SCIENCE & TECHNOLOGY Development of a Self-regulated Bubble Irrigation System to Control the Size and Shape of Wetting Fronts

Abstract

The main objectives of this study were to introduce a bubble irrigation system, compare the wetting fronts created by the bubble and free-flow systems, and test the viability of a bubble irrigation system. Two laboratory experiments were conducted using 2D flow to measure the wetting fronts. The first experiment measured the free-flow infiltration using an inverted, open plastic bottle. The second experiment tested the bubble-flow technique using an inverted, closed plastic bottle (ICPB). The results showed that the bubble-flow system created a larger width of wetting fronts at the beginning of the infiltration and then expanded less than that of the free-flow system. In contrast, the infiltration depth of the wetting fronts created by the bubble-flow system was much lower than that of the free-flow system. In conclusion, the wetting front width and depth in the bubble-flow system were slightly smaller than those in the free-flow system. In addition, the wetting fronts created by the ICPB were not moved upwards significantly, which proves the ability of specific distribution of the bubble-flow system on the wetting fronts. Therefore, the bubble irrigation system can be used as an alternative for distributing the moisture content in soil profiles.

Full text

Pertanika J. Sci. & Technol. 28 (4): 1297 - 1313 (2020)
ISSN: 0128-7680
e-ISSN: 2231-8526
SCIENCE & TECHNOLOGY
Journal homepage: http://www.pertanika.upm.edu.my/
Article history:
Received: 20 April 2020
Accepted: 18 August 2020
Published: 21 October 2020
ARTICLE INFO
E-mail addresses:
Yasir_alrubaye@hotmail.com (Yasir Layth Alrubaye)
nisa@upm.edu.my (Badronnisa Yusuf)
safaanori12@yahoo.com (Safaa Noori Hamad)
*Corresponding author
© Universiti Putra Malaysia Press
DOI: https://doi.org/10.47836/pjst.28.4.09
Development of a Self-regulated Bubble Irrigation System to
Control the Size and Shape of Wetting Fronts
Yasir Layth Alrubaye1*, Badronnisa Yusuf1 and Safaa Noori Hamad2
1Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM,
Serdang, Malaysia
2Engineering College, Uruk University, Baghdad, 10067 Iraq
ABSTRACT
The main objectives of this study were to introduce a bubble irrigation system, compare
the wetting fronts created by the bubble and free-ow systems, and test the viability of a
bubble irrigation system. Two laboratory experiments were conducted using 2D ow to
measure the wetting fronts. The rst experiment measured the free-ow inltration using
an inverted, open plastic bottle. The second experiment tested the bubble-ow technique
using an inverted, closed plastic bottle (ICPB). The results showed that the bubble-ow
system created a larger width of wetting fronts at the beginning of the inltration and then
expanded less than that of the free-ow system. In contrast, the inltration depth of the
wetting fronts created by the bubble-ow system was much lower than that of the free-ow
system. In conclusion, the wetting front width and depth in the bubble-ow system were
slightly smaller than those in the free-ow system. In addition, the wetting fronts created
by the ICPB were not moved upwards
signicantly, which proves the ability of
specific distribution of the bubble-flow
system on the wetting fronts. Therefore,
the bubble irrigation system can be used as
an alternative for distributing the moisture
content in soil proles.
Keywords: Air-water exchange, bubble irrigation,
plastic bottle, soil-water, uniformity and eciency

