沙漠研究 22-1, 283 -286 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Analysis of Changes in Soil Water Content under Subsurface Drip Irrigation
Using Ground Penetrating Radar
Hirotaka SAITO*1) and Masatoshi KITAHARA2)
Abstract: In arid or semi-arid regions high-performance irrigation systems are necessary to minimize the amount of water used foragriculture purposes. Among common irrigation systems, subsurface irrigation is known to increase the water use efficiency bydecreasing the water loss from the ground surface. For effective design and management of the subsurface irrigation systems,
non-destructive methods to observe changes in water contents in soils are essential. Ground penetrating radar (GPR), one of thegeophysical methods for subsurface measurement, has been used to observe subsurface water contents non-destructively. The main
objective of this study was to investigate whether or not changes in soil water content distributions under subsurface irrigation can beobserved using GPR. In this study, laboratory experiments were conducted using a soil lysimeter (120 cm × 75 cm × 60 cm) filledwith river sands. An irrigation pipe was placed at a depth of 23 cm to supply water at a given head for 60 minutes. A GPR systemwith 1000 MHz central frequency was used in this study. GPR common offset (CO) data were collected during and after irrigation.
CO profile data (radagram) show reflections from wetting fronts around the irrigation pipe. Vertical distributions of water contents atthe center were then estimated from two-way travel time of EM waves. This study demonstrates that GPR can be used to observechanges in water contents due to subsurface irrigation.
Key Words: Ground penetrating radar, Infiltration, Redistribution, Subsurface irrigation, Water content

1. Introduction
Arid or semi-arid areas cover more than one third of
Earth’s land surface. As water resources are limited in suchareas, efficient irrigation systems need to be introduced forfarming. In areas where evapotranspiration rates are muchgreater than precipitation rates, irrigation systems supplying
water directly to root zones are necessary to use limited water
resources efficiently. Subsurface drip irrigation is one of suchsystems installed and practiced in arid and semi-arid regions(e.g., Hanson et al., 1997; Kandelous and Simunek, 2010).
However subsurface drip irrigation has some shortcomings that
can happen frequently but are not easy to detect. For example,
although irrigation pipes can be easily cracked or emitters canbe readily clogged by soil particles, their exact locations cannot
be easily detected unless some subsurface imaging techniques
are used. To manage subsurface irrigation effectively,changes in soil moisture contents thus need to be monitored notat specific discrete locations but along subsurface irrigation
pipes (in two or three dimensions).
Ground penetrating radar (GPR), one of the geophysicaltools to prove subsurface, allows imaging near-surface
environment non-destructively using electro-magnetic (EM)waves. GPR emits pulse EM waves from a transmitter
antenna to the ground and receives the reflected or refracted
waves that travel in soils at a receiver antenna. From thearrival time of the EM wave, we can estimate the EM wave
velocity in the soil if the wave travel path is known. At thefrequency range usually used in GPR, the EM wave velocity
depends strongly on the electric property of the soil that is therelative permittivity, εr, as shown in Eq. (1).

v≅

cεr

(1)

where v is the EM wave velocity [m s-1] and c is the speed oflight in vacuum (3.0 × 108 [m s-1]). The relative permittivityof water is about 81, while that of soil particles is about 3 to 5
and that of air is 1. This indicates that the relative permittivityof the soil is very sensitive to its moisture content (Huisman etal., 2003). In fact, many studies have shown a significant andunique relationship between the relative permittivity and themoisture content (e.g., Topp et al., 1980). Such empiricalequations are known as petrophysical relationships.
The main objective of this was therefore to investigate theperformance of GPR to monitor changes in soil moisture
contents under subsurface drip irrigation.

* Corresponding Author: hiros@cc.tuat.ac.jp
3-5-8 Saiwai-cho, Fuchu, Tokyo, 183-8509 Japan
1) Institute of Agriculture, Tokyo University of Agriculture and Technology2) Department of Agricultural and Environmental Engineering, Tokyo University of Agriculture and Technology

during irrigation. CO profiles were then collected every 60
minutes for 300 minutes after irrigation was ceased. One CO
profile was finally collected at 1440 minutes.
In this study, Topp equation was used to estimate watercontents from relative permittivity values as it is known toperform well with sandy soils.

3. Results and Discussion

Fig. 1. Schematic diagram of the sand lysimeter.

