Agricultural Water Management 244 (2021) 106601

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Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat

Evaluating soil salt dynamics in a field drip-irrigated with brackish waterand leached with freshwater during different crop growth stages
Yuehong Zhang a, Xianyue Li a, *, Jirí Šimůnek b, Haibin Shi a, Ning Chen a, Qi Hu a, Tong Tian aabCollege of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, Huhhot 010018, China
Department of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USA

A R T I C L E I N F O

A B S T R A C T

Handling Editor: Dr. N. Jovanovic

Irrigation with brackish water is a possible solution to alleviate freshwater shortages in arid and semi-arid re­gions. However, necessary measures need to be taken to leach accumulated salts out of the root zone. This can beaccomplished by leaching with freshwater applied by surface irrigation (LFSI), which needs to be optimized to
improve the leaching efficiency. An experiment in a field drip-irrigated with brackish water was carried out
during 2017 and 2018 to evaluate the effects of leaching (or no leaching (T0)) during the corn elongation (T1),tasseling (T2), and grain-filling (T3) stages. The HYDRUS (2D/3D) model was calibrated and validated using
experimental data collected in 2017 and 2018, respectively. The results of numerical simulations were in good
agreement with measurements, which is electrical conductivity (ECw), with mean relative errors of 11.9%,
10.1%, 10.6%, and 9.2% for the T1, T2, T3, and T0 treatments, respectively, during the validation periods. LFSI
during the corn elongation stage (T1) had the shortest duration of “high salinity stress” which lasted 42, 56, 57,
and 82 days for the T1, T2, T3, and T0 scenarios, respectively. The highest rate of salt reduction in the root zone
(i.e., the desalination rate) occurred when LFSI was applied during the corn grain-filling stage (T3). The desa­
linization rates were 59%, 68%, and 69% in the T1, T2, and T3 treatments, respectively. The cumulative salt flux
at a depth of 40 cm increased by 2.0%, 16.3%, and 34.7% for T1, T2, and T3, respectively, compared to T0. Twodimensional simulations of ECe distributions revealed that the “high salinity stress” area in the soil profile after
LFSI was 4%, 48%, and 92% higher for T0 than for T1, T2, and T3, respectively. The simulations showed that
optimal leaching time was about 41 days after sowing, which led to the lowest duration of the “high salinity
stress” period and a relatively high desalination rate of 59%.

Keywords:
Drip irrigation
Leaching during different growth stages
Duration of “high salt stress”
HYDRUS (2D/3D)
Corn

1. Introduction
Shortages of water resources and soil salinization have become
critical problems facing arid regions. These problems have greatly
restricted sustainable agricultural development and directly influenced
the food security (Lambers, 2003; Liu and Diamond, 2005; Singh, 2012;
Jesus et al., 2018). In agriculture, water-saving and soil-salinization
prevention technologies that reduce deep percolation (Nassah et al.,2018; Wang et al., 2019a, 2019b), decrease soil evaporation (Hou et al.,2016; Wang et al., 2019a, 2019b), and control the groundwater table(Soppe et al., 2003) have often been used to relieve water-shortage
pressures. However, these strategies can reduce agricultural water
shortages only to some extent. A more direct and efficient approach is tosearch for new water sources.
Soil salinization and shallow salty groundwater (brackish water,

with a salt concentration of 2–5 g L − 1) (MWR, 1998), besides severe
drought, are usually signs of problems for agricultural production (Singhet al., 1989). Quantitatively, the supply of brackish water worldwide isabout 1.39 × 109 km3, which is as much as the supply of freshwater,
accounting for 1% of the water on Earth (Rozema and Flowers, 2008). In
China, the total volume of brackish water is about 13 billion m3, while
the Hetao Irrigation District has about 8.9 billion m3 of brackish water(Yang et al., 2020). Currently, brackish water has been recognized as analternative water source to make up for the shortage of freshwater inregions with limited water resources (Li, 2018; Cucci et al., 2019).
Therefore, brackish water has been used as an important supplement toagricultural irrigation in many water-scarce areas and countries, such as
Israel (Ghermandi et al., 2014), Afghanistan (Ragab and Christel, 2002),
Italy (Cucci et al., 2019), and China (Huang et al., 2019; Xu et al., 2019).
However, brackish water needs to be used in such a way so that its* Corresponding author.
E-mail address: lixianyue80@126.com (X. Li).
https://doi.org/10.1016/j.agwat.2020.106601
Received 7 July 2020; Received in revised form 2 October 2020; Accepted 17 October 20200378-3774/© 2020 Elsevier B.V. All rights reserved.