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INTRODUCTION
Recently, competition for water resources has increased for urban, industrial, and
agricultural users (Kandelous & Šimůnek, 2010). Agricultural water use will increase by
approximately 19% by 2050 (Kisekka et al., 2017). Therefore, more ecient and uniform
irrigation methods, such as subsurface irrigation systems, must be developed for the
distribution of irrigation water.
Recent studies have discussed subsurface irrigation systems to improve the design
of irrigation systems (Gu et al., 2017; Gunarathna et al., 2017; Gunarathna et al., 2018),
regulate the wetting fronts created by new products (Lima et al., 2019; Saefuddin et al.,
2019), evaluate the hydraulic performance (Ren et al., 2017; Ren et al., 2018; Ghazouani
et al., 2019), and measure the moisture distribution in the soil prole (Cai et al., 2017; Fan
& Li, 2018; Cai et al., 2019a; Elnesr & Alazba, 2019).
Designing ecient subsurface irrigation systems is the most challenging because the
irrigation water is applied directly to the soil prole. The design of subsurface irrigation
systems is inuenced by several factors including soil variety, environment, time, duration
of the irrigation process, and plant type (Sakaguchi et al., 2019). Designers consider
all these factors; however, these factors still reduce the eciency and uniformity at the
operation stage. To overcome the eects of these factors, new system components have
been invented. Cai et al. (2019b) tested a ceramic patch in a subsurface irrigation line
to control the saturation zone in the soil prole, which was created by the pressure head
applied by the emitters. They found that there was a relationship among the pressure head,
ceramic properties, and soil properties. Lima et al. (2019) showed that using a new irrigation
product, named a permeable membrane, could be the solution for increasing water use
eciency and maximizing irrigation management.
Continuous evaluation of subsurface irrigation systems is essential for obtaining a
more uniform irrigation process (Gunarathna et al., 2018). Evaluation of irrigation systems
indicates what is the best method for irrigation or which irrigation system is more suitable
under certain conditions. Nabayi et al. (2018) evaluated the performance of three dierent
irrigation types (sprinkler, drip, and capillary wick irrigation systems) for raising rubber
seedling crops. Their results indicated that the eciency and water productivity was
inuenced by the type of irrigation system used. They also showed that the capillary wick
and the drip irrigation systems had the highest water productivity, whereas the sprinkler
irrigation system had the lowest water productivity due to the canopy intercept. Al-Ghobari
and Dewidar (2018) evaluated decit irrigation strategies for surface and subsurface drip
irrigation systems and concluded that these improved water management by minimizing
the eects on production.
The measurement of the size of the wetting front in the soil prole is critical for
designing cost-eective and highly ecient subsurface irrigation systems (Fan et al.,

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2018b). Measurements of wetting fronts indicate the size and shape of the moisture
distribution in the soil. This measurement is used to reduce deep percolation to increase
the eciency of irrigation performance. Researchers have conducted experiments and
simulations to measure the wetting patterns in dierent soil types, applied discharge,
irrigation systems, and boundary conditions. Fan et al. (2018a) simulated a vertical line
source using HYDRUS and produced empirical forms to measure the wetting patterns
that were created. These authors concluded that the created empirical forms were required
for future eld studies to measure the wetting patterns and evaluate the created empirical
forms. Elnesr and Alazba (2019) simulated the wetting fronts created by subsurface drip
irrigation using 3D and 2D HYDRUS. They concluded that 2D simulation could be used
condentially by simulators.
The size and shape of the wetting patterns in the soil prole are inuenced by the applied
discharge, time of application, and the hydraulic properties of the soil (Amin & Ekhmaj,
2006; Dawood & Hamad, 2016; Moncef et al., 2002). Because the soil properties are the
natural conditions, the applied discharge and time of application are specied to control
the wetting patterns. In general, there are two traditional techniques for applying discharge
into the soil prole: a free ow and pressurized discharge. However, these techniques are
fully controlled, i.e., they are not inuenced by the soil conditions.
To control the wetting patterns, several studies have used the basic principle of the
buoyancy of bubbles for irrigation improvements. A study by Liu et al. (2019) aimed to
improve the yield and irrigation water eciency by evaluating a technique involving micro-
nano bubble—water oxygenation using a subsurface drip irrigation system. They noticed
that the speed and size of the bubbles in the saturated soil body increased the nutritional
quality of the crops. To use the basic principle of buoyancy in subsurface irrigation systems,
it is necessary to measure the movement and size of the bubbles in the hydraulic eld.
Mohagheghian and Elbing (2018) measured the size distribution of the bubbles within the
water column using 2D imaging with a high-resolution camera. Another study by Barkai
et al. (2019) investigated the hydrodynamics of bubble movement in a vacuum airlift
column to optimize its design and operation.
There is a global need to save water, especially during the irrigation process. Therefore,
the performance of subsurface irrigation systems must be made more ecient and uniform.
Wetting fronts created in soil proles by subsurface irrigation systems must be controlled.
Novel design criteria of subsurface irrigation systems can be outlined to overcome the
emissive losses owing to the uncontrolled size of wetting patterns. The key solution is to
use the buoyancy principle of bubbles. This principle can be used in subsurface irrigation
systems to improve irrigation performance and control the wetting fronts in the soil prole.
The main objectives of the present study were to introduce a bubble irrigation system,

