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Appropriate irrigation methods for using saline water have long been established and practiced [10, 11].
While these methods target field crops, the green roof is much more vulnerable due to its thin soil layer.
Complete management of the green roof will burden building owners with obligations such as frequent
observations for plant health or system errors with the automatic irrigation system. Thus, irrigation
management needs to allow for the possibility of stressful conditions for the vegetation, and reduce the
amount of oversight required; this will make the green roof sustainable and encourage its use. However, as
there has been scarce information on established irrigation methods that consider stressful conditions, it is
difficult to estimate what degree of stress plants can experience without significant impacts on their growth.
One of the reasons this information is scarce is that it is difficult to evaluate the total amount of stress
incurred by plants, which can vary during irrigation intervals; estimating biomass reduction due to these
stressors is also a challenge. Therefore, the purpose of this study is to qualify the stress based on the stress
factors [12] and propose strategies for green roof management with gray water irrigation.

Design of the Irrigation System

Figure 3. The concept of the stress factor induced by drought and salinity, which is expressed by time
integration with the soil matric potential exceeding the plant threshold for normal growth.
In this study, gray water is to be used for green roof irrigation based on the assumption that gray water can
be treated enough that it can be safely applied to vegetation as irrigation water when supported by rainfall
collection into primary tank in order to dilute the original wastewater as shown in Figure 2. In this system,
gray water is stored in a primary tank, where water will undergo biological purification under aerobic and
anaerobic conditions [13]. An electric pump is used to lift the water from the primary tank to the secondary
tank. The irrigation is performed by the water supplied from the secondary tank, driven by renewable energy
(e.g., windmill or solar panels). In green roof management, there are two kinds of irrigation methods,
extensive and intensive [14]. The vegetation and irrigation systems are different depending on their
applications. The extensive irrigation method is widely used due to the low maintenance requirements: it
mainly relies on rainfall and a small amount of irrigation water during severe drought conditions (Fig. 3a).
Therefore, plant needs to be selected from varieties with drought stress tolerance, such as sedum species,
which exhibit Crassulacean acid metabolism (CAM), and grasses, which might be insufficient for a bee
habitat. With gray irrigation, plants incur salinity stress (Fig. 3b) and the combination of extensive irrigation
methods with gray water does not require much water overall; however, plants are continuously at risk of
drought and salinity stressors (Fig. 3c), as seen in dry climate areas [10]. In contrast, the intensive green roof
is conducted by the application of frequent irrigation by the micro-sprinkler and/or drip emitter, which

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enables the planting of various kinds of vegetation (Fig. 1). When the proper irrigation method is applied
with saline water, drought and salinity stressors can be prevented by either increasing the frequency of
irrigation to keep the soil moist to some extent to avoid the condensation of soil water or by leaching to
exclude excess soil for drainage [15].

METHODOLOGY
Experimental Conditions
The irrigation experiment was conducted during the hot summer season in a glasshouse, where the average
temperature and humidity were 32°C and 66%, respectively. A plastic pot was used for simulating the real
green roof system shown in Figure 1; a small pot with a 200 cm2 inner surface was used. At first, growth
media exhibiting a highly developed aggregate with a depth of 20 cm was laid on perlite gravel with a depth
of 10 cm, functioning as a filter fabric. A plastic plug was placed near the bottom of the pot to collect the
drainage water. The bottom of the pot played the role of a root barrier while the heat from the ground
penetrated through the thin plastic bottom. The ground in the glasshouse was covered by sand that was
heated, showing over 50°C in fine days. Next, a plastic cushion was put under the pots. Diluted seawater was
used to simulate gray water, and its salinity ranged from 0.7 (groundwater) to 18 dS m-1. Two intervals of
irrigation were used: frequent irrigation at intervals of 1–3 days and intermittent irrigation at intervals of 5–
17 days. Irrigation treatment with groundwater was regarded as the normal condition in the frequent
irrigation (control) and only the drought condition without salinity stress received the frequent interval
irrigation treatment with the groundwater irrigation. Evapotranspiration (ET) was measured by weighing the
pots before every irrigation. An amount 1.2 times the evapotranspiration amount was applied for irrigation to
get leaching. The EC and volume of drainage were measured. A small pan was used for measuring potential
evaporation. The water content was calculated based on the known weights of the pot, perlite gravel, and
dried soil, ignoring the weight of plant. The soil salinity was calculated from the difference between the salt
input in the irrigation water and the output in the drainage water, with the consideration of the water
contents. To quantify the stress factor, the soil matric and osmotic potential were converted based on the
measured soil water retention curve and simple liner conversion from the EC value, respectively. The
detailed methodology is explained by Moritani et al., 2013 [12].