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Agricultural Water Management 244 (2021) 106601

impact does not significantly affect crop growth (Ayers et al., 1976; Liet al., 2019a, 2019b).
When leaching after irrigation with brackish water is insufficient,salts accumulate in the root zone, resulting in a significant decrease in
the osmotic potential (Aragüés et al., 2015), which produces a physio­
logical drought. Root vitality decreases under conditions involving salt
stress (Annunziata et al., 2017), which reduces crop root water uptake(Khan et al., 2004). Additionally, soil microorganisms are affected(Hodge et al., 2009; Chen et al., 2018). The utilization of brackish water
for irrigation, as an alternative to fresh water, is thus quite challenging.
If used inappropriately, the use of brackish water can produce severe
threats to agricultural production (Hassanli and Ebrahimian, 2016).
Drip irrigation has been used worldwide due to its advantageous
agronomic, water conservation, and economic aspects (Lazarovitch
et al., 2009; Wang et al., 2007; Zhang et al., 2019; Jia et al., 2020).
However, since drip irrigation applies water at specific points in space,salts and water are pushed towards the fringes of the wetting area (Chenet al., 2009), and crop root growth is limited to inside the wetted volumeof the soil (Dong et al., 2010). Additionally, salts continue to accumulate
in the root zone with an increase in irrigation time if brackish water isused for irrigation (and leaching does not occur), which inevitably af­
fects crop growth. Thus, an efficient and low-cost leaching strategy forbrackish water irrigation is needed to push salts out of the root zone andto reduce soil salt stresses (Forkutsa et al., 2009; Satchithanantham
et al., 2014; Hassanli and Ebrahimian, 2016).
The conjunctive use of brackish and freshwater for irrigation is an
important way to decrease soil salinity (Rhoades, 1984; Xue and Ren,2017). Blending and alternate use are two options adopted when watersof different qualities are available (Rhoades, 1984; Dudley et al., 2008;
Kulkarni, 2011; Ghermandi et al., 2014; Kondash et al., 2020). For
example, the initial and final salt concentrations in the soil profile were
not significantly different for alternate irrigation (Hassanli and Ebrahi­
mian, 2016). However, the final salt concentrations significantly
increased for irrigation with conjunctive use of seawater and freshwater(Machekposhti et al., 2017). Li et al. (2019a, 2019b) reported thatalternate use of fresh and brackish waters for irrigation was more effi­
cient than blending in alleviating adverse effects of saline water on thesoil-crop system.
In drip irrigation with brackish water, salts quickly accumulate in thecrop root zone due to low irrigation volumes and limited leaching(Hanson et al., 2008). Nevertheless, drip irrigation with brackish water(1.1–4.9 dS m− 1) and appropriate management strategies did not affectcrop growth (Wan et al., 2007). However, soil salts still accumulate in
the root zone with increasing irrigation times (volumes) when brackish
water is used. For example, soil water salinity at a distance of up to 30
cm from the dripper almost doubled after 7–10 irrigations with brackish
water (electrical conductivity of 0.3 dS m− 1) (Hanson et al., 2008). It is,therefore, a primary practice to apply excess freshwater for leaching to
avoid salt accumulation in the root zone of drip-irrigated soil (Raij et al.,2016). Alternative irrigation using brackish water and freshwater wasoften used to solve salt accumulation when only brackish water was usedwith drip irrigation (Burt and Isbell, 2005). However, high cost,complicated operation, and incomplete leaching remain unresolved
questions (Letey et al., 2011). The salt removal efficiency of drip irri­
gation is smaller than for surface irrigation due to spatial constraints(Hassanli and Ebrahimian, 2016). For the same irrigation duration,better results are likely to be obtained for LFSI than drip irrigation,
because of higher water volumes and complete coverage of the soil
profile. It is thus necessary to optimize leaching dynamics (i.e., times
and volumes) to improve the effects of LFSI. Additionally, it is critical toreduce the frequency of leaching events (with constant irrigation
depths) to decrease the required quantity of freshwater and to optimize
its timing to alleviate the effects of the soil salt stress on crop growth.
Compared with field experiments, mathematical models have theadvantage of saving time, expense, and labor. Mathematical models
have many advantages to evaluate the effects of different leachingstrategies on spatial and temporal distributions of soil salts (Sammis
et al., 2011; Ramos et al., 2012; Srivastava, 2019; Li et al., 2019a,
2019b). Many models exist that can be used to evaluate the movement ofwater and soil salt under conditions involving brackish water irrigationor saline shallow water tables. These include HYDRUS (Šimůnek et al.,2016), SWAP (Yuan et al., 2019), LEACHC (Ali and Elliott, 2000),SALTMED (Afzal et al., 2016; Soothar et al., 2019; Karandish andŠimůnek, 2019), and UNSATCHEM (Rasouli et al., 2013). Bradford andLetey (1992) used the modified van Genuchten-Hanks model to showthat the cyclic irrigation strategy produced higher simulated yields ofsalt-sensitive corn than the blending irrigation strategy. Yuan et al.
(2019) used the SWAP model to show that brackish water with the salt
concentration of 3–5 mg cm− 3 was not suitable for long-term irrigation.
Soothar et al. (2019) used the SALTMED model to reveal that the
optimal time of using brackish water for irrigation was during thevegetative stage of winter wheat. Since HYDRUS is flexible to accom­
modate different types of boundary conditions, it has been widely used
to simulate soil water and salt distributions under brackish water irri­
gation and to optimize management strategies (Phogat et al., 2012; Li,
2018; Liu et al., 2019). Liu et al. (2019) used HYDRUS to simulate
irrigation with brackish water and indicated that the amount of fresh­water should be increased to reduce the salinity and enhance saltleaching. Yang et al. (2019) showed that the higher the leaching volume,the lower the salinity around the wetted body of drip irrigation. HYD­
RUS simulations showed that about 1.2 kg m− 3 salt was leached out fromthe 100-cm root zone irrigated for 15 years by a blend of brackish andfresh water (He et al., 2017).
The main objectives of this study were: (i) to calibrate and validate
the HYDRUS (2D/3D) model for drip irrigation with brackish water (ii)to evaluate soil salt spatio-temporal dynamics, (iii) to compare actualand cumulative soil salt fluxes at depths of 40 and 100 cm, and (iv) to
optimize the timing of leaching with freshwater applied by surface
irrigation during different growth stages. The study aims to provide abasis for the better use of drip irrigation with brackish water andleaching using surface irrigation with freshwater.
2. Material and methods
2.1. Field experiment
The experimental station is located in the middle of the Hetao Irri­gation District, in the Yellow River basin of northwest China (40º41’N,
107º18’E). The region has an arid continental monsoon climate withmean annual precipitation of about 188 mm, mean annual potential
evaporation of about 1900 mm, and annual sunshine of about 3222 h.
The soil type is classified as silt loam with mean bulk density of 1.42 g
cm− 3 and average field capacity of 28.5% (volumetric water content)
within the top 100 cm soil layer (Table 1). Additionally, the initial soil
salt concentration in the surface soil layer (0–10 cm) was 2.80 and 3.20
dS m− 1 before sowing in the 2017 and 2018 seasons, respectively, andthe mean groundwater table depth during the crop growth seasons was
175 and 171 cm in 2017 and 2018, respectively.