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compare the wetting fronts created by the bubble and free-ow systems, and test the
viability of using a bubble irrigation system.
MATERIALS AND METHODS
Description of Self-Regulated Irrigation System
In the present study, a novel self-regulated irrigation system was introduced, called the
bubble irrigation system. The design of this system was dependent on the movement of
water through soil caused by the air–water exchange technique, which was produced in
the soil prole using inverted, closed plastic bottles (ICPBs; Figure 1). The system used
in the present study consisted of an elevated closed tank, a main valve, an air valve, a pipe
network, and an ICPB. The water movement in the bubble irrigation system started in the
elevated tank and moved through the pipe network until it reached the ICPB by air–water
exchange. The water owed into the ICPB whereas the air travelled to the elevated tank
through the pipe network. Water inltrated into the soil prole from the ICPB and generated
an inverted wetted bulb. The inltration process depended on the air–water exchange in
the soil voids. This exchange mechanism controlled the amount of water that inltrated
into the soil and thus the size of the wetting front was held under the control of the plants.
The bubble irrigation system is an alternative irrigation system that will improve the
eciency and uniformity of irrigation performance compared to traditional irrigation
methods. This system controls the water that is applied and is a self-regulated system. The
Wetting front
Inverted bulb
ICPB
Plant
Root zone
hb
Figure 1. Schematic representation of the locations of the ICPB and root zone and the shape of the inverted
bulb

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water holding capacity of the soil and the water delivery capacity of the irrigation system
are constraints on the ability to supply water to a crop (Klocke et al., 2009). Traditional
irrigation systems adjust the water applied into or onto the soil to t the required quantities.
However, the bubble irrigation system adopts an air–water exchange technique to control the
amount of applied water based on the soil moisture decit, i.e., based on the consumptive
use by the plants.
The major advantages of the bubble irrigation system are the avoidance of water
losses from spray drift losses when sprinkling (Hobson et al., 1993; Holterman, 2003;
Miller, 2003; Hilz & Vermeer, 2013), deep drainage losses (Paydar et al., 2009), surface
runo when surface irrigating (Hatiye et al., 2018; Masih & Giordano, 2014), and emitter
clogging problems when drip irrigating (Cararo et al., 2006; de Oliveira et al., 2020).
Adding water directly to the soil also decreases evaporation losses from the soil surface
(Alrubaye et al., 2018). The bubble system continuously applies irrigated water directly to
the soil depending on water inltration. A suggested layout of a bubble irrigation system
is shown in Figure 2. The operational cost of a bubble irrigation system depends on the
method used for relling the main tank.
Main air-pipe
Elevated closed tank.
Inverted Closed Plastic Bottles,
ICPB.
Sb
Sl
Valve
Hose
z
hw
ha
ht
Main pipe
Manifold
Air Valve
Figure 2. Suggested layout of a bubble irrigation system
Setup of Laboratory Experiments
Laboratory experiments were performed to compare the wetting fronts in the soil prole
created by free ow from inverted, open plastic bottles (IOPBs) and the bubble ow from
ICPBs. Bubble and free-ow experiments were designed to measure the ow rate and 2D
wetting fronts in the soil prole during the experiments. The experiments were performed
using a glass soil container that was 80 cm long, 80 cm deep, and 20 cm wide. An elevated