Plants
For sustainable green roofs with gray water, it is indispensable to select plants that have a high tolerance for
drought and salinity. For example, succulent plants that have Crassulacean acid metabolism (CAM) have
fleshy leaves, which can store a fair amount of water (Hsiao and Acevedo, 1974). CAM plants also open
their stomata during the night so that CO2 is fixed into organic acids and eventually stored in the vacuoles.
During daytime, the stomata mostly remain closed, while the CO2 is released from the stored organic acids
and refixed via the Calvin-Benson cycle in the form of photosynthetic products [17, 18]. These
characteristics prevent water loss from CAM plants and decrease the uptake of salt into plant bodies [19].
Such plants are therefore suitable for green roofs because of their low water requirements. In contrast, C.
dactylon has been predominantly used for green roofs due to its high salt- and drought-stress tolerances. This
grass was found growing in the vicinity of a natural salt lake in Pakistan, and it has an electrical conductivity
(EC) of 19.9 dS m-1 [20] (Hameed and Ashraf, 2008); however, it has less tolerance for salt and drought than
CAM plants [21]. We used three kinds of plants, a turf grass (Cynodon dactylon) and two CAM plant
species, S. kamtschaticum and S. oryzifolium, for irrigation experiments. At the end of the experiments, the
dry matter yield (DM) of each plant was observed by weighing the shoots after drying them at 80ºC for 3
days. Dry biomass was powdered using a mixer, then the powder was mixed with a certain volume of
deionized water and shaken for more than a day, and the EC of the supernatant liquid was measured. The EC
of a leaf (ECleaf) was calculated based on the water contents of the biomass, using the simple inverse
proportion of ECleaf to the water contents of the biomass. The ion content of Na+, K+, Ca2+, and Mg2+ was
measured using the same methods as those used by Moritani et al., 2017 [16].

Determination of Stress Incurred by the Plant
Unlike in the thick soil of a field, the soil moisture, referred to as the soil matric potential, in our experiment
fluctuated greatly due to the small amount of soil water in the thin substrate layer. Moritani et al (2013) [12]
proposed the qualification of drought stress as a stress factor by time integration with the soil water potential
exceeding the threshold for normal growth (e.g. pF3.0) (Fig. 3a). While the drought stress is simply
enhanced by ET, salinity stress, which could be indicated by the osmotic potential, would keep increasing
from the salt accumulation of every irrigation until it reached the soil salinity level controlled by the leaching
requirement and irrigation salinity. Additionally, the salt concentration in the soil is condensed by ET during

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the irrigation interval (Fig. 3b). Therefore, careful irrigation management by frequent irrigation is needed to
prevent salt condensation as well as the application of excessive irrigation water to promote leaching of the
salt into the drainage water (Fig. 3c). While both the salinity and drought stress factors were taken beyond
the threshold for normal growth, the total stress factor was calculated to identify the additive stress factor of
drought and salinity.

RESULTS AND DISCUSSIONS
Influence of Stress Factor on Water Use Efficiency

Figure 4 shows the relationship between the stress
factor (S) and the total amount of ET (TET) in all
studied plants. The TET in C. dactylon ranged from
172 to 574 mm and was higher than that of CAM
plants under the same conditions. Although a weak
coefficient of determination (0.47) was found for S.
kamtschaticum, a negative relationship between the S
value and the TET was observed. The rate of decline
in the TET for the stress factor in CAM plants was
more rapid than in C. dactylon. This sharp decline
was due to closure of the stomata to prevent the
plants from losing water and intake salt from the soil
via CAM photosynthesis. The S value in CAM plants
increased based on water salinity, with maximum
values of 33.5 and 34.0 MPa∙day for S.
kamtschaticum and S. oryzifoliu, respectively, but no
significant difference in S values was seen between

