Stable isotope-based approach to validate effects

of stand structure and understory on soil water in a Japanese forest plantationSaki Omomo¹ , Yuichi Onda¹ , Boutefnouchet Mohamed¹ , Chenwei Chiu² , Takashi Gomi² , Sean Hudson¹ , Yupan Zhang¹ , and Janice Hudson¹

・¹University of Tsukuba, Geoscience, Japan・² Department of International Environmental and Agricultural Science (IEAS), Tokyo University of Agriculture and Technology, Japan

Expected effects of thinning

Improving the forest environment

(Decreasing of canopy intercept,

Increasing of understory vegetation,

Increasing of drought flow, ect)

Plantation forest not properly

managed

Increasing of canopy blockage

Decreasing of understory vegetation

Decreasing of drought flow

Increasing of sediment discharge

Despite the complexity of the effects of thinning on groundwater recharge, such as the soil surface

becoming covered with vegetation over time after thinning, the effects of thinning on groundwater recharge have

not been studied yet.

Brooks et al. (2009) classified soil water into three types: tightly bound water, mobile water, and

preferential flow, showing that tightly bound water is absorbed by plants, while mobile water and preferential flow

recharge groundwater and rivers.

Isotope analysis of mobile water and preferential flow should be taken to determine the type of soil water

that actually recharges the groundwater.

Introduction

Zero tension lysimeterSuction lysimeter

Mobile water ? Preferential flow ?

Determine the impact of thinning on groundwater recharge

Separate collection of mobile water

and preferential flow to determine what

water recharges groundwater

To investigate the effects of thinning on

soil moisture in four plots with different overstory

and understory vegetation distributions.

Object

Method

Throughfall

Open rainfall

Mobile water

Suction lysimeter

Zero

tension

lysimeter

Preferential Flow

Various water samples

Collecting data and sampling

Evaporation Soil water potential

Various micrometeorological

data

Weighing

lysimeter Tensiometer

RadiationSoil moisture

content

Modular zero tension lysimeter

• It can be inserted without breaking

soil and have a wide catchment

surface

• This shape also prevents the

release of soil water and improves

collection efficiency.

Installing zero-tension lysimeter

P1O P1O+ P1C P1C+ P3O+ P3C+

10cm cm² 666 600 640 668 672 676

30cm cm² 700 628 552 700 626 600

表1：Catchment area of zero tension lysimeter

Field

(https://www.google.com/maps/place)

Japan, Tochigi-prefecture, Mt. Karasawa

(Field museum of Tokyo University of Agriculture and Technology)

Result : The time series of δO¹⁸ and precipitation

Fig.2：Time series change of δO¹⁸ for mobile water and preferential flow, and the daily precipitation. Fig.1：Time series change of δO¹⁸ for open rainfall and throughfall, and the daily precipitation.

・Analysis for soil water and throughfall sample of 8/6/19 isn’t finished yet.

・We couldn’t collect much water sample in P1O and P1C. Therefore, We discuss using the data in P1O+, P1C+, P3O+ and P3C+

・Soil water sampled by the zero tension lysimeter is assumed to be preferential flow, and soil water sampled by the suction

lysimeter is assumed to be mobile water.

Open rainfall and throughfall Soil water (suction lysimeter and zero-tension lysimeter)

0

50

100

150

200

250

300

-12

-10

-8

-6

-4

-2

0

6/1

7/1

9

6/2

4/1

9

7/1

/19

7/8

/19

7/1

5/1

9

7/2

2/1

9

7/2

9/1

9

8/5

/19

8/1

2/1

9

8/1

9/1

9

8/2

6/1

9

9/2

/19

9/9

/19

9/1

6/1

9

9/2

3/1

9

9/3

0/1

9

10/7

/19

10/1

4/1

9

10/2

1/1

9

Daily p

recip

ita

tio

n (m

m)

δO

¹⁸

date

●：5cm, ○：10cm ■：20cm, □：30cm🔶：50cm, ◇：80cm ▲：Preferential flow 10cm△：Preferential flow 30cm

―：P1O, ―：P1C, ―：P1O+, ―：P1C+―：P3O+, ―：P3C+

0

50

100

150

200

250

300

-12

-10

-8

-6

-4

-2

0

6/1

7/1

9

6/2

4/1

9

7/1

/19

7/8

/19

7/1

5/1

9

7/2

2/1

9

7/2

9/1

9

8/5

/19

8/1

2/1

9

8/1

9/1

9

8/2

6/1

9

9/2

/19

9/9

/19

9/1

6/1

9

9/2

3/1

9

9/3

0/1

9

10

/7/1

9

10

/14/1

9

10

/21/1

9

Daily p

recip

ita

tio

n (m

m)

δO

¹⁸

date

×：Throughfall○：Open rainfall

―：P1O, ―：P1C, ―：P1O+, ―：P1C+―：P3O+, ―：P3C+

-20

-10

0

10

20

30

40

50

60

70

80

90

-12-10-8-6-4-20

Dep

th (

cm

)

δO¹⁸

―：P3O+, ―：P3C+

●：Mobile water

×：Preferential flow

-20

-10

0

10

20

30

40

50

60

70

80

90

-12-10-8-6-4-20

Dep

th (

cm

)

δO¹⁸

―：P1O+, ―：P1C+

●：Mobile water

×：Preferential flow

Discussion : The change of the δO¹⁸ value

P1O+ and P1C+

Fig. 3: The weighted average of δO¹⁸ value for each water samples in P1O+, P1C+, P3O+ and P3C+.

‣ Mobile water (Suction lysimeter)

The average of δO¹⁸ value in P1O+ and P3O+ is closer to open rainfall than it in P1C+ and P3C+. The change of

δO¹⁸ value for mobile water become smaller as deeper points.

‣ Preferential flow (Zero-tension lysimeter)

The average of δO¹⁸ value for preferential flow at 30cm and The average of δO¹⁸ value for mobile water at 80cm is

very close in P1O+, P3C+.

Open rainfall

ThroughfallOpen rainfall

Throughfall

P3O+ and P3C+

Summary

Fig. 4: Schematic diagram of isotopic composition of soil water.

Since the soil water δ¹⁸O collected by the zero tension lysimeter are close to the δ¹⁸O values of open

rainfall, the soil water collected by the zero tension lysimeter can be judged as the preferential flow.

The value of δ¹⁸O for groundwater-recharging soil water is changed before and after thinning in response

to the changing of input rainfall δ¹⁸O values.

The 80cm soil water δ¹⁸O value can be judged as the mixture of the preferential flow and mobile water at

30 cm depth.

The impact of thinning on groundwater recharge could be clarified by determining the percentage of

preferential flow in soil water at each depth.

ThroughfallEvaporation

Mo

bil

e w

ate

r

Ground water

Pre

fere

nti

al fl

ow