Table 1
Soil bulk densities, soil particle size distributions, and field capacities of different
soil layers.

2

Depth
(cm)

Soil particle size distribution (%)<0.002mm

0.002–0.05
mm

>0.05
mm

0–10
10–20
20–40
40–60
60–80
80–100

5.8
5.1
2.4
2.4
8.5
1.4

71.1
67.2
74.6
76.7
72.8
59.6

23.1
27.7
23.0
20.9
18.7
39.0

Bulk density(g cm− 3)Fieldcapacity
(%)

1.43
1.45
1.38
1.38
1.43
1.48

27.7
28.8
28.7
29.2
28.5
27.2

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Agricultural Water Management 244 (2021) 106601

In the experiment, brackish water (drip irrigation) and freshwater(surface irrigation) were conjunctively used. Different scenarios withdifferent timing of leaching with freshwater applied by surface irrigation(LFSI) were evaluated while irrigating the field with brackish water bydrippers. Complete random design comprising of three replicates of four
treatments was conducted on 12 field plots. The four treatments
involved LFSI in the elongation (T1), tasseling (T2), and grain-filling(T3) stages, or no leaching (T0). Each plot had a length of 20 m, awidth of 7 m, and was surrounded by a bare area with a 2-m width to
avoid external influences on trial conditions. Groundwater with anaverage salinity of 2.54 g L− 1 (electrical conductivity of about 3.7 dSm− 1) was the water source for drip irrigation. Irrigation canal (Yellow
River water) with an average salinity of 0.54 g L− 1 was the source ofleaching water for surface irrigation. The irrigation quota for drip was
22.5 mm per irrigation event (8 events), and the leaching quota forsurface irrigation (only once during the entire growth season) was 112.5
mm. Additionally, large irrigation (about 225 mm in each year, called
autumn irrigation) was used after the harvest on October 16 and 20 in2017 and 2018, respectively, to leach salts accumulated during theseason out of the soil profile (Lu et al., 2019).
The corn (Zea mays L.) was sown on May 9 (1) and harvested on
September 10 (11) in 2017 (2018). One drip line (with a 30 cm emitter
spacing) and one plastic film (80 cm wide) were used for two corn rows
with a row spacing of 40 cm and an emitter discharge of 2.4 L h− 1
(Fig. 1). A water meter (with an accuracy of 0.001 L) monitored water
flow. The same irrigation amount of brackish water (22.5 mm for eachirrigation event) was applied to each plot every 7–15 days. Eight irri­
gation events per year were used throughout the two growing seasons.
Before sowing, 140 kg ha− 1 of diammonium phosphate ((NH4)2HPO4, N≥ 18%), 140 kg ha− 1 of potassium sulfate (K2SO4), and 50 kg ha− 1 ofcarbamide (CO(NH2)2, N ≥ 32%) were applied in the field as basal
fertilizer. The carbamide (CO(NH2)2, N ≥ 32%), solubilized as
topdressing, was applied during the elongation stage (30% of the total
N-fertilizer) and the grain-filling stage (30% of the total N-fertilizer),each time accompanied with drip irrigation.
2.2. Measurements and methods
2.2.1. Meteorological data
An automatic meteorological station (Onset Computer Inc.; U30,
Hobo, USA) was located 500 m away from the experiment field to record
air humidity, temperature, precipitation (Fig. 2), solar radiation, airpressure, and wind speed. The Penman–Monteith equation (Allen et al.,1998) was used to calculate reference crop evapotranspiration (ET0)during the 2017 and 2018 seasons (Fig. 2). Potential crop evapotrans­
piration (ETp) (Fig. 2) was then calculated as ETp = Kc × ET0, where Kc is
the crop coefficient for corn, which is 0.7, 1.2, and 0.35 during the early,
middle, and late growth stages, respectively (Allen et al., 1998). Finally,
ETp was divided into potential transpiration (Tp) and potential evapo­
ration (Ep) (Campbell and Norman, 1989) as follows:
ETp = Tp + Ep