Figure 4. Relationships between stress factors the frequent and intermittent irrigation treatments
and the total amount of evapotranspiration when comparing irrigation with water of the same
(TET). The formulae indicate a linear regression. salinity. This is because the water stress was not
incurred in the intermittent irrigation treatment, due
to closure of the stomata at an early stage of the
experiment, which did not require much irrigation
water. The S value of C. dactylon was highest, with
126.3 MPa∙day under only the water stress condition.
The S value of C. dactylon under frequent saline
irrigation was suppressed by 67% in comparison with
the intermittent condition with the same irrigation
salinity. The higher S value in the water stress
condition can be explained by the fact that water
stress escalates from just a small decrease in soil
water moisture due to logarithmical changes of the
soil water potential, while salinity stress increases
proportionally with the salinity input into soil. Thus,
when a green roof is designed to be covered by
plants, frequent salinity irrigation helps the plants
avoid a water deficit by maintaining the moisture
content and reducing the risk of salinity stress due to

Figure 5. The relative water use efficiency lower water requirement, which can reduce salt input
(WUE). The relative WUE under frequent into soil and avoid condensation of soil water.
irrigation with 0.7 dS m-1 is 1.0. Solid and
dashed lines are approximate lines for CAM Figure 5 shows the influence of the stress factor on
plants (S. kamtschaticum and S. oryzifolium) relative water use efficiency (WUE) when the relative
and turf grass (C. dactylon), respectively. WUE under frequent irrigation with tap water was
1.0. The relative WUE of CAM plants tended to
The formulae indicate a linear regression. increase with increasing irrigation salinity, and the

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The Heritage Chiang Rai, Thailand, May 27-29, 2020

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highest WUE values were observed at the highest values for th e stress factor since the amount of ET was
largely decreased by survival characteristics such as the closure of the leaf stomata during the stress
condition. In the case of C. dactylon, WUE did not increase significantly in the range of 1.2 to 1.5 since the
ET decreased with reduction of the plant biomass at almost the same rate. The rate of relative WUE for the
stress factor in CAM plants was approximately 12 times more than that in C. dactylon. Finally, stress
restricted the ET activity of CAM plants, although it increased the WUE. Therefore, the use of CAM plants
for green roofs with intermittent saline irrigation induces plant stress, although their high WUE enhances the
effectiveness in terms of reducing the amount of irrigation water required.

2+ 2+ + +
Figure 6. Cation contents of Mg , Ca , K , and Na in plant leaves and the EC of leaves under frequent

irrigation treatments with different irrigation salinity levels. The abbreviation of DW indicates dry mass.

Salt accumulation in leaves

Although salt accumulation in plant leaves could help in cellular osmotic adjustment under soil stress
conditions, it affects plant growth with harmful ions such as sodium. Figure 6 shows the inorganic cation
contents of the leaves of three plants. The total cationic contents increased with increasing irrigation salinity
due to salt uptake. This increase was influenced largely by the sodium contents in all the plants. The Na+ of
C. dactylon was highest among the three plants under the same irrigation salinity. This could be the result of
greater salt uptake from soil water due to higher transpiration activity. The ratio of sodium to all the cations
in all the plants was 0.04–0.21 times the sodium ratio in the irrigation water. This is because the plants
excluded sodium, which is important to avoid damage to cellular enzymatic activity. The potassium and
magnesium contents in leaves were 325–882 and 11.1–248 mmol kg−1 of the dry mass (DM), respectively,
as reported by Walter (2007) [22]. The potassium content under a no salt stress condition in the control was
higher than those of the other cations because a high potassium content is indispensable for the metabolic
processes and growth of plants (Leigh et al., 1984). The average potassium content in all the treatments of
the S. kamtschaticum and S. oryzifolium plants was 2.1 and 1.9 times higher than that of C. dactylon,
respectively. The ratio of magnesium and calcium to the other cations differed between the CAM plants and
C. dactylon. Although the magnesium content was the second largest amount following Na+ in the irrigation
water [16], it was the least prevalent cation in the plant leaves. The content of this cation increased with
increasing irrigation salinity and was greater in C. dactylon, ranging from 30.3 mmol kg−1 to 249 mmol kg−1
of the DM. The CAM plants had a constant calcium content with increasing irrigation salinity, in contrast
with C. dactylon, in which an increase in calcium was observed, ranging from 47.1–163 mmol kg−1 of the
DM. The average calcium content of the CAM plants in all the treatments was 4.4 times higher than that of
C. dactylon, a trend similar to that reported by Walter (2007) [22].

