沙漠研究 22-1, 271 -274 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Evaluation of Measurement Accuracy of in-situ Devicefor Measuring Soil Moisture Profiles in Arid Land
Atsushi SHINKAI*1), Noriyuki YASUFUKU1), Kiyoshi OMINE1),
Taizo KOBAYASHI3), Atsushi MARUI2) and Tomoaki NAGAFUCHI2)
Abstract: A medicinal plant called Licorice (Glycyrrhiza), which originally grows in arid lands, has been used for various purposes
including herbal medicine. Recently, overharvest of Licorice has caused a problem of land deterioration and/or desertification, andsome exporting countries have begun restricting the export of Licorice. To ensure a stable supply of Licorice, these study cultivation
techniques of high quality Licorice are developed.
To cultivate Licorice, it is necessary to know soil properties in the site where wild Licorice grows. In this study, the fundamental
characteristic of an in situ device that can measure moisture content of unsaturated ground over the ground water level wasinvestigated. The device consists of a sensor probe, an access tube and a data acquisition device. Moisture distributions and its
time-variations in a vertical direction can be obtained by pushing the access tube into the ground, followed by inserting the sensor
probe in it. Sliding the sensor probe in the access tube allows us to measure the data at any point, and insertion/extraction of thesensor probe can be performed at anytime by leaving the access tube in the ground, indicating that this probe makes it possible toconduct a long-term monitoring of a continuous moisture profile.
As a result of accurate evaluation tests, it became clear that a calibration curve that is appropriate for sandy soil. This calibration
curve was obtained by changing only the constant term in the calibration curve that is provided from the manufacturer of ADR sensor.
Key Words: ADR, Field survey, Licorice, Soil moisture
fundamental characteristic of an in situ device that can measuremoisture content of unsaturated ground was investigated.

1. Introduction
It is said that 45 million (km2) of the land are the desert
ground, and, in addition, desertification progresses at the speedof 60,000 (km2) every year now. Desertification is a serious
problem in the global environment maintenance, and measuresagainst desertification are pressing needs. An approach forthe greening of desertification area has been conducted by
Yasufuku et al. (2010) with a high additional value that uses astrong medicinal plant for arid land like the Licorice. Licorice(Glycyrrhiza), which originally grows in arid lands, has beenused for various purposes including herbal medicine andsweetening ingredients. Recently, overharvest of Licorice has
caused a problem of land deterioration and/or desertification,and some exporting countries have begun restricting the exportof Licorice. To ensure a stable supply of Licorice, these study
cultivation techniques of high quality Licorice are developed.
Moreover, it is believed that this will lead to a countermeasureagainst desertification.
To cultivate Licorice, it is necessary to know soil properties
in the site where wild Licorice grows. In this study, the2. Characteristics of ADR Sensor
The device consists of a sensor probe, an access tube and adata acquisition device as shown in Figure 1. Four moisturesensors, which can detect moisture content by means of themethod of Amplitude Domain Reflectometry (ADR), aremounted in the sensor probe. Moisture distributions and its
time-variations in a vertical direction can be obtained bypushing the access tube into the ground, followed by inserting
the sensor probe in the access tube. Sliding the sensor probe
in the access tube allows us to measure the data at any point.
And insertion/extraction of the sensor probe can be performedat anytime by leaving the access tube in the ground. That is,
this sensor prove makes it possible to conduct a long-termmonitoring of a continuous moisture profile, where an image ofthe monitoring is schematically shown in Figure 2.
The ADR method is the one to measure the electricalresistance when shuttling by the signal of the high frequency ofabout 100 (MHz) that went out of two stainless rings pass

* Corresponding Author: marco_cb400four@yahoo.co.jp
744 Moto-oka, Nishi-ku, Fukuoka, Japan 819-0395
1) Faculty of Engineering, Kyushu University, Japan
2) Faculty of Agriculture, Kyushu University, Japan

3) Faculty of Engineering, Fukui University, JapanFig. 1. ADR sensor.