(1)

Fig. 1. A schematic showing the modeling domain with a dripper and plants in the experimental field, the cropping pattern, the boundary conditions and the lo­
cations of sensors.

3

Y. Zhang et al.

Agricultural Water Management 244 (2021) 106601

Fig. 2. Rainfall, irrigation, potential evapotranspiration (ETp), and groundwater table depths during the crop growing season in 2017 (left) and 2018 (right).

)(
Tp = 1 − e− k⋅LAI ETp(2)

Ep = e− k⋅LAI ETp(3)logger (Onset Computer Inc.; U24-001, Hobo, USA), respectively,
installed in an observation well in the experimental field. The data wereregularly manually verified, and the 1-day statistics (average) werecomputed and recorded (Fig. 2).

where LAI is the measure leaf area index, k is the extinction coefficient.

2.2.5. Corn leaf area, height, and yield
Six plants in each plot were selected to measure the corn height, theleaf length, and the leaf width using a tape (with an accuracy of 0.1 cm)
once every 7–14 days. The corn leaf area was calculated as 0.75 × leaf
length × leaf width. The leaf area index (LAI) (Fig. 3) was calculatedusing the FAO method (Allen et al., 1998). Ten plants from each plot
were randomly chosen to measure the grain number per cob and the
100-grain weight in the harvest. Crop yield for each treatment was ob­
tained as the mean value of three replicates. Water use efficiency (WUE,
kg ha− 1 mm− 1) was defined as corn yield (Y, kg ha− 1) divided by total
actual evapotranspiration (ET, mm) during the growth stage.

2.2.2. Soil water content (SWC), water potential, and electricalconductivity
The SWC and the soil water potential were measured using TDR
probes (IMKO GmbH Inc.; IPH, TRIME-PICO, Germany) and tensiome­ters (Zhonghui Inc.; ZKNT-100, China), respectively. The distances be­
tween tensiometers and the drip emitter were 0 cm (under mulching)and 55 cm (outside of mulching, a bare area) (Fig. 1). The measuring
depths were 5, 15, 30, 50, 70, and 90 cm for SWC and 90 and 110 cm forsoil water potential, and measuring time intervals were about every 5–7
days. Additionally, auger boring was used to obtain soil samples. Since
the method is destructive, measurements were collected adjacent to the
TDR probes only every 10–14 days to calibrate TDR and salinity read­ings. Soil samples were divided into two groups, one for measuring SWC
and another one for soil salinity. The electrical conductivity (ECw) of all
soil samples was determined using 1:5 extracts of soil and water (byweight) using a conductivity meter (Zhongxi Inc.; DDSJ-308 F, China).
The electric conductivity of a saturated paste extract (ECe) was inferredfrom measured values of soil ECw using the following relationship:()
ECe = 5.8216ECw + 1.3403 R2 = 0.95
(4)

2.3. Model
2.3.1. Model description
The HYDRUS (2D/3D) model was used to simulate the twodimensional movement of soil water and salts by solving the Richards
equation (Richards, 1931) and the convection-dispersion equation. A
detailed description can be found in Šimůnek et al. (2016). Additionally,
it was assumed in this study that the drip line and crops acted as infinite
sources and sinks in the perpendicular direction.
Root water extraction was computed according to the Feddes model(Feddes et al., 1978; Karandish and Šimůnek, 2018) adapted for twodimensional conditions and simultaneous water and salinity stresses:

where ECe is the electrical conductivity of a saturated paste extract(dS m− 1), and ECw is the electrical conductivity measured on 1:5 extracts(dS m− 1).

S(h, hφ ) = α(h, hφ )β(x, z, t)Lt Tp

2.2.3. Cumulative soil salt leaching
Before sowing the corn, a tensiometer and a PVC tube opened at thebottom were installed at a depth of 100 cm to measure vertical water
fluxes (Wang et al., 2007) and collect soil solutions once every 7–14 days
(Gabriel et al., 2012; Chen et al., 2020). The titration method was used
to determine the soil salt concentrations (i.e., contents of Ca2+, Mg2+,
Cl–, SO2–4, HCO3–, and CO2–3) based on the specification of soil tests,
SL237-1999 (Ministry of Water Resources of the People’s Republic ofChina, 1999; Wang et al., 2019a, 2019b). Cumulative soil salt leaching
was determined by multiplying the soil salt concentration of the leach­ing solution by corresponding water fluxes after irrigation on days aftersowing (DAS) 33, 41, 55, 67, 78, 87, and 97.

