water
ArticleEvaluation of Subsurface Drip Irrigation Designs in a Soil
Profile with a Capillary Barrier
Koichi Noguchi 1,2 , Hirotaka Saito 1, *, Reskiana Saefuddin 1,3 and Jiří Šimůnek 4
1

2

3

4

*



Citation: Noguchi, K.; Saito, H.;
Saefuddin, R.; Šimůnek, J. Evaluationof Subsurface Drip Irrigation Designs
in a Soil Profile with a Capillary
Barrier. Water 2021, 13, 1300. https://Faculty of Agriculture, Tokyo University of Agriculture and Technology, 3-8-1, Harumicho, Fuchu,
Tokyo 183-8538, Japan; koichi.noguchi@wolfson.ox.ac.uk (K.N.); resk001@lipi.go.id (R.S.)
School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY,
Oxfordshire, UK
Research Center for Biology, Indonesian Institute of Sciences, JlRaya Jakarta-Bogor Km 46 Cibinong,West Java 16911, Indonesia
Department of Environmental Sciences, University of California, Riverside, Riverside, CA 92507, USA;jsimunek@ucr.edu
Correspondence: hiros@cc.tuat.ac.jp; Tel.: +81-42-367-5584

Abstract: Enhanced water use efficiency (WUE) is the key to sustainable agriculture in arid regions.
The installation of capillary barriers (CB) has been suggested as one of the potential solutions. CB
effects are observed between two soil layers with distinctly different soil hydraulic properties. A
CB helps retain water in the upper, relatively fine-textured soil layer, suppressing water losses by
deep drainage. However, retaining water in a shallow surface layer also intensifies water loss by
evaporation. The use of subsurface drip irrigation (SDI) with a CB may prevent such water loss.
This study evaluated the performance of SDI in a soil profile with a CB using a pot experiment andnumerical analysis with the HYDRUS (2D/3D) software package. The ring-shaped emitter was
selected for the SDI system for its low capital expenditures (CapEx) and maintenance. Strawberry was
selected as a model plant. The results indicated that the proposed SDI system with a CB was effective
in terms of WUE. The numerical analysis revealed that the CB’s depth influences the system’s waterbalance more than the ring-shaped emitter’s installation depth. While the CB’s shallow installation
led to more root water uptake by the strawberry and less water loss by deep drainage, it induced
more water loss by evaporation.

doi.org/10.3390/w13091300
Academic Editor: Teresa Afonso do

Keywords: subsurface drip irrigation; ring-shaped emitter; capillary barrier; water use efficiency(WUE); HYDRUS

Paço
Received: 5 April 2021
Accepted: 29 April 2021
Published: 6 May 2021

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Licensee MDPI, Basel, Switzerland.
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distributed under the terms andconditions of the Creative Commons
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction
In semi-arid and arid regions, agricultural production is dependent on vulnerable
water resources. Water insecurity is expected to intensify in the future due to climatechange, such as global warming [1,2]. Predictions suggest that the anticipated increase inevapotranspiration due to rising temperatures may be more significant than the increasein precipitation [3]. The severe effect of water shortages is already apparent in someregions [4]. The development of water-efficient agricultural practices is thus required forsustainable food production.
The installation of Capillary Barriers (CB) has been suggested as a promising engineering approach to improve agricultural water use efficiency (WUE) [5–10]. A CB, anatural barrier to unsaturated water flow in soils, can be observed at the boundary of two
soils with distinctly different hydraulic properties. When a fine-textured layer overlies acoarse-textured layer, the water in the fine-textured layer does not infiltrate into the coarsetextured layer if the capillary forces in the fine-textured layer are significantly stronger,
retaining moisture in the fine-textured layer. This is due to the coarse material having muchlarger pore sizes. This CB effect is maintained until a certain soil water pressure (breakthrough pressure) is reached at the interface of the two materials. At the breakthroughWater 2021, 13, 1300. https://doi.org/10.3390/w13091300

https://www.mdpi.com/journal/water

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pressure, the capillary forces of the fine-textured layer match those of the coarse-textured
layer so that liquid water can infiltrate from the layer above a CB into the CB layer [11].
CBs have been used for decades to contain and control hazardous wastes [12,13].
Many studies have been dedicated in recent years to the application of CBs to agricultural production. Ityel et al. [5] investigated the effect of introducing a CB layer into
surface-irrigated cultivation plots. This study reported that up to 70% more water was
retained in the root zone as a result of the CB suppressing downward water drainage, also
resulting in reduced water content fluctuations [5]. An increase in the root zone’s watercontent can be observed regardless of plant types [6–9]. These changes prevent plants frombeing exposed to water stress, especially in arid regions. Even though the response to the
CB installation varies crop by crop [6], it generally provides positive effects for biomass
and/or fruit production [6–8].
A downside of CBs is that water loss by evaporation may increase [8,9]. Miyake
et al. [8] conducted a cultivation experiment on a leafy vegetable with sprinkler irrigationand showed that a CB might fail to improve root zone conditions when the amount ofapplied water (AW) is insufficient, as evapotranspiration intensified in the CB’s presence.
Wongkaew et al. [9] also reported that water loss by evaporation increased by 8.0–62.3% in
a CB’s presence. The HYDRUS-1D simulation of leafy vegetable cultivation with surface
irrigation showed that up to 90% of AW was lost by evaporation in the presence of a CB [9].
The increase in water loss by evaporation is likely the result of water retained near thesurface, as the soil water content near the soil surface is one of the determinants of theactual evaporation rate [14,15].
One of the potential solutions to intense evaporation is to implement subsurface drip
irrigation (SDI). Under SDI, water loss by evaporation is significantly smaller than undersurface irrigation, as less water is directly exposed to the atmosphere [16,17]. Despite its
drawbacks, such as high capital expenditures (CapEx) and maintenance [16,18–20], SDI is
widely used in the large-scale production of horticultural crops [18]. Martines and Reca [16]compared the WUE of SDI and surface drip irrigation in olive cultivation. They revealed
that 8–20% water saving was achieved by the use of SDI. Bonachela et al. [15] measured
the evaporation rate in the olive field production and showed that the evaporation rate
could be suppressed by keeping the ground surface dry. O’Brien et al. [18] compared
the economic performance of SDI and sprinkler irrigation. They showed that the high
WUE of SDI offers an economic advantage, despite its high CapEx and maintenance that
may hinder its adaptation by farmers. These past studies indicate that the use of SDI may
help overcome the shortcomings of CBs. However, there are no studies investigating thefeasibility of SDI when used together with a CB.
In recent years, to further disseminate SDI by overcoming the aforementioned drawbacks, SDI with ring-shaped emitters (Figure 1) has been extensively studied [17,19–21].
Saefuddin and Saito [19], for example, showed that a ring-shaped emitter made of a rubber
hose could improve WUE during bell pepper cultivation. In their study, two to five holes
were drilled into the ring-shaped rubber hose, through which water was applied [19].
The ring-shaped hose was covered with textile so that water infiltrated in all directions
when buried subsurface [19]. The simple design and material of the ring-shaped emitter
makes it easy for small-scale low-income farmers to use the emitter in practice [19,20]. The
feasibility of the ring-shaped emitter has been confirmed for the field production of various
crops [21]. Saefuddin and Saito [19] investigated the effect of the number of holes in the
ring-shaped emitter and revealed that the emitter with five holes enabled the highest yield,
while the emitter with two holes produced the highest WUE. Saefuddin et al. [20] analyzed
the ring-shaped emitter’s performance experimentally and revealed that downward waterflow is more dominant than radial flow in the sand profile. Saefuddin et al. [20] further
evaluated the ring-shaped emitter’s performance numerically using the HYDRUS (2D/3D)
software package. They showed that non-uniformity in the wet volume occurred in thesand profile, putting some roots under water stress. However, the system used in theiranalysis was relatively simple as the pot was filled uniformly with sand. A more detailed

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observation of soil water dynamics in a layered soil profile may be needed to further
improve the design and operation of SDI with the ring-shaped emitter in practice.

