, this isequivalent to the wilting point of the grass.
3. The difference between the wilting point
VWC and the field capacity VWC is theamount of plant available water in the soil.

With that information, one can attempt to maintain the VWC at any desired level, or within any
desired range.

Morning vs. afternoon irrigation
Is there a time of day when it is best to irrigate theturf? I think that the best time to irrigate the turf
is at the time we identify that the grass requires
water. Practically it may not be possible to irrigate
at that time, so the best time to irrigate would thenbecome as soon as possible.
Guertal and Han (2009) compared the effect ofmorning (08:00) irrigation to afternoon (16:00) irrigation on soil temperatures. They found thatmorning irrigation sometimes had a greater effectin cooling soil temperatures than did afternoon irrigation.
However, when fans were used, the soil temperatures were consistently lower than when fanswere not used. The use of fans also increased theroot length density, no matter whether irrigation
was applied in the morning or afternoon.

Syringing to Cool Bentgrass
Greens

4. If we are using a 10 cm rootzone for our calculations, then each change of 1% in VWC is For this discussion, I will consider syringing (orequivalent to 1 mm of water and equivalent to misting) to be the application of a small amount
1 L of water per m2 .
of water to the turfgrass leaves for the purpose ofcooling the surface. If I were managing bentgrass
1 If the grass never wilts, or the grass never starts to experience drought stress, then too much water is being applied. Or, greens in the summer, I would not syringe. Here’s
it just rains too much!
why.
3

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the soil and surface temperatures by a few degrees
for a few hours. Application of tap water reduced
the surface temperature by about 0.5°C when applied in the morning, and caused a slight increasein surface temperature when applied at sunset.
The difference in canopy temperature between
transpiring turf and non-transpiring turf can be
more than 10°C (Figure 4). Application of water tothe turf, in the extensive studies of DiPaola (1984),
has minimal to no effect on canopy temperature,unless the grass is wilting, in which case the application of water does result in a reduction of canopy
temperature.
Figure 5: In an experiment conducted on a bentgrass green in Chiba, ice, ice water, and 26°C tapwater were applied and the soil and surface temperature were measured.

Irrigation Water Quality

When irrigation water is applied, it is important toknow what is in the water. The content of the waTemperature of transpiring vs. wilting ter, and the chemical properties of the water, canturf
have an impact on turfgrass performance and on
soil physical properties. These potential problems
Figure 4 shows the surface temperature of four surcan be managed, but only if one knows what is infaces on a sunny day at Bangkok when the air temthe water. There are two important water quality
perature was 38°C. When the roots cannot takeparameters.
up enough water from the soil, the grass wilts.
And when the grass wilts, the surface temperature on a sunny day can rise much higher than the Salinity
air temperature. However, when there is adequate
soil moisture, the transpiring grass usually has a The first is salinity, which is the amount of inorcanopy temperature within 1 or 2°C of air temper- ganic ions (salts) in solution. Table salt is sodiumchloride, which in water dissolves to be Na+ andature.
Cl− . There will also be sulfate and calcium andmagnesium and potassium and ammonium andTemperature change after syringing
nitrate dissolved in the water, along with other
Given that turf supplied with adequate soil mois- ions; these ions, taken together, are called total disture will have a canopy temperature very close solved solids (TDS) and are a measurement of theto air temperature, what happens when the turf amount of salt (salinity) in the water.
The TDS is reported as mg L−1 and is measured
is syringed? How does the temperature change?
DiPaola (1984) conducted an extensive investiga- directly by taking 1 L of water, evaporating all themuch solid matter retion and found that water application of 50 or water, and weighing how
3−2mains
inthecontainer.
Whenirrigation water
100 mL m resulted in no change in canopy temwithsaltisapplied
toturfgrass,thegrass uses theperature 30 minutes after application. Adding−2water,
butitmaynottakeupallofthe salts, andmore than 1.4 mm (1.4 L m ) resulted in a decrease
thosesalts
accumulate
inthesoil.
For example,of about 0.7°C in canopy temperature 30 minutes
irrigationwaterwith
TDSof
1000
mg
L−1 , applied
after application. By 1 hour after application, there−1
was no change in canopy temperature between for one−2month at a rate of 4 mm d , could cause
syringed and non-syringed plots, no matter how 120 g m of salt to accumulate in the the rootzone.
That is a lot! Fortunately, irrigation water in Japanmuch water had been applied.
−2
When more than 100 mL m is applied, I hesi- rarely contains that much salt. But unless it is meatate to call it syringing, because most of the applied sured, one doesn’t know how much salt is being
water is going to the soil rather than staying on the applied to the turf.
Salinity is managed by applying more watercanopy. In that case, I call it irrigation.
thanthe grass can use. This results in leaching,I measured (Figure 5) the effect of ice, ice waterandas
some of the applied water leaches below(1.5°C), and tap water (26°C) on the soil and sur2 the rootzone, some of the salt also leaches below
face temperatures of a bentgrass green in Chiba.
4This was not syringing, because the amount of the rootzone.
H2 O applied in each treatment was equivalent to
7.8 mm. Application of ice or ice water reduced
2 The results of these experiments were described in articles
I wrote for Golf Course Seminar magazine in the July 2012, October 2012, and August 2013 issues.