(5)where α(h,hϕ) is the water stress and osmotic stress function (–), β(x, z, t)
is the normalized spatial root water uptake distribution (cm− 2), Lt is thewidth of the soil surface associated with transpiration (cm), and Tp is thepotential transpiration rate (cm day− 1). The multiplication model wasused to account for combined water and salinity stresses. The root water
uptake reduction due to the water stress was simulated using the piecewise linear function of Feddes et al. (1978), with default HYDRUS pa­
rameters for corn. The root water uptake reduction due to the salinity
stress was simulated using the salinity threshold (ECt) and slope function(Maas, 1990). The salinity threshold and slope for corn were set to
1.7 dS m− 1 and 12%, respectively (Karandish and Šimůnek, 2018).
The soil hydraulic properties (i.e., θ(h) and K(h)) are represented in
the HYDRUS (2D/3D) model using the analytical functions of van
Genuchten (1980):

2.2.4. Groundwater
The groundwater table and its electrical conductivity were measured
at 1-hour intervals by an automatic water level logger (Onset Computer
Inc.; U20L-01, Hobo, USA) and an automatic water conductivity data
4Agricultural Water Management 244 (2021) 106601

Y. Zhang et al.

Fig. 3. The leaf area index (LAI) for scenarios with leaching during the corn elongation (T1), tasseling (T2), grain-filling (T3) stages, and no leaching (T0) in 2017 (a)and 2018 (b).

θ(h) =⎧⎨ θr +⎩⎡

θs − θrm[1 + |αh|n ]

h<0

θs

h≥0

⎛

⎞m ⎤2

1⎟ ⎥
⎢
⎜
K(h) = Ks ⋅Sle ⎣1 − ⎝1 − Sem ⎠ ⎦ ,35–55 cm top boundary (Fig. 1), respectively, depending on whether thesoil surface was covered or not by a plastic film. A 2 cm wide timevariable flux BC was specified at the left corner of the soil surface torepresent the emitter. The potential transpiration (Tp) flux was specified
to account for plant water uptake. Time-variable head BC was used at
the bottom boundary. Left and right boundaries were assigned a no-flow
BC. Additionally, a third-type BC was used to describe concentration
fluxes along the top and bottom boundaries.

(6)

m = 1−1θ − θr, Se =nθ s − θr(7)

where θr is the residual volumetric water content (cm3 cm− 3), θs is thesaturated volumetric water content (cm3 cm− 3), Ks is the saturated hy­
draulic conductivity (cm day− 1), α (cm− 1), n (–), and m (–) are empiricalparameters, Se is the effective saturation (–), and l is the pore connec­
tivity parameter (–).
The root distribution function is implemented in HYDRUS (2D/3D)
as follows (Vrugt et al., 2001a, 2001b):()()() − pz |z∗ − z|+ pr |x∗ − x|
X
Z
m
m
z
x
(8)
b(x, z) = 1 −
1−
e
Zm
Xm

2.3.3. Model parameters
Soil hydraulic parameters (i.e., θr, θs, α, n, and Ks) were obtained
according to the percentage content of sand, silt, and clay in the soil
particle distribution curve using the Rosetta module of HYDRUS. These
values of soil hydraulic parameters were further manually calibrated bycomparing simulated and observed values of SWCs and soil salt contents(Table 2). The longitudinal dispersivities of two soil layers were 50 and
20 cm. The transversal dispersivities were set to one-tenth of the lon­
gitudinal dispersivities (Ramos et al., 2012). Since molecular diffusion
can usually be neglected, the molecular diffusion coefficient was set tozero (Radcliffe and Šimůnek, 2010). The root distribution parameters
were obtained from actual measurements (Table 3).

where b(x,z) is a two-dimensional spatial distribution of the potential
root water uptake rate (–), Zm is the maximum rooting depth in the z
direction (cm), Xm is the maximum rooting length in the x direction(cm), x and z are distances in the x and z directions (cm), and px (–), pz(–), x* (cm), and z* (cm) are empirical parameters (Vrugt et al., 2001b).
The parameters x* and z* indicate the location in the profile with the
maximum rooting density, while px and pz are assumed to be zero forx > x*and z > z* (Šimůnek et al., 2018).