.
Figure 1. Configuration of the ring-shaped emitter (adapted from Saefuddin et al. [20]).
Figure 1. Configuration of the ring-shaped emitter (adapted from Saefuddin et al. [20]).

The SDI setups with the ring-shaped emitter were numerically investigated by FriedThe
setups
with[17].
the ring-shaped
numericallyinvestigatedby FriedmanSDI
andGamlielHowever, itemittershouldwere
be noted
that a tree
was the model
plant inman andGamliel[17].
However,itshould
be
noted
that
a
tree
was
the
model
plant
their thetheir study. Hence, the SDI design (several ring-shaped emitters were placedinaroundstudy.tree)
Hence,SDI designdifferent(several fromring-shaped
emitters
were placedaroundthe tree)wasthesignificantly
other studies[19–21].
The study
revealed
that thewas significantly
different
fromother studies[19–21].
The study
that thepreferpreferable depthof thering-shaped
emitterdepends
on therevealed
evaporation
rate.
The study
able depthof thering-shaped
emitter installation
depends on depth
the evaporation
rate. Theinstudy
reportedthata deeper emitter
led to a decreaseroot reported
water uptakethat a(RWU)
deeper emitter
installation
depth
led
to
a
decrease
in
root
water
uptake(RWU)
when thewhen evaporation was assumed negligible. On the other hand, increasing
evaporation
wasassumednegligible.
On
the
other
hand,
increasingthering-shaped
emitring-shaped emitter’s installation depth to a certain point increased RWU in thepresence
ter's installation
depth
to
a
certainpoint
increased
RWU
in
the
presenceofintenseevapof intense evaporation. The deepening of the emitter, however, is not always possible. For
oration.
The deepeningof theemitter,
however,
is notpossible.
For instance,
when
instance,
when a CB
layer
is installed
below
the always
root zone,
the emitter’s
deep installationa CB layer
is installed
the root zone,
the emitter's
installation
mayin the andmay resultin thebelow
accumulationof wateron a CB,deep
whichwould leadto resultits breakage
accumulationof wateron adeepCB, percolation.
which wouldThe
leadeffects
to its breakage
hence water
loss bydepth
hence water
loss byof the SDIandemitter’s
installation
deep percolation.
effectsof theSDI needemitter's
installation
depth
CB onwith
water
loss
and a CB on The
waterloss andRWU
to be
investigatedin aandsoil aprofile
a CB.
and RWU To
need
to
be
investigated
in
a
soil
profilewithaCB.
facilitate the development of the SDI design with the ring-shaped emitter, weTofacilitate
the development
SDIprofile
designwith
theFirst,
ring-shaped
emitter, emitter’s
weaimed
to evaluate
its feasibilityofinthea soil
witha CB.
the ring-shaped
aimedperformanceto evaluate its
inwitha soila profilea CB. bothFirst,experimentallythe ring-shaped
emitter's
in afeasibility
soil profile
CB waswith
analyzedandnumerically.
performancein a soil profile
with
CB was analyzed
bothexperiment.
experimentallyand numeriThe experimental
analysis
wasa conductedusing a pot
The numericalanalysis
performedanalysis
to evaluate
on theusing
plantawater
stress and water
losses using thecally. wasThethenexperimental
waseffects
conducted
pot experiment.
The numerical
HYDRUS(2D/3D)softwarepackageeffects[22] withdata
obtainedpotexperiment
analysis
was thenperformed
to evaluate
on theplant
waterfromstresstheandwater
losses andliterature.
The
HYDRUS(2D/3D)softwarepackagehas
been
proventobeable
to provide
using the HYDRUS (2D/3D) software package [22] with data obtained from the pot experpredictionsof water (2D/3D)
flow forsoftware
SDI [20,23,24]
andhasa CB[5,13].
Finally,
the effectimentaccurate
and literature.
The HYDRUS
packagebeen
provento be ableof changing
thepredictions
installationofdepth
the forring-shaped
emittera CB
on overall
to provide
accuratewaterofflow
SDI [20,23,24]
andanda CB[5,13].
Finally,waterbalance(i.e., RWU,
evaporation,and deepwasevaluated
HYDRUSthe effectof changing
the installation
depthof thedrainage)
ring-shaped
emitter
and ausing
CB ontheoverallpackage[22]. and deep drainage) was evaluated using the HYwater(2D/3D)
balance software(i.e., RWU,
evaporation,
DRUS (2D/3D) software package [22].
2. Materials and Methods
2.1. A Laboratory
Experiment with a Ring-Shaped Emitter and a CB
and Methods
2. Materials
A pot experiment was conducted in a lab using a plastic pot as shown in Figure 2.
◦ C fortemperaturein thelaba waskept at 24
entire duration of the experiment.
2.1. AThelaboratory
Experiment
with
Ring-Shaped
Emitterandthea CB
The height of the pot was 55 cm. The pot was first filled with Tottori dune sand from theup to a heightof 10 cm. The
dune
sand (D= shown
292 µm)inhad
0.1%
Abottompot experiment
was conductedin a Tottorilab usinga plastic
pot50asFigure2. of its
particles
smaller
than
75
µm,
0.2%
in
the
range
of
75–106µm,
36.6%
in
106–250µm,
The temperature in the lab was kept at 24 °C for the entire duration of the experiment. The55.2%
8.0%Thein 425–850
two-layered
CB layerthenplacedupon theheightinof250–425
the potµm,
wasand55 cm.
pot wasµm.
firstAfilledwith Tottori
dunewassandfromthe botTottoridune sandThe
CB layercomprisedcmµm)
graveldiameter >tom upto a heightof 10 layer.
cm. TheTottoridune
sand (D50 a= 3292
hadlayer
0.1%(particleof its particles
2 mm) and a 2 cm coarse sand layer (particle diameter range of 0.85–2 mm) overlying thegravel layer. While the gravel layer’s function was to suppress downward water flow as
a CB, the coarse sand layer’s role was to prevent small Tottori dune sand particles from

Water 2021, 13, 1300

smaller than 75 µm, 0.2% in the range of 75–106 µm, 36.6% in 106–250 µm, 55.2% in 250–
425 µm, and 8.0% in 425–850 µm. A two-layered CB layer was then placed upon the Tot4 of 16
tori dune sand layer. The CB layer comprised a 3 cm gravel layer (particle diameter>2mm) and a 2 cm coarse sand layer (particle diameter range of 0.85–2 mm) overlying thegravel layer. While the gravel layer's function was to suppress downward water flow as
a CB, thesandlayer
layer's
was to theprevent
small Tottori
particlesfromfalling
intocoarse
the gravel
androlenullifying
CB function[8,9]. Adune
30 cmsandTottoridune sandinto
thegravellayer
andnullifying
theCB
function[8,9].
A
30
cm
Tottorifallingdunelayer was then placed on the CB layer. The bulk density of all Tottori dune sand layers was
sand layer
was thenon the CBbulk
layer.
The ofbulk
all Tottoridune sandadjusted
uniformly
at aplaced
predetermined
density
1.50densityg cm−3of. The
pot’s diametersatlayerswasadjusted
uniformly
at a thepredetermined
densitywereof 1.50cm-3and
. The
the
sand
layer’sheight(45 cm frombottom) andbulk
the bottom
44.5g cm
38pot'scm,◦ C forand
thethreesand materials
layer's height
cm from thebottom)the bottom
were
44.5thecm
diameters atAll
respectively.
were(45oven-driedat 105at least
24 hours
beforeand38cm,
respectively.
All
three
materials
wereoven-dried
at
105
°C
for
at
least
24
hours
experiment.
before the experiment.