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closed cubic tank edge with a length of 50 cm was used with an air valve at the top and a
control valve at the bottom to assure the ow conditions of the bubbles (Figure 3).
Figure 3. Elevated closed tank
The experimental setup of the free-ow experiments is shown in Figure 4. The
equipment used included a soil container lled with sandy soil, a 2000 mL IOPB that was
implanted 5 cm below the soil surface, and transparent paper. The constant head of water
used in the experiment was 10 cm above the soil surface. Throughout the experiment, a
transparent sheet was pasted on the front of the soil container. Wetting fronts were drawn
manually on the transparent sheet at 5-min intervals and the ow rate was measured
volumetrically every 5 min.
The bubble-ow experiments were conducted using the same soil container, a 2000 mL
ICPB, a 1 m long hose, an air tube, and a closed elevated tank. The elevated tank was fully
closed with two valves. The rst valve was for water ow control and the second was for
airow control (Figure 3). The ICPB was connected to the elevated tank using the water
and air tubes, in which these tubes were fully sealed to the ICPB by silicon and Teon
tape (Figure 5). The experimental setup is shown in Figure 6. Therefore, the condition
inside the ICPB was fully controlled by the condition in the closed tank. When the water
was raised in the ICPB until it reached the air tube, the water tube automatically stopped
providing water into the ICPB. This is because the water level in the ICPB plugged the
airow to the closed tank. Water would raise in the air tube when there is a high vacuum
pressure in the elevated tank. Therefore, the closed tank was determined as being in the
right condition that allowed the mechanism of air–water exchange to control the supply of

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water into the ICPB. This can be solved using a check valve at the head of ICPB connected
to the air tube. In this paper, a slight vacuum was produced in the elevated tank to generate
the air-water exchange condition. Thus, a slight water raised in the air tube did not aect
the mechanism during the experiment.
Figure 4. Experimental setup of the free-ow inltration from the inverted open plastic bottle (IOPB)
Figure 5. Bubble-ow inltration from the inverted closed plastic bottle (ICPB)

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Figure 6. Experimental setup of bubble-ow inltration from the inverted closed plastic bottle (ICPB)
Measurement of the Wetting Fronts
Two laboratory experiments were performed in the uid mechanics laboratory at Al-
Mansour University College. These experiments involved measuring the ow rate and
drawing the movement of the wetting front throughout the bubble and free-ow experiments.
For the free-ow experiment, the container was lled with soil that was placed in layers
to maintain a uniform and homogeneous soil density. Then, the IOPB was implanted in the
soil and the transparent paper was pasted on the front face of the soil container. To allow
water to ow through the soil, a constant head of water in the IOPB was maintained at the
same level by a continual supply of water. The wetting front was traced on the transparent
paper at 5-min intervals. The ow rate was measured volumetrically by dividing the volume
of the inltrated water by the time at 5-min intervals.
For the bubble-ow experiments, the container was lled with soil that had been placed
in layers to maintain a uniform and homogeneous soil density. Then, the plastic bottle
was connected to the hose by a plastic connection and the hose, in turn, was connected
to the elevated tank. After that, transparent paper was pasted on the front face of the soil
container. The control valve was opened to allow water to ow through the soil. At 5-min
intervals, the wetting front was traced on the transparent paper. The ow rate was measured
volumetrically by dividing the volume of the inltrated water by the time in 5-min intervals.
For both the bubble and free-ow experiments, width measurements were taken at
5 cm below the plane at the point of inltration throughout experimental period. Depth
measurements were also taken along the central line of the wetting fronts over time.

