0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Green area Bare roof

He

at f

lux fr

om

air

to b

uild

ing(M

J d

-1) 8/11

8/12

Dawnstream

tank

Rice plant area Upstream

tank

Float Pump

Water

Water flowWater supply

Effects of a rice plant hydroponics system on the thermal environment

over an urban rooftop during summer

Yoshikazu Tanaka1*, Shigeto Kawashima1, Takehide Hama2, Kimihito Nakamura1

1Graduate School of Agriculture, Kyoto University, 2 Graduate School of Science and Technology, Kumamoto University.

Introduction

Material and Methods

Results and Discussion

Conclusion

Address correspondence to Yoshikazu Tanaka, Graduate School of Agriculture, Kyoto University

Email: tanaka.yoshikazu.25u@st.kyoto-u.ac.jp

Picture of experimental area This system is installed on a building

rooftop. This building has 8 floors,

and located in central of Osaka.

Experimental area is about 12m2 .

Fig.1 Heat island effect in Osaka, 2013

(Japan Meteorological Agency, 2013) .

Fig.2 Rice plant hydroponics

system

Fig.3 Observation points

Air temperature（℃）

Water temperature（℃）

Surface temperature（℃）

Heat flux（Wm-2）

Wind speed (m/s)

Wind direction (°)

Solar radiation (Wm-2)

Precipitation (mm)

Dew point

Air temperature(˚C)

Weather station

Green area

Bare roof area

The problem of increasing hot day during summer in major cities is

forming heat islands. In Osaka, for example, the days when daily

mean air temperature over 30˚C continued in 2013. It was occurred

heat island effect in Osaka (Fig.1). There are a lot of treatment for

mitigating heat island effect. Our study focuses on green rooftop using

hydroponics of rice plant. It was already founded that green rooftop

lowered air and surface temperatures by some previous studies. Rice

plant has a good evapotranspiration. We expect more cooling effect by

rice plant hydroponics system. Fig.5 Hourly mean temperature

Fig.4 Daily mean air temperature(7/1-8/31)

Temperature of green are is lower than that of bare roof

during observation term excepting rainy day.

In maximum temperature, the difference of temperature is

approximately 3.0 ˚C

Air temperature of green area is lower

than that of bare roof in all day.

In maximum air temperature, the

difference of the temperature is

approximately 8.0 ˚C.

In air temperature of green area, the

hydroponics system decreased air

temperature.

In maximum surface temperature, the difference

of the temperature is approximately 19℃.

In this study, we demonstrated through air and surface temperatures and

heat flux measurements that a building can be cooled with the addition

of a rice hydroponics system on its roof. Fig.11 Heat flux (8/11, 8/12)

Fig.7 Hourly mean surface temperature Fig.6 Daily mean air temperature(7/1-8/31)

Fig.9 Estimate of heat flux

(6:00 – 18:00)

0 10 20

Green area

Bare roof

Estimated heat flux（MJ m-2d-1）

G H ΔS lE

37% 63%

29% 50%17%

4%

86%

Down

Fig.12 Estimated heat balance(8/11)

Fig.10 Heat balance (8/11-8/12)

Fig.8 Relationship between daily

solar radiation and ΔTa, ΔTs.

We defined two parameter such as

ΔTa(˚C) = Air temp.of green area – that of bare roof

ΔTs(˚C) = Surface temp. of green area

– that of bare roof

Both of ΔTa and ΔTs depend on solar radiation.

Lower radiation occurs on a rainy day.

Maximum of ΔTa is approximately -3.0˚C

Maximum of ΔTa is approximately -6.0˚C

This experiment clarified the followings.

Air and surface temperatures of green area are lower than that of

bare roof except rainy days.

Estimate of heat flux from the air to the roof in green area

decreased about 86% of bare roof on Aug.11.

ΔTa and ΔTs depend on solar radiation.

G and H decreased by this system.

Green area: 𝑅𝑛= 𝐻 + 𝑙𝐸 + 𝐺 + ∆𝑆

Bare roof: 𝑅𝑛 = 𝐻 + 𝐺 where 𝑅𝑛: Net radiation, 𝐻: Sensible heat flux, 𝑙𝐸: Latent heat flux, 𝐺: Ground heat flux, ∆𝑆: Water heat storage.

G and H decreased by using this system.

In green area, sum of ΔS and lE occupied

more than half of net radiation.

Roof

Roof

Rice plant

Water

Container

Block

0

20

40

60

80

10020

25

30

35

40

7/1

7/5

7/9

7/1

3

7/1

7

7/2

1

7/2

5

7/2

9

8/2

8/6

8/1

0

8/1

4

8/1

8

8/2

2

8/2

6

8/3

0

Pre

cip

ita

tio

n (

mm

)

Air

Te

mp

era

ture

(℃）

Precipitation

Bare roof area

Green area

20

25

30

35

40

7/1

7/5

7/9

7/1

3

7/1

7

7/2

1

7/2

5

7/2

9

8/2

8/6

8/1

0

8/1

4

8/1

8

8/2

2

8/2

6

8/3

0

Su

rfa

ce

te

nm

pe

ratu

re （

℃）

Bare roof area Water

y = -0.2948x + 1.9417

R² = 0.8342

y = -0.0987x - 0.0417

R² = 0.6807

-7

-6

-5

-4

-3

-2

-1

0

1

2

0 5 10 15 20 25 30

ΔT

a,Δ

Ts

（℃

）

Daily solar radiation （MJ m-2 d-1）

-400

-200

0

200

400

1:0

03

:00

5:0

07

:00

9:0

01

1:0

01

3:0

01

5:0

01

7:0

01

9:0

02

1:0

02

3:0

01

:00

3:0

05

:00

7:0

09

:00

11

:00

13

:00

15

:00

17

:00

19

:00

21

:00

23

:00

He

at f

lux (

W m

-2)

Bare roof Green area

8/11 8/12

20

25

30

35

40

45

50

1:0

03

:00

5:0

07

:00

9:0

01

1:0

01

3:0

01

5:0

01

7:0

01

9:0

02

1:0

02

3:0

01

:00

3:0

05

:00

7:0

09:0

01

1:0

01

3:0

015:0

01

7:0

01

9:0

02

1:0

02

3:0

0

Air

te

mp

era

ture

（℃

）

Bare roof Green area

8/11 8/12

25

30

35

40

45

50

55

60

1:0

0

4:0

0

7:0

0

10:0

0

13:0

0

16:0

0

19:0

0

22:0

0

1:0

0

4:0

0

7:0

0

10:0

0

13:0

0

16:0

0

19:0

0

22:0

0

Su

rface te

mpera

ture

(℃

)

Bare roof area Water

8/11 8/12

-400

-200

0

200

400

600

800

1000

1:00

4:00

7:00

10:00

13:00

16:00

19:00

22:00

1:00

4:00

7:00

10:00

13:00

16:00

19:00

22:00

1:00

Rn G H

8/11 8/12

-400

-200

0

200

400

600

800

1000

1:00

4:00

7:00

10:00

13:00

16:00

19:00

22:00

1:00

4:00

7:00

10:00

13:00

16:00

19:00

22:00

1:00

He

at f

lux (

Wm

-2)

Rn G ΔS lE H

8/11 8/12