FigureFigure2.2.(a)(a)AAverticalverticalcross-section
cross-sectionandand(b)(b)top
topview
viewofofthe
thesetup
setupofofthe
thepotpotexperiment.
experiment.Red
Redpoints
S1–S6
show
the
positions
of
the
soil
moisture
sensors.
Points
P1–P4
show
the
horizontal
positionspoints S1–S6 show the positions of the soil moisture sensors. Points P1–P4 show the horizontal
positionssoil
samples
wereatcollected
at the experiment.
end of the experiment.
wheresoil where
sampleswerecollected
the end of

A ring-shaped emitter with a diameter of 20 cm was placed 20 cm above the CB
layer to inject water into the Tottori dune sand profile. The ring-shaped emitter had
five evenly spaced holes (water outlets), all facing downwards. The holes were covered
with a permeable textile, following the 5F design of Saefuddin and Saito [19]. Water wasWater 2021, 13, 1300

5 of 16

injected every 24 hours by applying a constant water pressure head of 0 cm at the inlet
connected to a Mariotte tank. A total of 1080 cm3 of water was injected during each water
application cycle. The amount of water supplied during each water application cycle wasdetermined based on the strawberry’s typical potential transpiration rate (=720 cm3 ) [20].
The injected water was then left to redistribute in the soil profile for the rest of the 24hours. This water application process was repeated 16 times in the pot experiment (i.e., thepot experiment continued for 16 days). The time required for each water application wasrecorded manually. The recorded water application time was used to calculate the water
flux values at the emitter surface in the inverse estimation of soil hydraulic properties (See
Section 2.2).
Soil water dynamics were monitored using six soil moisture sensors installed in the
Tottori dune sand layers. The red dots in Figure 2a show the positions of the soil moisturesensors. The soil water content was recorded every 10 minutes using the EM50 probes(METER Environment) and the MIJ-01 datalogger (Environmental Management Japan).
The EM50 soil moisture sensor, a standard capacitance sensor, measures soil dielectriccharacteristics, which are further used to estimate soil volumetric water contents. In thisstudy, a calibration equation provided by the METER Environment was used because it is
known that this calibration equation performs well for sands.
After Day 16, core samples of the gravel, coarse sand, and Tottori dune sand werecollected at a 5 cm interval to measure the volumetric water content gravimetrically. At
each depth, five soil samples from five different horizontal positions (depicted as P1, P2,
P3, P4, and center in Figure 2b) were collected to reveal the horizontal water content
distribution. It should be noted that the inlet of the ring-shaped emitter was between P1
and P2. The collected samples were oven-dried at 105 C for at least 24 hours to measure
the volumetric water content.
2.2. The HYDRUS (2D/3D) Inverse Estimation of Soil Hydraulic Parameters and Simulation of
the Pot Experiment
Some of the Tottori dune sand’s soil hydraulic parameters were optimized using
the inverse solution of HYDRUS (2D/3D). The HYDRUS (2D/3D) software package [22]simulates soil water dynamics in a two-dimensional, three-dimensional, and axisymmetricdomain by solving the Richards equation with a source/sink term S(h):hihi∂θ (h)
∂
∂h
∂
∂h
=
K
h
+
K
h
(
)
(
)
∂t
∂x h
∂x
∂y i
∂y
(1)
∂
∂h
+ ∂z K (h) ∂z +1 −S(h)where θ(h) denotes the volumetric water content [-], h is the soil water pressure head [L], t
is time [T], S(h) signifies a sink term [T−1 ], K(h) is the unsaturated hydraulic conductivity
function [L T−1 ], and x, y, and z are the axes of the Cartesian coordinate system [L]. The
sink term S(h) represents RWU.
The parameters were optimized by minimizing an objective function, which expresses
discrepancies between observed data and simulated results. The objective function is
first calculated using the initial parameter estimates. The parameters are then iteratively
modified so that the objective function is minimized. The iteration process continues until apredetermined precision is achieved. More details about the parameter estimation method
can be found in the technical manual of HYDRUS (2D/3D) [25].
The authors attempted to conduct the inverse parameter estimation in a fully threedimensional domain. However, the calculation stopped before the parameters were optimized due to computational instability. For this reason, the inverse estimation of soil
hydraulic parameters and the simulation of the pot experiment were conducted in an
axisymmetric domain. This axisymmetric modeling approach has been adopted in manyother studies on SDI (e.g., [20,26,27]). The simulation domain was finally discretized into
1390 nodes and 2664 two-dimensional elements. The finite element mesh around thering-shaped emitter was made fine to stabilize the computation of HYDRUS (2D/3D).

Water 2021, 13, 1300

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The solution of the Richards equation requires an appropriate soil water retention
curve model and initial and boundary conditions. In coarse materials such as coarse sandand gravel, non-capillary-type flow occurs after water is drained quickly by capillary
flow [9]. A dual-porosity (DP) model may be more appropriate than single-porosity (SP)models to describe such soils’ soil hydraulic properties. Brunetti et al. [28] statisticallyevaluated the accuracy of the Durner [29] model, a DP model, in comparison to the van
Genuchten [30] model, a typical SP model. They reported that the use of the Durner [29]model led to more accurate predictions of volumetric water contents (VWC) and theK(h) function of a coarse material, especially in the dry range (4 < log|h| < 6) and nearsaturation. The use of the SP model resulted in an unrealistically small estimation of K(h)even in a relatively high soil water pressure range [28]. Wongkaew et al. [9] also compared
the predictive accuracy of the soil hydraulic conductivity function for coarse sand andgravel using the van Genuchten [30] and Durner [29] models, revealing that the Durner [29]model showed a better agreement with measured data. Based on these studies’ findings,
the Durner [29] model shown below was used in this study Equation (2,3):Se =θ − θr11
= w1
n1  + w 2
θs −θ r1+|α 1 h|
1+|α 2 h|n21m

K (h)= K s

(2)
2

1
m

(w 1 Se1 +w2 Se2 )l (w1 α1 (1− (1 − Se11 )m1 )+w2 α2 (1 − (1 − S e22 )m2 ))(3)( w 1 α1 + w2 α2 )2where Se1 and Se2 are effective water contents [-], θ r and θ s are the residual and saturated
water contents [-], respectively, w1 and w2 are the weighting factors for two overlapping
regions with different water retention characteristics, respectively, and Ks is the saturated
hydraulic conductivity [L T−1 ]. Parameters α1 , α2 [L−1 ], m1 , m2 , n1 , and n2 [-] are the fitting
parameters. Initial estimates of soil hydraulic parameters of each soil are presented inTable 1. It should be noted that the van Genuchten [30] model was applied for Tottori dunesand by setting the w2 value to zero.
Table 1. Initial estimates of the soil hydraulic parameters of Tottori Dune sand, coarse sand, and gravel for the Durner [29]model.
Materials

θr [-]

θs [-]α1[cm−1 ]

n [-]Ks
[cm/min]w1 [-]

w2 [-]

α2[cm−1 ]

n2 [-]l [-]Tottori Dune sandCoarse sandGravel

0.006
0.004
0.005

0.384
0.371
0.274

0.030
0.133
0.204

6.675
20.8
15.5

2.15
4.97
13.8

1.0
0.99
0.99

0
0.01
0.01

0
0.25
0.25

1.5
1.5
1.5

0.5
0.5
0.5

The parameters for the coarse sand and gravel obtained by Wongkaew et al. [9], whichwere estimated by the inverse solution of HYDRUS-1D [31], were adopted. The saturated
hydraulic conductivity Ks and the saturated water content θ s of the Tottori dune sandwere measured by the constant-head permeability test and gravimetrical measurement,
respectively. The θ r value was determined based on the initial soil water content recordedin the pot experiment. Finally, the values of α, l, and n were estimated by the HYDRUS(2D/3D) inverse solution.
The initial VWC of the soil profile was set to 0.02 based on the measured data in the
pot experiment. No-flux boundary condition was applied to the bottom and sides of thepot. An atmospheric boundary condition with an observed potential evaporation rate of
1.36 mm d−1 was applied at the soil surface. A time-variable flux boundary condition
was assigned as a boundary condition at the ring-shaped emitter. As the rate of water
application was known from the pot experiment, the water flux values at the emittersurface were determined such that the application of 1080 cm3 of water is completed in thesame duration as in the pot experiment.