The EC of the leaf solution tended to increase proportionally with the total cation amount (Fig. 6). However,
the EC in CAM plants was 0.28 to 0.55 times less than that of C. dactylon. This is because the abundant
water contained in CAM plants may dilute the salinity in leaves. For C. dactylon and S. kamtschaticum, the
biomass decreased according to the ECleaf increment, with a determination coefficient of linear regression
over 0.90. The biomass of S. oryzifolium had not decreased due to possible stomata closure after salinity
stress was applied in the first irrigation in the experiment, while S. kamtschaticum might have been damaged
by salinity stress during the process of shifting from C3 to CAM [16]. Thus, measurement of ECleaf is
recommended as an alternative simple method to examine the degree of stress to the plant (except pure CAM

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plants) because measuring soil salinity is a time- and labor-consuming operation, as it involves sampling of
soil in the deep-profile range of the root zone.

CONCLUSION

Saving water for irrigation is essential

due to limited water volume, especially

in water-scarce areas; this is true even if

irrigation water with gray water is

available. Hence, green roof irrigation

should be applied to reduce the gross

water requirement. Additionally,

choosing adequate plants for the water

and salt stresses of the environment is

important for maintaining high plant

biomass to provide a habitat for bees

and a good urbanscape. Based on this

study, a sustainable green roof with

gray irrigation and four sections (as

shown in Fig 7.) is proposed. 1) A basic

green roof is limited by the thin layer of

soil available due to the necessity to

Figure 7. Sustainable green roof system with gray irrigation pass the loading regulations for roofs,
water in four sections based on two axes. implying that there will be only a small
amount of readily available water for

root uptake. Therefore, a highly

developed soil aggregate is needed to

enhance both water retention and drainage of rainfall or excess soil salinity. 2) Limited water in the soil with

gray water stresses the plant. The kinds of plants possessing the characteristics of drought and salt tolerance
could be screened by preliminary tests to measure the plant threshold for the stress factor. The plant’s health

under high salinity could be diagnosed easily by sampling leaves and measuring the EC of leaves 3) The

climate should be considered severely hot and dry to avoid a worst-case scenario in which the chosen plants

and vegetation cannot survive. 4) The irrigation system should utilize electricity driven by solar or wind

power in case of a blackout. Proper scheduling of the frequency and amounts of each irrigation is crucial.

The irrigation amount can be calculated from the ET based on the rainfall amount and leaching requirement.

However, ET changes according to the stress factor, climate condition, and leaf area index. These

measurements require soil sensors, and weather and plant observations. Therefore, irrigation can be applied

under fixed ET input with the leaching requirement calculated from the premeasured soil EC of the plant

threshold and irrigation salinity [10]. Further research using herbaceous plants will be required for a more

attractive environment of bee habitats.

ACKNOWLEDGEMENT
The authors gratefully acknowledge T. Fujita for providing the CAM plants used in this study, and T.
Shimizu for technical support. This work was partially funded by JSPS KAKENHI Grant Number
18K05895.

REFERENCES

[1] Sanchez, M. and Wang, D. 2020. Carrying the ball: A portrait of leadership in urban rooftop
agriculture Journal of Living Architecture. 7(1): 1-26.

[2] Goulson, D., Lye, G. C., Darvill, B. 2008. Decline and conservation of bumble bees. Annual Review
of Entomology. 53: 191-208.

[3] Hennig, E. I. and Ghazoul, J. 2012. Pollinating animals in the urban environment. Urban Ecosystems.
15: 149-166. DOI: 10.1007/s11252-011-0202-7.

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The Heritage Chiang Rai, Thailand, May 27-29, 2020

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[4] Kratschmer, S., Kriechbaum, M. and Pachinger, B. 2018. Buzzing on top: Linking wild bee diversity,
abundance and traits with green roof qualities. Urban Ecosystems. 21: 429-446. DOI: 10.1007/s11252-
017-0726-6.