2. Materials and Methods
In this study, a laboratory experiment was carried out using
a 120 cm × 75 cm × 60 cm sand lysimeter that was made fromtransparent PVC (Fig. 1). Air dried river sand with a medianparticle size (d50) equal to 0.34 mm was filled at a
pre-determined dry bulk density of 1.55 g cm-3 on top of the2-cm gravel layer at the bottom. The texture of the soil used
was classified as sand from the particle size analysis.
A 2-cm diameter PVC pipe was placed horizontally at thedepth of 23 cm with a 3-mm diameter water emitting inlet
faced up at the middle of the lysimeter. To avoid any
clogging at the inlet, a metal mesh with a diameter muchsmaller than the sand particle size was placed on top of theinlet before installing to the lysimeter. The PVC pipe wasconnected to a water reservoir tank in which a water level canbe held constant. In the remainder of the manuscript, thePVC pipe is referred to as the irrigation pipe as it supplies
irrigation water to the soil lysimeter. This system drawn in
Figure 1 represented a subsurface irrigation system in ourexperiment. In this experiment, a 5-cm constant head wasmaintained at the water tank during 60-min irrigation. Avalve at the bottom of the water tank was closed after 60
minutes to cease irrigation. The water tank was also a
Mariotte bottle tank so that water supply rates could beobserved.
To monitor movements of wetting fronts in the lysimeter,time-lapse common offset (CO) profiles were acquired using asurface GPR system. All measurements were carried out
using a pulseEKKO PRO system (Sensors and Software,
Canada) with 1000 MHz central frequency antennas. CO
profile data can be acquired by keeping an antenna separation
constant. In our experiment, both transmitter and receiverantennas were moved along the CO survey line (x-axis) with afixed antenna separation of 15 cm. CO traces were acquired
at every 2 cm along the survey line. After an initial profile
was acquired, CO profiles were collected every 5 minutes

Figure 2 shows ground penetrating radar common offset(CO) profiles acquired at 0 (initial), 20, 40, 60, 360, and 1440
minutes. The CO profile is also called “radagram” which is acollection of waveform traces recorded at all acquisition
positions. The vertical axis of the profile represents thearrival time of the EM waves, while the horizontal axis
represents the antenna position. Each trace has positive andnegative amplitudes. Black and white colors in the radagram
therefore correspond respectively to the positive end and thenegative end of the amplitude range.
In the initial CO profile (Fig. 2(a)), there are three distinct
strong signals; two parallel ones and one in a hyperbolic shape.
The first parallel strong signal accounts for the pulse wave
traveled directly between two antennas. This kind of wave isreferred to as a direct wave. The direct wave includes waves
that travel through air and along the interface of air and ground,which are called air wave and ground wave, respectively.
Because the velocity of air wave is equal to the speed of lightin the vacuum, an actual travel time can be calculated if theantenna separation is fixed. Then the air wave arrival time
can be used to calibrate time zero of the profile data.
Another strong horizontal signal in Figure 2(a) at 9 ns isdue to the reflection of the EM wave at the bottom of thelysimeter. If the moisture content of the soil is uniform inside
the lysimeter, the velocity of the EM wave also becomes
constant. In such a situation, reflected signals from thehorizontal boundary such as the bottom of the lysimeter
become horizontal. As the depth of the soil profile is constantat 75 cm in this experiment, a two-way travel time of the EM
wave can be used to estimate the EM wave velocity. Once,the velocity is estimated, the average volumetric water content
can be calculated from Topp equation (Topp et al., 1980).
The hyperbolic shape signal peaked at x = 60 cm in Figure2(a) represents a reflection of the EM wave at the irrigation
pipe installed 23-cm below the soil surface at x = 60 cm.
When a point reflection source exists, the shape of the reflectedwave in the radagram becomes hyperbolic (e.g., Kearey et al.,2002). In this experiment, the irrigation pipe is a pointreflection source. From the hyperbolic shape signal in theradagram, the average EM velocity to the reflected point canbe uniquely determined (e.g., Kearey et al., 2002).

Fig. 2. Ground penetrating radar common offset profile data, radagrams, acquired at a) 0 min, b) 20 min, c) 40 min, d) 60 min, e) 360 min, and f)
1440 min (24 hr).