Water 2021, 13, 1300

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Finally, the agreement between observed and simulated water dynamics was evaluated based on four types of statistical errors: the mass balance error [%], the mean weighted
error (ME) [-], the mean weighted absolute error (MAE) [-], and the root mean square
weighted error (RMSE) [-] between the observed and predicted values. HYDRUS (2D/3D)
calculates these statistical error values during the computation process.
2.3. Numerical Analysis of the Optimal SDI Setup
2.3.1. The RWU Model of HYDRUS (2D/3D)
One of this study’s objectives was to investigate the effect of changing the installation
depth of the ring-shaped emitter in a soil profile with a CB on the plant water stress andwater loss under arid or semi-arid climatic field conditions. The plant water stress wasevaluated based on the method proposed by Saefuddin et al. [20]. RWU simulations werefirst conducted using the HYDRUS (2D/3D) software package [25]. RWU was then treatedas a proxy variable of the water stress that a plant is exposed to. Following Saefuddin
et al. [20], strawberry was selected as a model plant for its high profitability. Simulations of
RWU in HYDRUS (2D/3D) are based on two models: a spatial root distribution model anda water stress response model.
The root distribution model of Vrugt et al. [32] is implemented in HYDRUS (2D/3D)
as follows Equation (4):




py
px
pz
∗
∗
∗
y
z
x
(4)
1−
1−
e−( Xm | x − x|+ Ym |y −y|+ Zm |z −z|)b( x, y, z) = 1−
Xm
Ym
Zm
where b(x, y, z) [-] is the three-dimensional root distribution function, Xm [L], Ym [L], andZm [L] are the maximum root lengths in the x, y, and z directions, respectively, and px [-],
py [-], pz [-], x* [L], y* [L], and z* [L] are empirical parameters. The root distribution model
parameters of strawberries were obtained from Saefuddin et al. [20] (Table 2). The root
distribution was assumed to be constant for the 28-day simulation.
Table 2. The Vrugt et al. [32] parameters for strawberries [20].
Vrugt et al. (2001) Parameter Description

Variable

Value

Maximum rooting radius [cm]Radius of maximum intensity [cm]
Maximum rooting radius [cm]Radius of maximum intensity [cm]
Maximum rooting depth [cm]Depth with maximum rooting density [cm]Non-symmetry coefficient [-]Surface area associated with transpiration [cm2 ]

Xm
x*
Ym
y*
Zm
z*
Px , Py , Pz
-

20
0
20
0
30.5
5
1, 1, 1
1200

The water stress response of plants is simulated using the Feddes et al. [33] model in
HYDRUS (2D/3D). In this model, RWU is represented as a sink term in Equation (1) and isgiven using the following Equation (5) with the water stress response function α(h):S(h)= α(h)S p(5)

where S(h) and Sp are the actual and potential RWU rates, respectively. The α(h) value
is determined based on the water pressure head around the roots, which exemplifies theplant’s water stress. The plant starts extracting water when the soil water pressure head is
above the wilting point (h4 ). The RWU rate linearly increases as the soil water pressure
head increases from h4 until h3 . Between h3 and h2 , the RWU rate is constant at the potential
RWU rate (Sp ). The RWU rate decreases as the soil water pressure head increases further to
h1 , the anaerobiosis value, where RWU halts. The threshold values for strawberries used inthis study are listed in Table 3. These values were obtained from Saefuddin et al. [20] andthen adjusted to improve the computational stability.

Water 2021, 13, 1300

8 of 16

Table 3. The Feddes et al. [33] parameters for strawberries (modified from those presented in
Saefuddin et al. [20] to improve the computational stability).
Feddes et al. [33] ParametersValueh1 [cm]h2 [cm]h3 [cm]h4 [cm]

−5
−10
−40
−45

2.3.2. Simulation Domain Setups
The simulation domain is shown in Figure 3. Since a ring-shaped emitter with fiveevenly spaced holes was considered, the authors were compelled to use a simulation
domain that could fully account for these features. The simulations to analyze the effects
of changing the ring-shaped emitter and CB’s installation depths were thus conductedin a fully three-dimensional domain. The size of the domain was 60 × 60 × 75 cm3 . The
height of the domain was determined according to the vertical root distribution of the
strawberry plant. On the other hand, the width and length were set so that the domain’s
side boundaries would not affect the water dynamics around the roots in a long-term
simulation. The strawberry plant was placed in the center of the domain. The red points inWater 2021, 13, x FOR PEER REVIEW Figure 3 show the positions of observation nodes inserted at a 5 cm interval.9Computationsof 17
were stable for all simulations using this simulation domain.

.

Figure3.3.TheThe
simulation
domainthe HYDRUS(2D/3D).
Theof the ring-shaped
emitter
Figuresimulation
domainof theofHYDRUS(2D/3D).
The depths
ofdepths
the ring-shaped
emitterandCB
CBwerewerevaried.
andvaried.

assumedbe filledwith Tottori
sand witha CB layer
AThetotaldomain
volumewasof 1080cm3 of towater
was applied
everydune
day during
the 28-day
simu-consistinglation.
Intheinverseestimationofsoil
hydraulicproperties(Section
2.2),
theboundaryof a 2 cm coarse sand layer and a 3 cm gravel layer placed at a given depth of the Tottori
conditionat thering-shaped
washydraulica time-variable
flux boundary
dune sandprofile.
Whileemitterthe soilparameterslisted because
in Tablethe1 water
were used for
application
rate
had
been
previously
known
from
the
pot
experiment.
However,in (2D/3D)
practhe gravel and coarse sand layers, those inversely estimated by HYDRUS
were
tice,
an
irrigationwater
application
rate
isnormally
controlled
by
a
water
pressure
headdune sandused for Tottori dune sand. The ring-shaped emitter was installed in the Tottoriat the emitter. Thus, in the investigation of the effect of the emitter and CB’s installation
layer above a CB to inject water into the Tottori dune sand profile. The installation depths
depth on water balance, a time-variable pressure head boundary condition was assignedof the ring-shaped emitter and CB were varied so that their influence on WUE could beto the ring-shaped emitter. The original HYDRUS (2D/3D) software package was modievaluated.
Thering-shaped
emitterwasplaced at
a depthwasof 10,
20, 30toora 40
cm, while thefied
so that the
time-variable
pressureheadboundary
condition
switched
no-flux
CB
layer
was
placed
at
a
depth
of
45,
50,
55,
60
or
65
cm,
resulting
in
a
total
number ofboundary condition when the cumulative infiltration flux reached or exceeded a given
20 combinations.
The
CBcmdepths
wereon themaximum3). Water
amountof water (i.e.,1080
wasdetermined
applied at thebased
beginningof strawberry’s
each day by assigning a zero water pressure head to the ring-shaped emitter. Infiltrated water was thenleft to redistribute in the soil profile during the rest of the day.
A uniform soil water pressure head of -30 cm was used as the initial condition for theentire soil profile (VWC = 0.084 for Tottori dune sand). This initial condition was determined based on the assumption that the soil profile is reasonably wet when the strawberry
plant is at the maturity level shown by the root distribution parameters (Table 2). The