2.4. Model performance criteria
The paired t-test analysis for the statistical comparison of theobserved and HYDRUS (2D/3D)-simulated data demonstrated no sig­
nificant difference between the observed and simulated SWCs and sa­
linities during 2017 and 2018. Agreement between simulated andobserved values was evaluated using the root mean square error (RMSE),
the mean absolute error (MAE), and the mean relative error (MRE) for

2.3.2. Initial and boundary conditions
A rectangular two-dimensional transport domain (55 cm wide and220 cm deep; the latter corresponds with the maximum observed
groundwater table depth) was defined between the emitter and the
middle of the no mulched area (Fig. 1). The simulation domain was
discretized using the non-uniform finite element mesh generated by the
HYDRUS (2D/3D) model. Measured SWCs and soil salinities at the
beginning of the experiment of each year were used as initial conditions
for simulations. The top and bottom water contents were 0.19 and0.23 cm3 cm− 3, and ECw were 0.25 and 0.32 dS m− 1. The no-flow andatmospheric (to apply precipitation/irrigation and potential evapora­tion Ep) boundary conditions (BCs) were assigned to the 0–35 andTable 2
Soil hydraulic parameters in the 0–40 and 40–100 cm soil layers. θr is the re­sidual soil water content (cm3 cm− 3), θs is the saturated soil water content (cm3
cm− 3), α (cm− 1) and n (–) are shape parameter, Ks is the saturated hydraulicconductivity (cm day− 1), and l is the tortuosity parameter in the conductivity
function (–).

5

Soil layer(cm)θr (cm3
cm− 3)θs (cm3
cm− 3)

α

n (–)

Ks
(cm day− 1)

l
(–)

0–40
40–100

0.042
0.040

0.371
0.366

0.00640.0067

1.64
1.63

50.8
49.4

0.5
0.5

(cm− 1)Agricultural Water Management 244 (2021) 106601

Y. Zhang et al.

values of different soil layers were analyzed for different timings of LFSI
during the elongation (T1), tasseling (T2), and grain-filling (T3) stages,
and under no leaching conditions (T0) in the 2017 and 2018 seasons(Fig. 4). ECe in the 0–40 cm soil layer had an average ECe of 6.2 dS m− 1
during the growth period. The ECe in the 0–20 and 20–40 cm soil layers
decreased after irrigation with brackish water (Fig. 4) and then gradu­
ally increased. ECe in the deep soil layer (40–100 cm) was practicallyunaffected by drip irrigation. Overall, ECe of each soil layer gradually
increased during the entire growth stage, and it quickly increased above
5.9 dS m− 1, when corn yield was expected to be reduced by about 50%
from optimal yield (FAO29, 1976). Additionally, the maximum ECereached values of 9.0 and 12.1 dS m− 1 in the shallow soil layer (0–40 m)
in 2017 and 2018, respectively, and 6.5 and 7.0 dS m− 1 in the deep soil
layer (40–100 cm) when no leaching was applied during the growthstage.
When ECe is higher than 5.9 dS m− 1 (“high salt stress,” HSS), cornyield is expected to decrease by more than 50% (FAO29, 1976). It is thus
important to extend the duration of the “low salt stress” conditions (ECelower than 5.9 dS m− 1, LSS) during the corn growing season. Significant
decreases in ECe in all soil layers were produced after LFSI (leaching withfreshwater). Average ECe decreased to 2.4 dS m− 1, which represented anaverage decrease of 74% and 52% in the shallow and deep soil layers forthree leaching treatments in both years, respectively. The effects of thetiming of LFSI on ECe were different. Early LFSI caused the lowest
average ECe in the experiment and a longer duration of LSS. The averageECe in the 0–100 cm soil layer in the T1, T2, T3, and T0 scenarios was
5.3, 5.4, 5.5, and 6.9 dS m− 1 during the entire growing season in both
years, respectively. The average duration of LSS was 83, 69, 68, and 43days, respectively. The average ECe in the 0–40 cm soil layer was 6.1,
6.2, 6.3, and 7.8 dS m− 1, respectively, and the average duration of LSS
was 69, 56, 55, and 41 days, respectively. However, later LFSI led tolower ECe at the end of the growing season (DAS 125). ECe in the shallow
soil layer was 10.2, 10.2, 8.6, and 13.1 dS m− 1 in the T1, T2, T3, and T0
scenarios in both years, respectively, and 6.7, 6.2, 5.3, and 8.6 dS m− 1 in
the deep soil layer. LFSI in the elongation stage (T1) produced a longer
duration of LSS by 21%, 19%, and 45% compared with LFSI in the T2,
T3, and T0 scenarios, even though ECe at the end of the growing season
was a little higher than in T2 and T3. LFSI applied during any growthstage can significantly decrease ECe in the soil in all soil layers. LFSI in
the elongation stage can produce the lowest average soil salinity and thelongest duration of LSS.