[5] Hofmann, M. M. and Renner, S. S. 2018. Bee species recorded between 1992 and 2017 from green
roofs in Asia, Europe, and North America, with key characteristics and open research questions.
Apidologie. 49: 307-313.

[6] Penny, J. 2019. Beehives for Green Roofs, stamats communications, INC. Buildings
https://www.buildings.com/news/industry-news/articleid/22001/title/beehives-green-roofs
(accessed April 5, 2020).

[7] Friedler, E. and Hadari, M. 2006. Economic feasibility of on-site greywater reuse in multi-storey
buildings. Desalination. 190: 221-234.

[8] Solodar, A., Bawab, O., Levy, S., Kadas, G.J., Blaustein, L. and Greenbaum, N.. 2018. Comparing
grey water versus tap water and coal ash versus perlite on growth of two plant species on green roofs.
Science of The Total Environment. 633: 1272-1279. DOI: 10.1016/j.scitotenv.2018.03.291.

[9] Williams, N.S.G. Rayner, J.P. and Raynor, K.J. 2010. Green roofs for a wide brown land:
opportunities and barriers for rooftop greening in Australia. Urban Forestry & Urban Greening. 9(3):
245-251.

[10] Ayer, R.D. and Westcot, D.W. 1985. Water quality for agriculture. FAO Irrigation and Drainage Paper
29: FAO, Rome. 141.

[11] Allen, R.G., Pereira, L.S., Raes, D., and Smith, M. 1998. Crop evapotranspiration: guidelines for
computing crop water requirements. Irrigation and Drainage Paper. 56: 177, FAO, Rome, Italy.

[12] Moritani, S., Yamamoto, T., Andry, H., Inoue, M., Kato, K. and Saito, H. 2013. Effect of combined
water and salinity stress factors on evapotranspiration of Sedum kamtschaticum Fischer in relation to
green roof irrigation. Urban Forestry & Urban Greening. 12: 338-343. DOI:
10.1016/j.ufug.2013.04.005.

[13] Ichinari, T., Ohtsubo, A., Ozawa, T., Hasegawa, K., Teduka, K., Oguchi, T. and Kiso, Y. 2008.
Wastewater treatment performance and sludge reduction properties of a household wastewater
treatment system combined with an aerobic sludge digestion unit. Process Biochemistry. 43(7): 722-
728. DOI: 10.1016/j.procbio.2008.02.016.

[14] Van Mechelen, C., Dutoit, T. and Hermy, M. 2015. Adapting green roof irrigation practices for a
sustainable future: A review. Sustainable Cities and Society. 19: 74-90.

[15] Smith, R. J. and Hancock, N. H. 1986. Leaching requirement of irrigated soils. Agricultural Water
Management. 11: 13-22.

[16] Moritani S., Yamamoto T., Andry H., and Saito H. 2017. Evapotranspiration and mineral content of
Sedum kamtschaticum Fischer under saline irrigation. Communications in Soil Science and Plant
Analysis. 48: 1-10. DOI: 10.1080/00103624.2017.1358743.

[17] Kluge, M. 1977. Is Sedum acre L. a CAM plant? Oecologia. 29: 77-83.
[18] Black, C. C., 1973. Photosynthetic carbon fixation in relation to net CO2 uptake. Annual Review of

Plant Physiology and Plant Molecular Biology. 24: 253-286.
[19] Gravatt, D. A. and Martin, C. E. 1992. Comparative ecophysiology of five species of Sedum

(Crassulaceae) under well-watered and drought-stressed conditions. Oecologia. 92: 532-541.
[20] Hameed, M. and Ashraf, M. 2008. Physiological and biochemical adaptations of Cynodon dactylon

(L.) Pers. from the Salt Range (Pakistan) to salinity stress. Flora. 203: 683-694.
[21] Marcum, K. B. 2006. Use of saline and non-potable water in the turfgrass industry: Constraints and

developments. Agricultural Water Management. 80(1-3): 132-146.
[22] Larcher, W. 2003. Physiological plant physiology (4th edition). Springer-Verlag, Berlin, Heidelberg,

New York, pp. 513.

9th International Conference on Environmental Engineering, Science and Management
The Heritage Chiang Rai, Thailand, May 27-29, 2020