Percent finer by weight ( % )100Fig. 3. Calibration test.

Decomposed
granite soil

80

Silica sand No. 3
Silica sand No. 760402001

10100

1000

10Grain size ( μm )4105Fig. 4. Grain size distribution.
Table 1. Soil condition.
D50

Sample(1)

Where θ is volumetric water content and V is voltage obtained.
This equation is the calibration curve provided by themanufacture of the ADR sensor.

UC’0.60

13.3

1.9

Silica sand No. 3

1.50

1.6

1.1

Silica sand No. 7

0.25

2.1

1.1w(%)Height( cm )0, 1, 3, 5,7, 9, 11, 13,15, 17, 20,302530

Volumetric water content ( % )𝜃 = −0.057 − 0.66𝑉 + 8.00𝑉 2 − 27.91𝑉 3+49.23𝑉 4 − 42.46𝑉 5 + 14.47𝑉 6 (𝑚3 ⁄𝑚3 )UC

Decomposed granite soil

Fig. 2. Moisture profiling method.

through the soil. The electrical resistance depends on thepermittivity of the material in the soil, and the permittivity isdifferent according to each material. If the permittivity of air
is assumed to be 1, the permittivity of the soil is about 4 andwater is about 81. The measured permittivity is converted
into the voltage, and the volumetric water content in the soil iscalculated for the following polynomials;( mm )

2520

Decomposed
granite soil
Silica sand No. 3
Silica sand No. 7
Company's data
Calibration curve

1510
50200

400

600

800

1000

Output voltage ( mV )Fig. 5. Calibration curve.

3. Evaluation of Moisture Profiling Method
3.1. Calibration
First, to use the ADR sensor in local soil ground, theaccuracy of calibration needs to be examined.
Theexamination was carried out in a column of 25.7 (cm) in theinside diameter and 30 (cm) in height shown in Figure 3.
Decomposed granite soil, silica sand No. 3 (k - 3), and silica
sand No. 7 (k - 7) were used as the calibration samples.
Although a not less than 20 (kg) sample was needed for thiscalibration test, the sample collected by field survey was little.

Therefore, this examination was carried out using Japanese
sandy soil. The characteristic of grain size distribution and soil
condition are shown in Figure 4 and Table 1, respectively.
These sample used are assumed as the sandy soil of the desert
area where the investigation carried out. First, the sample thatwas used for adjusting the water content was packed into thecolumn. Next, the part of a stainless ring of the ADR sensor isinserted in the central portion of the sample, and the output
voltage is measured. After that, the sample is taken out, andwater content is directly measured by The JapaneseGeotechnical Society (JGS) standards. Finally, the relation
between the output voltage and the volumetric water content isformulated.
Figure 5 shows the relation between the output voltage andthe volumetric water content. The calibration curve that isprovided by the manufacturer of the ADR sensor isoverestimating the volumetric water content about 5 ~ 10%.
Therefore, it is necessary to obatain a new calibration curve thatis appropriate for sandy soil. It is understood that there is aclear relation between the volumetric water content and theoutput voltage of each sample. That is, each plot is on thesame curve, and this curve is expected to have a similar shapeas the calibration curve provided by the manufacture. Then,the next expression is presented as a calibration curve that is
appropriate for the sandy soil by moving Eq. (1) in parallel
downward direction.
𝜃 = −0.1352 − 0.66𝑉 + 8.00𝑉 2 − 27.91𝑉 3+49.23𝑉 4 − 42.46𝑉 5 + 14.47𝑉 6 (𝑚3 ⁄𝑚3 )(2)

It is necessary for θ - intercept of Eq. (1) to be changed in orderto move Eq. (1) in parallel downward direction. Therefore,Eq. (2) is characterized as an expression obtained by changing
only the constant term of the Eq. (1) as a result of the datafitting. This expression is able to estimate volumetric watercontent more accurately for the sandy soil used in this study.

Fig. 6. Multi-layer tank.