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3 The TDS is also estimated by measuring the electrolytic
conductivity (EC) of the water. An EC of 1 dS m−1 is approximately equivalent to a TDS of 640 mg L−1 .
4 The leaching fraction with example calculations was discussed in an article I wrote in the July 2011 issue of Golf Course
Seminar magazine.

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4Sodium adsorption ratio
If there is a high amount of sodium in the watercompared to the amount of calcium and magnesium in the water, there can be some problems inthe soil. Over time, that water applied to the soil
will cause sodium to accumulate on the cation exchange sites, and as this happens, the small claysized soil particles can deflocculate and swell. This
reduces air space in the soil and slows the water infiltration. The sodium adsorption ratio (SAR) is anindication of how the water may cause a problem
with infiltration and soil structure.
One can consult various interpretation tables toevaluate how a particular SAR may be a problem; for most irrigation water in Japan, an SAR ofabout 6 or above would be an indication of potential problems. To manage the potential problemsof high SAR, calcium is applied to the soil, or injected into the irrigation water, usually in the formof gypsum (calcium sulfate).

Additional information

Figure 6: On 8 September 2008, the average temperature for for the previous 39 days had been
26.8°C including an average of 26.4°C in the first
8 days of September; even with these high temperatures, well above the optimum for creeping bentgrass, new roots are growing where air space hadbeen created in the soil by a slicing treatment at theend of August.

• On the subject of irrigation water quality, a References
particularly comprehensive yet concise guide
is Interpreting Turfgrass Irrigation Water Test Re- J.M. DiPaola. Syringing effects on the canopy temsults by Ali Harivandi: http://anrcatalog.
peratures of bentgrass greens. Agronomy Journal,ucdavis.edu/pdf/8009.pdf
76:951–953, 1984.
• For a clear explanation of leaching require- Jinmin Fu and Peter H. Dernoeden. Creeping bentment to maintain soil salinity at a tolerablegrass putting green turf responses to two sumlevel, I recommend Leaching for Maintenancemer irrigation practices: rooting and soil temfrom the University of Arizona: http://ag.
perature. Crop Science, 49:1063:1070, 2009a.
arizona.edu/pubs/water/az1107.pdf
Jinmin Fu and Peter H. Dernoeden. Creeping bent• An annotated chart of soil VWC withgrass putting green turf responses to two irrigamore detail than shown in Figures 2tion practices: quality, clorophyll, canopy temand 3 is available for viewing or downperature, and thatch-mat. Crop Science, 49:1071–
load:
http://www.flickr.com/photos/1078, 2009b.
asianturfgrass/8922147839/
E.A. Guertal and D.Y. Han. Timing of irrigation
for cooling bentgrass greens with and without• During the seminar, I showed some video
fans. USGA Turfgrass and Environmental Research
clips from the classic Water Movement in Soils
Online, 8(17):1–5, 2009.
movie. This film is available for purchasefrom the Department of Crop and Soil Sciences at Washington State University: http: Paul G. Johnson. The influence of frequent or infrequent irrigation on turfgrasses in the cool-arid//css.wsu.edu/merchandise/West. USGA Turfgrass and Environmental Research
Online, 2(6):1–8, March 2003.
Figure 6 shows that new roots can develop evenduring the hot temperatures of summer, provided
J.E. Jordan, R.H. White, D.M. Vietor, T.C. Hale, J.C.
there is adequate air space in the soil. By monitorThomas, and M.C. Engelke. Effect of irrigation
ing the VWC, it is possible to supply the grass withfrequency on turf quality, shoot density, and root
just enough water while still maximizing air space
length density of five bentgrass cultivars. Cropin the soil.
Science, 43:282–287, 2003.
In many parts of Japan, the average temperatureduring August is more than 28°C and creeping
bentgrass is subjected to severe heat stress. Managing the soil moisture at an optimum level, so thatdry spots (water deficit) are avoided, while still
keeping the soil as dry as possible so that air spaceis at a maximum, is an essential part of successful
greenkeeping during summer.
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