Table 3
The root distribution parameters. Xm is the maximum rooting length in the x
directions (cm), Zm is the maximum rooting depth in the z directions (cm), and px(–), pz (–), x* (cm), and z* (cm) are empirical parameters.
StagesXm (cm)Zm (cm)x*(cm)

z*(cm)

px

pz

Elongation
Tasseling
Filling
Maturation

17.9
33.6
41.2
35.6

37.5
62.4
80.3
68.9

5.2
14.8
22.7
21.2

10.4
28.9
43.6
40.8

4
4
4
4

1
1
0.5
0.5

each treatment:[
RMSE =

MAE =

]1/2
(9)

n
1∑
|Pi − Oi |
n i=1

1
n

MRE =

n
1∑
(Pi − Oi )2
n i=1

n
∑

(10)

|Pi − Oi |

i=1

Oi

(11)

× 100

where n is the number of data points, and Pi and Oi are simulated andmeasured values, respectively.
3. Results
3.1. Model evaluation
The HYDRUS (2D/3D) model was calibrated and validated using the
SWC and ECw experimental data from the years of 2017 and 2018,
respectively. Various statistical tests (RMSE, MAE, and MRE) were car­ried out to investigate the model performance, and they showed good
agreement between simulated and measured values (Table 4). The rangeof RMSEs for the calibration periods for SWCs and ECw was
0.03–0.08 cm3 cm− 3 and 0.08–0.12 dS m− 1, respectively. Correspond­
ing ranges for MAE were 0.01–0.02 cm3 cm− 3 and 0.05–0.06 dS m− 1, for
MRE were 10.2–11.2% and 8.6–13.0%, and for R2 were 0.9–0.95 and
0.85–0.91, respectively. Additionally, the range of RMSEs for the vali­
dation periods for SWC and ECw was 0.02–0.09 cm3 cm− 3 and
0.10–0.13 dS m− 1, respectively. Corresponding ranges for MAE were0.01–0.02 cm3 cm− 3 and 0.05–0.08 dS m− 1, for MRE were 8.9–11.6%
and 9.2–11.9%, and for R2 were 0.89–0.95 and 0.88–0.92, respectively.
Simulation errors for both SWC and ECw were slightly higher under themulch than in the bare area, likely because of damaged plastic film.

3.3. Actual and cumulative salt fluxes at depths of 40 and 100 cm fordifferent LFSI treatments
In the study, the maximum depth of the wetted soil after drip irri­
gation was 40 cm, while the maximum measured corn rooting depth
during the grain-filling stage was 80.3 cm. We therefore measured,simulated, and analyzed actual and cumulative salt fluxes at depths of
40 and 100 cm (Fig. 5). Overall, soil salts at depths of 40 and 100 cm

3.2. Effects of LFSI timing on electrical conductivity ECe
The soil ECe under drip irrigation varied quite dynamically. ECe

Table 4
Statistical measures for SWCs and ECw in the 2017 (calibration) and 2018 (validation) years. MAE is the mean absolute error (cm3 cm− 3, dS m− 1), MRE is the mean
relative error (%), RMSE is the root mean square error (cm3 cm− 3, dS m− 1), R2 is the coefficient of determination (–), and t is the calculated t value.
Index
SWC

ECw

2017
MAE (cm3 cm− 3)
MRE (%)RMSE (cm3 cm− 3)
R2t
MAE (dS m− 1)
MRE (%)RMSE (dS m− 1)
R2
t

2018

T1

T2

T3

CK

T1

T2

T3

CK

0.01
10.8
0.03
0.90
–0.3
0.06
11.5
0.09
0.91
1.0

0.01
10.3
0.03
0.92
–0.5
0.05
8.6
0.08
0.90
1.4

0.02
10.2
0.03
0.95
1.2
0.06
13.0
0.12
0.85
1.3

0.02
11.2
0.08
0.91
–1.1
0.06
9.1
0.12
0.90
1.3

0.01
9.7
0.02
0.89
1.1
0.05
11.9
0.10
0.90
–1.3

0.02
8.9
0.04
0.90
–0.3
0.07
10.1
0.13
0.88
0.1

0.02
9.8
0.03
0.92
0.8
0.05
10.6
0.12
0.90
1.1

0.02
11.6
0.09
0.93
1.3
0.08
9.2
0.13
0.90
0.9

6

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Agricultural Water Management 244 (2021) 106601

Fig. 4. Simulated and observed saturated extract electrical conductivities (ECe) at the dripper position in the 0–20 (top), 20–40, 40–60, and 60–100 (bottom) cm soil
depths in 2017 (left) and 2018 (right) when leaching with freshwater (LFSI) was applied during the elongation (T1), tasseling (T2), and grain-filling (T3) stages, orwith no leaching (T0).

always moved upward during periods without irrigation or rainfall anddownward (salt leaching) after drip irrigation with brackish water. The
average peak of the salt leaching flux at 40 cm was 8.9 mg cm− 3 day− 1(negative sign in Fig. 5 indicates leaching), which was about 80% higher
than fluxes at 100 cm. Salt leaching fluxes significantly increased after
LFSI in each treatment. The average peak of the salt leaching flux at a
depth of 100 cm was higher than the one at a depth of 40 cm (i.e., 30.2
and 36.9 mg cm− 3 day− 1 at depths of 40 and 100 cm, respectively) in alltreatment due to accumulated salts in the 40–100 cm soil layer during

drip irrigation without leaching. Later LFSIs caused higher maxima inthe salt leaching flux. For example, the maximum salt leaching flux at a
depth of 40 cm was 19.3, 32.2, and 39.1 mg cm− 3 day− 1 for LFSI during
the elongation (T1), tasseling (T2), and grain-filling (T3) stages,
respectively. The salt fluxes increased to 25.1, 36.2, and 49.4 mg cm− 3day− 1 at a depth of 100 cm due to salt accumulation in the soil before
LFSI. Except for small positive values (capillary rise) during the earlycorn growing season (DAS 0–41), the cumulative salt flux (CSF) at the
100 cm soil depth was always negative (i.e., soil salt leaching). The
7