Water 2021, 13, 1300

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rooting depth [20]; it is known that the roots may penetrate through the CB layer when it isinstalled in the root zone [8].
A total volume of 1080 cm3 of water was applied every day during the 28-day simulation. In the inverse estimation of soil hydraulic properties (Section 2.2), the boundary
condition at the ring-shaped emitter was a time-variable flux boundary because the water
application rate had been previously known from the pot experiment. However, in practice,an irrigation water application rate is normally controlled by a water pressure head at theemitter. Thus, in the investigation of the effect of the emitter and CB’s installation depth
on water balance, a time-variable pressure head boundary condition was assigned to thering-shaped emitter. The original HYDRUS (2D/3D) software package was modified sothat the time-variable pressure head boundary condition was switched to a no-flux boundary condition when the cumulative infiltration flux reached or exceeded a given amountof water (i.e., 1080 cm3 ). Water was applied at the beginning of each day by assigninga zero water pressure head to the ring-shaped emitter. Infiltrated water was then left toredistribute in the soil profile during the rest of the day.
A uniform soil water pressure head of -30 cm was used as the initial condition for
the entire soil profile (VWC = 0.084 for Tottori dune sand). This initial condition wasdetermined based on the assumption that the soil profile is reasonably wet when the
strawberry plant is at the maturity level shown by the root distribution parameters (Table 2).
The potential evaporation rate varies depending on factors such as the growing season
and climatic conditions. Stroosnijder [14] reported that the potential evaporation rateis typically between 1 and 6 mm d−1 under semi-arid conditions. For this reason, thepotential evaporation rate of 6 mm d−1 was selected and applied to the upper boundary
as an atmospheric boundary condition. Hence, combined with the strawberry’s potential
transpiration rate of 6 mm d−1 (=720 cm3 d−1 ) [20], the potential evapotranspiration rate
was equal to 12 mm d−1 . The atmospheric boundary condition switches from a timevariable flux boundary condition to a constant pressure head boundary condition when
the soil water pressure head reaches a predefined minimum pressure head (i.e., hCritA =−80 cm for Totorri dune sand). The hCritA value was determined heuristically based on
the agreement between the observed and simulated water contents and the computational
stability. No flux was assumed through the lateral sides of the domain. The bottom
boundary condition was set as free drainage, representing a zero pressure head gradient at
the profile bottom (i.e., allowing for gravity flow).
3. Results
3.1. A Laboratory Experiment with a Ring-Shaped Emitter and a CB
Changes in the soil water content observed by the soil moisture sensors, S1 to S6, aredepicted in Figure 4. At S2, S4, and S5, the soil water dynamics pattern can be explained by
the following two phases; the soil water content steeply increased immediately after thewater application and then leveled off or gradually decreased because of redistribution. The
soil water dynamics at S3 followed the same pattern after a quasi-linear increase in VWC
before Day 8. No obvious increase in the soil water content was observed at S1 throughout
the entire experiment duration. An obvious increase in the soil water content was notobserved at S6 until Day 12, indicating that the CB successfully suppressed downward
water flow during the first 12 days.
Soil water contents measured on soil samples collected after the experiment aresummarized in Table 4. VWCs of the soil samples near the surface (5 cm and 10 cm) slightly
varied depending on the horizontal position. The relatively high variability in VWCs near
the surface may be attributed to the fact that the effect of evaporation is dominant near
the soil surface, making the water dynamics more complicated. However, the horizontal
variability in VWCs was small at other depths.

Water 2021, 13, 1300

was not observed at S6 until Day 12, indicating that the CB successfully suppressed downward water flow during the first 12 days.
Soil water contents measured on soil samples collected after the experiment are summarized in Table 4. VWCs of the soil samples near the surface (5 cm and 10 cm) slightly
varied depending on the horizontal position. The relatively high variability in VWCs near
10 of 16
the surface may be attributed to the fact that the effect of evaporation is dominant near
the soil surface, making the water dynamics more complicated. However, the horizontal
variability in VWCs was small at other depths.

Figure 4. Soil water dynamics in the pot experiment. S1–S6 indicate soil water contents measured
FigureSoilwaterdynamics
in thepotpotexperiment.
S1 –while
S6 indicate
soil watercontents
measuredusing4.thesoilmoisture
sensors
in theexperiment,
Obs1–Obs6
show
corresponding
soil
using
the
soil
moisture
sensorsinthepotexperiment,
while
Obs1
–
Obs6
show
correspondingsoil
water contents simulated by HYDRUS (2D/3D) using the optimized soil hydraulic parameters.
water contents simulated by HYDRUS (2D/3D) using the optimized soil hydraulic parameters.
Table 4. VWC distribution measured at the end of the pot experiment.

The vertical water distribution shown in Table 4 also confirms that supplied water
accumulated
upon the CB layer as the samples
VWCcollected 5 cm above the CB were mostMeasurement
Ave.
SD.
wet. ThisDepth
result verified that
downward
water
P1
P2
P3flow was
P4 suppressed
Centerat the CB. However,it should also be noted that the CB seems to have been broken during the 16-day experi5 cm
0.071
0.031
0.061
0.023
0.014
0.040
0.022
ment, as 10
thecm
VWC 5 cm 0.085
below the0.071
CB is as0.087
high as in
the layers
above theCB. The
CB
0.049
0.053
0.069
0.016
breakage appears
tohaveoccurred
on
Day
12
when
the
soil
water
content
at
S6,
located
15 m
0.136
0.150
0.177
0.178
0.145
0.157
0.0175cm below20theincreased0.238(Figure 0.255
4). The CB0.244
successfully
cm CB layer, suddenly
0.213
0.224
0.235suppressed
0.015
25 watercm
0.294Day 12.
0.331
0.338
0.331
0.326
0.324
0.016
downward
flow until
Coarse sand layer
Gravel layer
40 cm

0.084
0.032
0.212

0.083
0.041
0.199

0.085
0.039
0.194

0.082
0.030
0.163

0.089
0.036
0.218

0.085
0.036
0.197

0.003
0.004
0.019

The vertical water distribution shown in Table 4 also confirms that supplied water
accumulated upon the CB layer as the samples collected 5 cm above the CB were mostwet. This result verified that downward water flow was suppressed at the CB. However, it
should also be noted that the CB seems to have been broken during the 16-day experiment,
as the VWC 5 cm below the CB is as high as in the layers above the CB. The CB breakage
appears to have occurred on Day 12 when the soil water content at S6, located 5 cm below
the CB layer, suddenly increased (Figure 4). The CB successfully suppressed downward
water flow until Day 12.
3.2. Inverse Estimation of Soil Hydraulic Parameters and Simulation of the Pot Experiment
Table 5 shows the α, n, and l values of Tottori dune sand optimized using the inverse
solution of the HYDRUS (2D/3D) software package. These inversely estimated parametervalues were then used together with the remaining Durner’s [29] parameters to simulate thesoil water dynamics of the pot experiment. Figure 4 shows the changes in the VWC observed
at six observation nodes (Obs1-Obs6) in the HYDRUS (2D/3D) simulation along with the
VWC observed in the pot experiment (S1–S6). At a depth of 5 cm (S1 and Obs1), the soil
water contents predicted by HYDRUS (2D/3D) were in good agreement with observed data,

Water 2021, 13, 1300

11 of 16

except for predicted temporary increases immediately after water applications. While theobserved data at S2–S5 showed that soil water contents varied depending on whether thesoil moisture sensor was at the center or near the periphery of the ring-shaped emitter, the
HYDRUS (2D/3D) prediction showed no such differences between the observation nodes
at the same depth. The CB breakthrough (observed at S6) occurred two days later in the
HYDRUS (2D/3D) simulation than in the pot experiment. Besides, the VWCs below the CB
were largely underestimated after the breakthrough in the HYDRUS (2D/3D) simulation.
Given that the agreement between VWCs measured in the pot experiment and simulatedby HYDRUS (2D/3D) was reasonably good at other points, these discrepancies imply that
preferential flow (or fingering) might have occurred when water flew through the CB layer.
Table 5. The initial estimates and optimized parameter values of Tottori dune sand.
α [cm−1 ]

n [-]

l [-]