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Statistical Analysis
Three cases were statistically tested to determine the signicant dierences for the variables
of the inltration rate, the width of the wetting patterns, and the depth of the wetting
patterns. These cases were tested using a one-sided paired t-test with a 95% condence
level to check the signicance of the dierences between free ow and bubble ow. The
null hypothesis, Ho, assumed that the mean variable of free ow was equal to or less than
that of the bubble ow, and the alternative hypotheses, H1, assumed that the mean variable
of free ow was greater than that of bubble ow. Statistical results were performed using
R Studio software.
RESULTS AND DISCUSSION
The bulk density of the soil used in the laboratory experiments was 1.3 gm/cm3 and was
classied as sandy soil based on the Unied Soil Classication System according to the
ASTM Standards. The grain size distribution of the sandy soil is shown in Figure 7.
Sieve op ening (mm)
0.01 0.1 110 100
Pa ssing (prc ent)
0
20
40
60
80
100
Figure 7. Grain size distribution of the sandy soil used in the laboratory experiments
The experiments showed that the wetting front area attained its maximum width
approximately 5 cm below the plane at the point of inltration, as can be seen in Figure
8A and 8B.
The inverted bulbs obtained from the bubble and free-ow experiments showed that
the width and depth variation decreased over time. The wetting fronts for both the bubble
and free-ow systems appeared 10 min after the start of the experiment. The wetting fronts
for both bubble and free-ow experiments were almost similar in their general shape.

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Figure 8. Wetting fronts of A) free ow and B) bubble-ow inltration from an inverted bottle throughout
the experimental period
The wetting fronts that were obtained from the IOPB showed that the free-ow
inltration extended in all directions and reached the soil surface. This was mainly because
free ow occurs due to gravity and the pressure head in the IOPB. However, the wetting
fronts obtained from the ICPB did not signicantly move upwards and did not reach the
soil surface; such distribution would decrease the evaporation from the soil surface and is
one of the advantages of the bubble irrigation system.
The variation in the inltration rate throughout the experimental period for both the
bubble and free-ow systems is shown in Figure 9. The inltration rate decreased with
time for both ow types. The free-ow inltration rate was, in general, larger than that
for the bubble ow because bubble inltration depends mainly on air–water exchange.
This reduction in inltration rate may indicate the eectiveness of the bubble irrigation
system. Additionally, this comparison showed that the free-ow inltration depended on
pushing water through the soil prole. This phenomenon indicated the ability of the bubble
irrigation system to control the irrigation water in the soil prole itself.
The results of the t-test for the rst case involving the inltration rate between the
free-ow and bubble-ow systems found a t-value of 10.24, degrees of freedom of 7, and a
p-value of 9.1 e-06. Therefore, the null hypothesis was rejected and the alternative hypothesis
was accepted, i.e., the mean inltration rate of the free-ow system was greater than that
of the bubble-ow system. The mean of the dierences was 0.34 Lph. The statistical tests
showed that the inltration rate of the free-ow system was signicantly greater than that
of the bubble-ow system. Because the bubble-ow system is regulated by the air–water
exchange technique, the inltration rate of the bubble-ow system was lower than that of
the free-ow system.

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The growth of the wetting front width over time for both the bubble and free-ow
systems is shown in Figure 10. At the beginning of the irrigation process, the wetting front
width in both the bubble and free-ow systems was sharply expanded and then the rate of
increase decreased over time. The two irrigation systems showed a reasonable phenomenon
of reduction in the inltration rate; however, there was a slight reduction in the wetting
width for the bubble-ow irrigation system.
Figure 9. Variation in the inltration rate throughout the experimental period for the bubble and free-ow
irrigation systems
Time (mi n)
020 40 60 80 100 120
Wi dth ( cm)
0
5
10
15
20
25
30
Width of Wetting Fronts of Free Flow.
Width of Wetting Fronts of Bubble Flow.
Figure 10. Variation in the wetting front width with time in the bubble and free-ow irrigation systems