200
Earth tank A ( 21 hours )Earth tank A ( 240 hours )160

Earth tank B ( 21 hours )Height ( cm )3.2. Application of ADR sensor to multi - layer groundThe calibration test was based on the single sample.
However, it is thought that arid land has not only homogeneous
ground but also heterogeneous ground which consists ofvarious grain sizes. Therefore, in this examination, the ADR
sensor was verified to confirm the moisture profile in theground by making multi-layer ground as an approach toinvestigation on the heterogeneous ground.
The multi-layer ground of 2 (m) of height, where theheterogeneous ground on the arid land was imitated, is madeshown in Figure 6. The samples k - 7 and silica sand No. 8(k - 8) were used as model samples to prepare the laminatedsandy ground shown in Figure 6, where depth of each layer
was around 20 (cm). It is shown in Figure 6 that the order ofthe laminated layers are different from model A and model B.
Water was supplied from the bottom after the earth tank was
made, and the groundwater level was kept constant in theposition of 10 (cm) from the bottom.
Figure 7 shows the water status from a water supply fromthe beginning to 21 hours to 240 hours (in 10 days) later. It is
confirmed that the volumetric water content rises with thepassage of time. Moreover, the volumetric water content risesin the layer of k - 8, and the change in the volumetric water

120Earth tank B ( 240 hours )804000102030

405060Volumetric water content ( % )Fig. 7. Moisture condition. Black plots (●, ■, ▲, and ◆) are thelayer of k - 8.

content of the laminated ground is caught in the following
water status. It can be seen that the precision of ADR sensor
to each layer is high.
3.3. Determination of the calibration curve in situ
Although Japanese sandy soil was used for the calibration
test as the arid land imitation, if this calibration curve isinapplicable, it is necessary to obtain a new calibration curve.
In this section, reference was made about the proofreading
method in a field survey by using the characteristic of the ADRsensor that it can obtain a new calibration curve by paralleltranslation.
It is shown that a new calibration curve is obtained whenchanging only the constant term in the calibration curve that isprovided by the manufacturer of the ADR sensor. Then, theprocedure to proofread in situ volumetric water content byusing the sensor is shown below.
(1) The output voltage of the sensor in each depth is recordedat an original position.
(2) At some depth, undisturbed sample is collected, and thevolumetric water content is requested to be directly
measured through the laboratory experiments.
(3) The average of the difference of the volumetric watercontents by a calibration curve provided by themanufacturer and the laboratory experiments is requested.
(4) It is assumed to be a new calibration curve for sum or
difference between the average and constant term.
𝜃 = 𝛼 + 𝑓(𝑉) → 𝜃 = 𝛽 + 𝑓(𝑉) (𝑚3 ⁄𝑚3 )(3)

Where α is constant value as shown in Eq. (1), and β is constantvalue that was obtained, shown in Eq. (2) for example, and f(V) is a function of output voltage. This work is an effective
method in sandy soil, and application with different types ofsoils is future subject. If shapes of these calibration curves are
similar to the calibration curve provided by the manufacture,application is possible.

4. Conclusions(1) In this study, the calibration curve that is appropriate forsandy soil was obtained. This method is simple andconsists of changing only the constant term of thecalibration curve provided by the manufacturer. This

expression reflects the change in the volumetric watercontent by the difference of samples in the multi-layer
ground. That is, it is expected to be applied in aheterogeneous sandy soil.
(2) It was possible to measure the change of soil moisturewith the passage of time. A lot of access tubes were set
up on the investigation ground, the probe was inserted inevery case at an arbitrary place, depth, and time. In thismethod, it is suggested that the moisture amount in acertain profile and the monitoring of the measurement in
the direction of depth become simpler and economicallyfeasible.

Acknowledgement
This research was supported by the Grant-in-Aid for
Scientific Research (A) Ministry of Education, Culture, Sports,
Science and Technology, Japan, Grant number: 22246064,
Project leader: Prof. Yasufuku Noriyuki, Kyushu University.
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