0.03
0.0502

6.665
4.748

0.5
0.00227

Initial estimates
Optimized values

Overall, the statistical errors between the observed and simulated soil water contents
were not significant; the mass balance error [%], the ME [-], the MAE [-], and the RMSE [-]between the observed and predicted values were 0.022%, −0.008, 0.017, and 0.024, respectively.
3.3. Numerical Analysis of the Effect of the Ring-Shaped Emitter and CB Installation Depths
Figure 5 shows the cumulative RWU fluxes expressed as a fraction of AW simulated in
the analysis investigating the effect of the installation depth. The ring-shaped emitter and
CB were installed at four (10, 20, 30, and 40 cm) and five (45, 50, 55, 60, and 65 cm) different
depths, respectively. Two-dimensional soil water distributions under different scenarios
can be found in the Supplementary Materials. The RWU value showed a monotonous
decrease as an increase in the CB installation depth. While roots were able to extract 41.9%
Water 2021, 13, xof
FOR
PEERwhen
REVIEWthe emitter and CB were at depths of 10 cm and 45 cm, respectively,
12 of 17theyAWextracted only 14.1% of AW when the emitter and CB were at depths of 10 cm and 65 cm,
respectively. This difference shows that the CB installation depth is crucial for the SDI
14.1at%aofdepth
AW whenthe emitterand CB
were attripled
depths ofwhen
10 cm theand CB
design. Whenthey
theextracted
emitteronly
was
of 10
cm, RWU
almost65 cm, respectively. This difference shows that the CB installation depth is crucial for thewas moved froma depth of 45 cm to 65 cm. In contrast, RWU increased only by 6.8% by
SDI design. When the emitter was at a depth of 10 cm, RWU almost tripled when the CB
changing the ring-shaped
emitterfromto 10 cmwiththe ring-shaped
was moved froma depthdepthof 45 cm
to 6540cm.cm
In contrast,
RWU
increased
only by 6.8 %emitterbychanging
at a depth of 45
cm. the ring-shaped emitter depth from 40 cm to 10 cm with the ring-shaped emitterat a depth of 45 cm.

.

Figure 5. Cumulative RWU expressed as a fraction of AW during the 28-day simulations when the
Figure 5. Cumulative RWU expressed as a fraction of AW during the 28-day simulations when the ring-shaped emitterring-shaped emitter and CB are at different depths. The results for different ring-shaped emitterand CB are at different depths. The results for different ring-shaped emitter depths (10, 20, 30, and 40 cm) are shown using
depths colors.
(10, 20, 30, and 40 cm) are shown using lines of different colors.
lines of different
Figure 6 shows the simulated cumulative water loss by evaporation expressed as afraction of AW when the ring-shaped emitter and CB are at different depths. Similarly,
the CB depth had a more significant effect on evaporation than the ring-shaped emitter
depth did. With the ring-shaped emitter installed at a depth of 10 cm, 3.3 times more water

Water 2021, 13, 1300

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Figure 6 shows the simulated cumulative water loss by evaporation expressed as afraction of AW when the ring-shaped emitter and CB are at different depths. Similarly, the CB
depth had a more significant effect on evaporation than the ring-shaped emitter depth did.
With the ring-shaped emitter installed at a depth of 10 cm, 3.3 times more water was lost due
to evaporation when the CB was at a depth of 45 cm than at 65 cm. However, the ring-shaped
emitter depth strongly influenced the amount of water lost by evaporation as well; with CB ata depth of 45 cm, 16.6% more water was lost by evaporation when the ring-shaped emitter is
at a depth of 10 cm than at 20 cm. The effect of the emitter depth was rather small when it
installedofoftheWater 2021, 13, wasx FOR PEER
REVIEWdeeper than 20 cm. This result exhibits that not only the depth 13
17 CB but
also the ring-shaped emitter installation depth is paramount to enhancing WUE.
Water 2021, 13, x FOR PEER REVIEW

13 of 17.

6. Cumulative
evaporation
expressed
a fractionof AW
duringthethe28-daysimulations when
Figure 6.Figure
Cumulative
evaporation expressed
as a fractionof AW asduringthe 28-day
simulations
when
ring-shaped
emitter andare at different
depths.and
The results
foratdifferent
ring-shaped
depths (10,30, and 40ring-shapedcm) arethe CB
ring-shaped
emitter
CB aredifferent
depths.emitter
The results
for20,different
emitter.
shown using lines of different colors.
depths (10, 20, 30, and 40 cm) are shown using lines of different colors.

Figure 6. Cumulative evaporation expressed as a fraction of AW during the 28-day simulations when the ring-shaped
The water loss by deep drainage is also influenced by the installation depth of theemitter and CB are at different depths. The results for different ring-shaped emitter depths (10, 20, 30, and 40 cm) are
The
water
loss
by emitter
deep anddrainageis alsoinfluenced
the
depth
ring-shaped
CB (Figure7). With
the CB at a bydepth
of installation
45 cm, increasing
the of theshown using lines of different colors.

ring-shaped
emitter’s
depth fromcm to 40cmCB
resultedover three
times
more
water
ring-shaped emitterand
CB (Figure7).10Withtheat aindepth
of 45
cm,
increasing
theloss
by waterdeep drainage.
When
the ring-shaped
emitter wasatthe
a depthof 10 cm,the CB
layer
The
loss
by
deep
drainageisalso
influenced
byinstallation
depthofring-shaped emitter’s depth from 10 cm to 40 cm resulted in over three timesthemore waterat a depth of
65 cmandinsteadof 45 cm7).
ledWith
to approximately
six times
lost dueemitter
CB (Figure
the CB at a depth
45morecm, water
the toloss by deepring-shaped
drainage.
Whenthering-shaped
emitterwasat a ofdepth
ofincreasing
10rootcm,zone.
the
CB layer
deep drainage.
Deepening
the CB
accumulating
below
the
ring-shaped
emitter’s
depth from10 ledcm toto more40 cmwater
resultedin over threetimes
more waterat a depth ofloss
65bycminstead
of
45
cm
led
to
approximately
six
times
more
water
lost
due todeep drainage. When the ring-shaped emitter was at a depth of 10 cm, the CB layer
at a depthof 65 cm instead
of 45
cmto
ledmoreto approximately
six times more water
losttheduerootto zone.
deep drainage.
Deepening
the CBledwater accumulating
below
deep drainage. Deepening the CB led to more water accumulating below the root zone.

.
Figure 7. Cumulative deep drainage expressed as a fraction of AW during the 28-day simulations when the ring-Scheme
10. 20, 30, and 40 cm) are shown using lines of different colors.

Figure 7. Cumulative deep drainage expressed as a fraction of AW during the 28-day simulations
when the ring-Scheme 10, 20, 30, and 40 cm) are shown using lines of different colors.

Water 2021, 13, 1300

13 of 16

4. Discussion
4.1. Water Dynamics in a System with a CB under SDI
The dynamics of the VWC at S3, S4, and S5 in the pot experiment can be explainedin the following way: first, the VWC increased when the wetting front reached the soil
moisture sensors (Figure 4); then, the VWC gradually decreased or leveled off due toredistribution. Saefuddin et al. [20] studied the VWC fluctuations in the soil profile withthe ring-shaped emitter and also reported the surge in the VWC immediately after thewater application. However, a decrease in the VWC caused by redistribution was rather
steep in the case of Saefuddin and Saito [19]. The gradual decrease or leveling-off observedin this study was likely due to the CB’s effect of suppressing deep percolation. The weak
capillary force may explain the limited upward water movement in a relatively coarse
material such as Tottori dune sand. Saefuddin et al. [20] also reported that the VWC 5 cm
above the ring-shaped emitter in the sand profile did not increase during the 24-hour
experiment.
Water accumulation upon the CB layer and its eventual breakage was observed bothin the pot experiment and in the HYDRUS (2D/3D) simulation (Figure 4). Between Day 2
and Day 12, most water was retained in the sand layer immediately above the CB. This
result verifies that CB’s installation is effective in keeping water in the root zone under
SDI, enhancing WUE. The CB breakage was confirmed by an increase in the water content
beneath the CB layer on Day 12. The CB breakage occurred after the soil above the CB
reached quasi-saturation. Yunasa et al. [11] also reported that water infiltrated into the CB
layer when the soil layer immediately above it reached near-saturated conditions. Thiswater accumulation above the CB resulted in the reduction of capillary forces in this upper
soil layer and consequent gravitational flow into the coarse sand and gravel layer, causing
a partial breakage of CB. After the breakage of CB, an increase in the VWC immediately
above the CB became more gradual. This slowdown occurred because accumulated water
started infiltrating into the lower layers. Even after the CB breakage, water was retained in
the layer immediately above the CB, as the breakage was likely only partial. This result
validates the effectiveness of CB under SDI in general.
4.2. The Comparison between the Pot Experiment and the HYDRUS (2D/3D) Simulation
The overall accuracy of the prediction by the HYDRUS (2D/3D) software package
was satisfactory. The vertical VWC distribution during the pot experiment was successfully
reproduced (Figure 4). The timing of the CB breakage was in good agreement between the
pot experiment and the simulation. The obtained error values also confirmed the modelingaccuracy. The values were comparable to those reported by the previous studies on theprediction accuracy of HYDRUS (2D/3D) [20,23,24], even though the system in this study
was more complex than in the earlier studies. This proves the ability of HYDRUS (2D/3D)
to simulate the soil water dynamics in a soil profile with a CB under SDI.
The comparison between the pot experiment results and the HYDRUS (2D/3D) simulation showed two distinct differences. First, while the data obtained in the pot experimentindicated the occurrence of preferential-type non-uniform flow, this process was not considered (and thus could not be reproduced) in the HYDRUS (2D/3D) simulation (Figure 4).
The occurrence of non-uniform flow was indicated by the difference in the VWCs obtainedat the observation nodes in the same depth. While non-uniform flow might have been
induced by the fine-textured textile [34], which covered the ring-shaped emitter, the differences between the observed and simulated VWCs were small (less than 0.033) at the end of
the experiment, demonstrating the ability of the ring-shaped emitter to spread water in all
directions. The second difference was that the VWC below the CB was underestimated in
the HYDRUS (2D/3D) simulation. This may be because the CB breakage occurred only
partially, limiting downward water flow through the CB layer. In addition, it is possiblethat inaccurate soil hydraulic parameters of coarse sand and gravel were used as they were
not optimized by HYDRUS (2D/3D). Although the authors attempted to optimize theparameters of the CB materials, this was not successful as the calculations became unstable