Figure 2(b)-(d) show respectively the CO profile obtained
at 20, 40, and 60 minutes. In addition to three signals shownin Figure 2(a), a strong hyperbolic-shape reflection overlappingwith the direct wave near 0 ns is observed in all threeradagrams. This is due to reflection of the EM wave at theinfiltration front created above the irrigation pipe, from wherewater was infiltrated. In Figure 2(b)-(d), there is an additionalhyperbolic-shape reflection peaked respectively at 7.5 ns, 9 ns,
and 10 ns. This is a reflection from the wetting front beneaththe irrigation pipe. The position changes because the wetting
front moves downward with time. Water infiltrated towardboth above and below the irrigation pipe. Because theaverage EM velocity can be uniquely determined from thehyperbolic-shape signal, the depth of the reflected point, inother word, the infiltration front can be estimated.
Because the horizontal reflection from the bottom wasdistorted at the center in Figure 2(b)-(d), it is difficult identified
the reflection from the bottom around x = 60 cm. However,
the extent of infiltration in the horizontal direction can be easily
identified. The distortion happens approximately between x =45 cm and x = 75 cm in all three figures. This indicates that,outside of this boundary, the soil condition is about the same as
the initial condition, which means there is no water infiltrated
in these areas. GPR profile data, therefore, provide not only
the vertical extent of the wet area but also the horizontal extent
of the wet area due to subsurface drip irrigation.
Figure 2(e) shows the CO profile at 360 minutes (300
minutes after irrigation was ceased). Although it is not clear
as it is for Figure 2(b)-(d), there is still a hyperbolic shapereflection near 0 ns. This represents a reflection from thewetting front above the irrigation pipe. While the reflection atthe irrigation pipe is clearly seen as others, there is no reflection
observed for the bottom end of the wetting front. This may
be because the wetting front beneath the irrigation pipe reachedto the bottom of the profile prior to 360 minutes.
Figure 2(f) represents the CO profile obtained at 1440
minute. Since irrigation lasted only 60 minutes, this figure
shows the CO profile 1380 minutes after irrigation was ceased.
Due to redistribution of water, there is no clear
hyperbolic-shape reflection near the surface unlike that shownin Figure 2(b)-(e). As for the reflection at the irrigation pipe,
we can again see the hyperbolic shape signal clearly because itcalculated based upon average EM velocities obtained fromthe CO profile data using Topp equation along with initial andfinal water contents directly measured by the gravitational
method. As expected both GPR-based water contents anddirectly measured water contents for the initial condition andthose at 1440 minutes match generally well and show low anduniform water content values along the center of the profile.
Water content values drastically increased around theirrigation pipe during irrigation. By converting GPR profiled
data to water content distribution data, we can clearly andquantitatively obtain the vertical extent of the wet area due tosubsurface drip irrigation.

4. Conclusion

Fig. 3. Water content distributions at x=60 cm obtained from GPR
profile data.

is located at the same depth even after 1440 minutes.
Compare to the location of the hyperbolic-shape signal for theirrigation pipe in the initial profile (Fig. 2(a)), that is locatedslightly lower at 1440 minutes (Fig. 2(f)). This indicates thatthe average EM velocity from the surface to the irrigation pipe
is lower at 1440 minutes than the initial average EM velocity to
the irrigation pipe. It means that, even after redistribution ofwater, the water content above the irrigation pipe was stillslightly greater than the initial water content. Knowing how
much water left near the soil surface without disturbing soil at
any given location is very critical and useful to optimize
irrigation schedules in practice.
As for the reflection from the bottom of the lysimeter, there
is no signal observed by 12 ns between x = 40 cm and x = 80
cm at 1440 minutes (Fig. 2(f)). It means that it took morethan 12 ns for the reflected wave to be recorded at the receiverantenna. This is clearly the effect of redistribution of waterwhere the water content beneath the irrigation pipe increased.
From the CO profiles presented in Figure 2, vertical watercontent distributions can be obtained at the center of the profile
at x = 60 cm. This can be achieved by estimating the average
EM velocity to the point of reflection and converting them tointerval velocities. As mentioned above, for the hyperbolic
shape reflection, the average EM velocity can be uniquelydetermined. As for the horizontal reflection, if the depth of
the boundary is known, then the average EM velocity can beestimated. For all calculation, time zero was calibrated using
the air wave arrival time. Figure 3 shows water content
distributions at the center of the lysimeter (x = 60 cm)

This study demonstrates that ground penetrating radar(GPR) can be an effective tool to measure changes in soil
water content non-destructively during subsurface drip
irrigation. In our experiment, the extent of the wet area wasquantitatively estimated from GPR profile data. For future
work, it may be possible to inversely estimate soil hydraulicproperties from changes in GPR estimated water content
distributions with time.
Acknowledgement
This study was partially supported by Grant-in-Aid forYoung Scientists (B) (No. 22780218) from JSPS.

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