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The wetting front width in the bubble-ow system was generally smaller than that in
the free-ow system (Figure 11). The bubble and free-ow systems distributed water close
to each other in the width direction.
Figure 11. Comparison between the wetting front width of the bubble and free-ow irrigation systems
The results of the t-test for the second case of the wetting front width between the free-
ow and bubble-ow systems showed that a t-value of 3.377, degrees of freedom of 21,
and a p-value of 0.00142. Therefore, the null hypothesis was rejected and the alternative
hypothesis was accepted, i.e., the mean wetting front width of the free-ow irrigation system
was greater than that of the bubble-ow system. The mean of the dierences was 0.772 cm.
The statistical tests showed that the wetting front width of the free-ow irrigation system
was signicantly greater than that of the bubble-ow irrigation system. The inltration
rate of the free-ow system was also larger than that of the bubble-ow system; therefore,
the response of the wetting front width was consistent.
The variation of the wetting front depth with time in both the bubble- and free-ow
irrigation systems is shown in Figure 12. The wetting front depth increased sharply at the
beginning of the irrigation process and then slowed down as time increased. In contrast,
the wetting front depth in the bubble-ow irrigation system was, in general, lower than
that of the free-ow irrigation system (Figure 13). This is one of the advantages of the
bubble-ow system because it will reduce deep percolation losses and retain the nutrients
within the root zone of the irrigated crop.

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Figure 13. Comparison of the wetting front depths for the bubble and free-ow irrigation systems
Figure 12. Variation in the wetting front depths in the bubble and free-ow irrigation systems
Time (min)
020 40 60 80 100 120
Dep th o f the wetting fronts (cm)
0
5
10
15
20
25
30
35
Depth of Wetting Fronts of Free Flow.
Depth of Wetting Fronts of Bubble Flow.

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The results of the t-test for the third case of the wetting front depth between the free- and
bubble-ow systems showed a t-value of 11.014, degrees of freedom of 21, and a p-value
of 1.738e-10. Therefore, the null hypothesis was rejected and the alternative hypothesis was
accepted, i.e., the mean wetting front depth of the free-ow irrigation system was greater
than that of the bubble-ow system. The mean of the dierences was 4 cm. The response
of the wetting front depth was consistent with the results of the inltration rate, whereby
the free-ow irrigation system was larger than that of the bubble-ow system. However,
there was a great dierence in depth and a slight dierence in the width of wetting patterns
between the free-ow and bubble-ow systems.
The statistical results indicated the dierences between the behavior of these systems.
The free-ow system pushes the water into the sandy soil using the free pressure head
in the IOPB, which produces a higher depth than width. Yet, the bubble-ow system
applies the air–water exchange and capillary pressure to reduce the pressure in the ICPB,
and thus produce a higher width compared to depth. Therefore, the bubble-ow system
can save water by self-regulating the wetting patterns. The experimental results from the
present study showed that the bubble irrigation system controlled and almost held the
wetting front compared to the free-ow irrigation system. Further studies are required to
determine the bubble ow in dierent soil types, the contact area of inltration, and other
design parameters.
SUMMARY AND CONCLUSIONS
The main aim of the present study was to investigate the viability of using what was named
‘bubble irrigation,’ where water ow through the soil depends on capillary pressure and
air–water interchange using an ICPB. Water ows into the soil and bubbles of airow back
into the bottle to replace the space of the inltrated water.
The wetting front width and depth in the bubble and free-ow irrigation systems
increased rapidly at the beginning of the irrigation process and then the rate of increase
of the wetting front width and depth in both systems decreased with time. The wetting
front width and depth in the bubble-ow system were smaller than those in the free-ow
irrigation system.
Preliminary laboratory experiments indicated that the bubble irrigation system can be
used as an ecient method for distributing irrigation water in the soil prole, eliminating
surface runo and deep percolation losses, and increasing water application eciency.
Field applications should be investigated in future studies to test the feasibility of such an
irrigation technique and develop systematic procedures to design bubble irrigation systems.
ACKNOWLEDGEMENT
The authors are grateful to Al-Mansour University College for providing a suitable place
for experiments.

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