Water 2021, 13, 1300

14 of 16(due to the parameters of the very coarse CB material). Nevertheless, it should be notedthat this discrepancy between the observed and simulated soil water content occurred
below the CB. Hence, it does not compromise the effectiveness of the CB or this study’s
relevance.
4.3. Effects of the Installation Depths of the Ring-Shaped Emitter and CB on Water Balance
It was shown by the numerical analysis that the depth of the ring-shaped emitter did
not influence RWU or water loss as much as the CB depth did. Its effect on RWU wasespecially limited. However, water losses were influenced by the emitter depth when it was
installed shallowly. When the emitter was at a depth of 10 cm, water loss by evaporation
largely increased (Figure 6), while water loss by deep drainage decreased (Figure 7). The
shallow installation of the ring-shaped emitter led to the high soil water content near thesurface, which is known to increase the actual evaporation rate [15]. Thus, there seems to
be a trade-off regarding the way water is lost depending on the emitter installation depth.
In practice, the installation depth should also be determined in light of other factors suchas the installation cost and irrigation water quality.
In contrast, the CB depth substantially influences both RWU and water loss by evaporation and deep drainage. The shallow installation of the CB at a depth of 40 cm led tothe most active strawberry RWU, which indicates that water stress on the strawberry’s
root was alleviated. This is because strawberry plants actively extract water near the soil
surface (5 cm below the surface) [20]. Therefore, the installation depth of the CB needs tobe determined considering the type of plants, as root characteristics largely affect RWU
and water loss by deep drainage [35]. Field cultivation experiments with various kinds ofplants will provide further guidance on the ideal CB depth.
As shown in Figure 6, the shallow installation of the CB also caused higher water
loss by evaporation because the soil near the surface was kept wet. Ityel et al. [5] also
reported that the shallower the CB, the more water was retained near the surface. As thesoil water content near the surface enhances the evaporation rate [15], evaporation is likely
to intensify in the presence of a shallow CB. This phenomenon was also reported by Miyake
et al. [8], who reported that evapotranspiration intensified when an effective CB layer waspresent. The numerical analysis in this study revealed that the evaporation rate, as well as
the VWC just above the CB, are affected by its installation depth and that the evaporation
rate depends not only on the presence of the CB but also on its installation depth. This
result indicates that the CB should be installed deep if the goal is to reduce evaporation.
Water loss by deep drainage was also affected by the CB depth. While the shallow CB
led to an increase in evaporation, the deep CB caused higher water losses by deep drainage
due to the CB breakage. When the CB layer is installed deep below the root zone, lesswater can be retained in the root zone. More water then flows downwards without being
absorbed by the root, resulting in water accumulation upon the CB layer and hence its
eventual breakage. Vertical water movement depends on the soil types under SDI [20]. The
effect of soil types on the preferable CB installation depth will further aid the developmentof the designs of SDI with a CB.
Overall, these findings strongly indicate that the agricultural water demand in arid
regions can be significantly reduced without harming crop production or additional costs
by improving on-farm SDI designs. Such improvements will greatly alleviate pressures on
scarce water resources, as 70% of freshwater resources are used globally for agricultural
production [1]. Given that this study has validated the effectiveness of a CB under SDI,
its application under various conditions (soil types, plant types, etc.) should be exploredfurther. Such exploration will contribute to disseminating water-efficient agricultural
practices and tackling water scarcity in semi-arid and arid regions.
5. Conclusions
Enhancing agricultural WUE is critical in both arid and semi-arid regions. The installation of a CB has been suggested as a promising engineering approach to reduce water

Water 2021, 13, 1300

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loss by deep drainage. This is the first study that evaluated the effectiveness of a CB
installation in a soil profile under SDI to the best of our knowledge. The pot experimentresults showed that a CB could aid in retaining water in the upper soil layer under SDI,
even after its eventual breakage. The inverse estimation of soil hydraulic parameters andstatistical analysis proved that HYDRUS (2D/3D) [22] could predict soil water dynamics
in a relatively complex soil profile with a ring-shaped emitter and a CB. The numericalanalysis indicated that CB’s shallow installation increased water retention near the soil
surface and the strawberry’s RWU. However, it also increased water loss by evaporation [15]. In contrast, water loss by deep drainage increased when a CB was installed
deep. The effect of the ring-shaped emitter’s depth on the water balance was relatively
limited. Overall, this study’s findings agree well with past studies on the ring-shaped
emitter [17,19,20] and CB [5,6,8,9]. Further testing and application of the proposed SDI
systems under various conditions (e.g., crop types, climatic conditions) will aid in ensuring
water and food security.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/
10.3390/w13091300/s1. Figure S1. Two-dimensional soil water distribution profiles with a CB
at a depth of 45 cm and 65 cm and with the ring-shaped emitter at a depth of 10 cm and 40 cm,
respectively.
Author Contributions: Conceptualization, H.S., K.N., and R.S.; methodology, K.N.; software, J.Š.;validation, K.N.; writing—original draft preparation, K.N.; writing—review and editing, H.S. andJ.Š.; funding acquisition, H.S. All authors have read and agreed to the published version of themanuscript.
Funding: This study was financially supported by JSPS Grant-in-aid Scientific Research Program(17H03885 and 20H03097) and by the Joint Research Program of Arid Land Research Center, Tottori
University.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data that support the findings of this study are available from thecorresponding author upon reasonable request.
Conflicts of Interest: The authors declare no conflict of interest.

References
1.

2.
3.

4.
5.
6.
7.
8.
9.

Steduto, P.; Faurès, J.-M.; Hoogeveen, J.; Winpenny, J.; Burke, J. Coping with Water Scarcity: An Action Framework for Agriculturaland Food Security; FAO WATER REPORTS 38; Food and Agricultural Organization of the United Nations: Rome, Italy, 2012; pp.
11–13, ISSN 1020-1203.
Grasham, C.F.; Korzenevica, M.; Charles, K.J. On considering climate resilience in urban water security: A review of thevulnerability of the urban poor in sub-Saharan Africa. WIREs Water 2019, 6, e1344. [CrossRef]
Niang, I.; Ruppel, O.C.; Abdrabo, M.A.; Essel, A.; Lennard, C.; Padgham, J.; Urquhart, P. Climate Change 2014: Impacts Adaptation,
and Vulnerability. Part B: Regional Aspects. Contribution of Working Group 2 to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change; Cambridge University Press: Cambridge, UK, 2014; pp. 1199–1265. [CrossRef]
Sousa, P.M.; Blamey, R.C.; Reason, C.J.C.; Ramos, A.M.; Trigo, R.M. The ‘Day Zero’ Cape Town drought and the poleward
migration of moisture corridors. Environ. Res. Lett. 2018, 13, 124025–124035. [CrossRef]
Ityel, E.; Lazarovitch, N.; Silberbush, M.; Ben-Gal, A. An artificial capillary barrier to improve root zone conditions for horticultural
crops: Physical effects on water content. Irrig. Sci. 2010, 29, 171–180. [CrossRef]
Ityel, E.; Lazarovitch, N.; Silberbush, M.; Ben-Gal, A. An artificial capillary barrier to improve root zone conditions for horticultural
crops: Response of pepper, lettuce, melon, and tomato. Agric. Water Manag. 2012, 30, 293–301. [CrossRef]
Ityel, E.; Lazarovitch, N.; Silberbush, M.; Ben-Gal, A. An artificial capillary barrier to improve root zone conditions for horticultural
crops: Response of pepper plants to matric head and irrigation water salinity. Irrig. Sci. 2012, 105, 13–20. [CrossRef]
Miyake, M.; Otsuka, M.; Fujimaki, H.; Inoue, M.; Saito, H. Evaluation of an Artificial Capillary Barrier to Control Infiltration and
Capillary Rise at Root Zone Areas. J. Arid Land Stud. 2015, 25, 117–120. [CrossRef]
Wongkaew, A.; Saito, H.; Fujimaki, H.; Šimůnek, J. Numerical analysis of soil water dynamics in a soil column with an artificial
capillary barrier growing leaf vegetables. Soil Use Manag. 2018, 34, 206–215. [CrossRef]

Water 2021, 13, 1300

10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.

26.
27.
28.
29.
30.
31.
32.
33.
34.
35.

16 of 16

Mohammed, S.A.; Rashid, A.; Said, S.A.; Anvar, R.K.; Hamed, A. Use of Soil-Structured Capillary Barrier can Mitigate the Impactof Saline Irrigation Water on Marigold Grown Under Field Condition. J. Agri. Mar. Sci. 2020, 25, 9–19. [CrossRef]
Yunasa, G.H.; Kassim, A.; Umar, M.; Talib, Z.A.; Abdulfatah, A.Y. Laboratory investigation of suction distribution in a modified
capillary barrier system. IOP Conf. Ser. Earth Environ. Sci. 2020, 476, 012047. [CrossRef]
Morris, C.E.; Stormont, J.C. Evaluation of numerical simulations of capillary barrier field tests. Geotech. Geol. Eng. 1998, 16,
201–213. [CrossRef]
Fala, O.; Molson, J.; Aubertin, M.; Bussière, B. Numerical Modelling of Flow and Capillary Barrier Effects in Unsaturated Waste
Rock Piles. Mine Water Environ. 2005, 24, 172–185. [CrossRef]
Stroosnijder, L. Soil evaporation: Test of a practical approach under semi-arid conditions. Neth. J. Agri. Sci. 1987, 35, 417–426.
[CrossRef]
Bonachela, S.; Orgaz, F.; Villalobos, F.J.; Fereres, E. Soil evaporation from drip-irrigated olive orchards. Irrig. Sci. 2001, 20, 65–71.
[CrossRef]
Martinez, J.; Reca, J. Water use efficiency of surface drip irrigation versus an alternative subsurface drip irrigation method. Irrig.
Drain. Eng. 2014, 140, 04014030. [CrossRef]
Friedman, S.P.; Gamliel, A. Wetting Patterns and Relative Water-Uptake Rates from a Ring-Shaped Water Source. Soil Sci. Soc.
Am. J. 2019, 83, 48–57. [CrossRef]
O’Brien, D.M.; Rogers, D.H.; Lamm, F.R.; Clark, G.A. An Economic Comparison of Subsurface Drip and Center Pivot Sprinkler
Irrigation Systems. Appl. Eng. Agric. 1998, 14, 391–398. [CrossRef]
Saefuddin, R.; Saito, H. Performance of a ring-shaped emitter for subsurface irrigation in bell pepper (Capsicum annum L.)
cultivation. Paddy Water Environ. 2019, 17, 101–107. [CrossRef]
Saefuddin, R.; Saito, H.; Šimůnek, J. Experimental and numerical evaluation of a ring-shaped emitter for subsurface irrigation.
Agric. Water Manag. 2019, 211, 111–122. [CrossRef]
Sumarsono, J.; Setiawan, B.I.; Subrata, I.D.M.; Saptomo, S.K. Ring-typed emitter subsurface irrigation performances in dryland
farmings. Int. J. Civ. Eng. Technol. 2018, 9, 797–806.
Šimůnek, J.; van Genchten, M.T.; Sejna, M. Recent Developments and Applications of the HYDRUS. Computer Software Packages.
Vadose Zone J. 2016, 6, 1–25. [CrossRef]
Skaggs, T.H.; Trout, T.J.; Šimůnek, J.; Shouse, P.J. Comparison of HYDRUS-2D Simulations of Drip Irrigation with Experimental
Observations. Irrig. Drain. Eng. 2004, 130, 304–310. [CrossRef]
Kandelous, M.M.; Šimůnek, J. Comparison of numerical, analytical, and empirical models to estimate wetting patterns for surface
and subsurface drip irrigation. Irrig. Sci. 2010, 97, 1070–1076. [CrossRef]Šimůnek, J.; van Genuchten, M.T.; Šejna, M. The HYDRUS Software Package for Simulating Two- and Three-Dimensional Movement of
Water, Heat, and Multiple Solutes in Variably-Saturated Media, Technical Manual, 3rd ed.; PC Progress: Prague, Czech Republic, 2018;
pp. 103–106.
Provenzano, G. Using HYDRUS-2D simulation model to evaluate wetted soil volume in subsurface drip irrigation system. Irrig.
Drain. Eng. 2007, 133, 342–349. [CrossRef]
El-Nesr, M.N.; Alazba, A.A.; Šimůnek, J. HYDRUS simulations of the effects of dual-drip subsurface irrigation and a physicalbarrier on water movement and solute transport in soils. Irrig. Sci. 2014, 32, 111–125. [CrossRef]
Brunetti, G.; Šimůnek, J.; Piro, P. A comprehensive analysis of the variably saturated hydraulic behavior of a green roof in a
Mediterranean climate. Vadose Zone J. 2016, 5, 1–17. [CrossRef]
Durner, W. Hydraulic conductivity estimation for soils with heterogeneous pore structure. Water Resour. Res. 1994, 30, 211–223.
[CrossRef]
van Genuchten, M.T. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J.
1980, 44, 892–898. [CrossRef]Šimůnek, J.; van Genuchten, M.T.; Sejna, M. Development and applications of the HYDRUS and STANMOD software packages
and related codes. Vadose Zone J. 2008, 7, 587–600. [CrossRef]
Vrugt, J.A.; van Wijk, M.T.; Hopmans, J.W.; Šimůnek, J. One-, two-, and three-dimensional root water uptake functions fortransient modeling. Water Resour. Res. 2001, 37, 2457–2470. [CrossRef]
Feddes, R.A.; Kowalik, P.J.; Zaradny, H. Simulation of Field Water Use and Crop Yield; John Wiley & Sons: New York, NY, USA, 1978;
ISBN 9789022006764.
Jury, W.A.; Horton, R. Soil Physics, 6th ed.; John Wiley & Sons: New York, NY, USA, 2004; pp. 139–146. ISBN 978-0-471-05965-3.
Kandelous, M.M.; Kamai, T.; Vrugt, J.A.; Šimůnek, J.; Hanson, B.; Hopmans, J.W. Evaluation of subsurface drip irrigation design
and management parameters for alfalfa. Agric. Water Manag. 2012, 109, 81–93. [CrossRef]