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Agricultural Water Management 244 (2021) 106601

Fig. 5. Measured and simulated actual (mg cm− 2 day− 1) and cumulative (mg cm− 1) salt fluxes at depths of 40 and 100 cm during the 2017 season when leachingwith freshwater (LFSI) was applied during the elongation (T1) (a), tasseling (T2) (b), and grain-filling (T3) (c) stages, and when no leaching was applied (T0) (d).

timing of LFSI had a noticeable effect on CSF, and later LFSI caused
higher cumulative salt leaching. For example, the CSF values at a depthof 40 cm were − 4.9, − 5.0, − 5.7, and − 6.6 t ha− 1 for T0, T1, T2, and T3
at the end of the growing season (DAS 125), respectively, and − 3.0,
− 3.2, − 4.5, and − 5.4 t ha− 1 at a depth of 100 cm. The later LFSI wasapplied, the more salts were leached. Larger differences were recorded
at a depth of 100 cm than 40 cm.

soil profile completely disappeared following LFSI. Additionally, when
LFSI was applied later in the season after more salts had accumulated in
the soil profile, a decrease in soil salt concentrations was more obvious.
For example, there was a 4%, 48%, and 92% reduction in average ECeafter LFSI was applied on DAS 43 (T1), 68 (T2), and 89 (T3) comparedwith those of DAS 41 (T1), 66 (T2), and 87 (T3), respectively (Fig. 6a, b,
i, j, q, r).

3.4. The effects of LFSI timing on two-dimensional ECe and salt stress
distribution

3.5. The effects of the LFSI time on water and salt balance components
Soil water and salt balance components were greatly influenced by
the LFSI events (Table 5). Actual ET fluxes for the LFSI treatments (T1,
T2, and T3) were higher than for T0, and increased on average by 14.2%
and 13.9% in 2017 and 2018, respectively. LFSI applied later in theseason caused lower ET and corn yield due to longer HSS. Actual ET was,on average, 71.7, 57.8, and 49.4 mm higher in the T1, T2, and T3
treatments than in the T0 treatment, respectively. Compared with T0,
corn yields increased on average in two growing seasons by 18.5%,
7.9%, and 0.8% (significant differences) in the T1, T2, and T3 treat­ments, respectively. However, except for T1, which showed a small
improvement, average WUE decreased due to higher irrigation andleaching compared T0. Additionally, the T0 treatment, due to the lowest
amount of irrigation water, utilized more groundwater (12.3% and25.5% more in 2017 and 2018, respectively) compared with the LFSI
treatments. LFSI applied later caused a slight reduction in groundwateruse. For example, there was a difference of 8.5 mm in the cumulative
upward water flux between T1 and T3 during 2017.
LFSI treatments produced more salt leaching compared with T0. The
later LFSI was applied, the higher the salt leaching. The averageFig. 6 shows the two-dimensional distributions of simulated ECe for
all four treatments to demonstrate the effects of the different timing of
LFSI on the salt stress in the soil profile. Fig. 6 shows the ECe profiles
before (DAS 41, 66, and 87) and after (DAS 43, 68, and 89) either drip
irrigation or LFSI during the 2017 season. Generally, ECe gradually
increased with time in all treatments due to the input of salts withbrackish water and salted groundwater. For example, the average ECe in
the soil profile was 5.67, 8.22, and 9.36 dS m− 1 on DAS 41, 66, and 87 in
the T0 treatment (Fig. 6s, u, w), respectively, when the HSS areas
accounted for 10%, 62%, and 91% of the total area. Although salt
concentrations in the surface soil decreased after drip irrigation withbrackish water, the overall mass of salts in the soil profile remained
similar due to the lack of leaching, and the average ECe in all treatments
increased. For example, the average ECe increased 21%, 15%, and 8% on
DAS 43, 68, and 89 compared with those on DAS 41, 66, and 87 in the T0
treatment (Fig. 6s, t, u, v, w, x), respectively.
There was a dramatic decrease in soil salt mass and ECe concentra­tion in the soil profile after LFSI was applied (Fig. 6). The HSS area in the8

Y. Zhang et al.

Agricultural Water Management 244 (2021) 106601

Fig. 6. Simulated two-dimensional distributions of ECe in a field drip-irrigated with brackish water and leached with freshwater applied by surface irrigation (LFSI)
during the elongation (T1, top), tasseling (T2, second row from top), and grain-filling (T3, second row from bottom) stages, or with no leaching (T0, bottom) on DAS
41 (before surface irrigation for T1, and drip irrigation for T2, T3, and T0), DAS 43 (after irrigation), DAS 66 (before surface irrigation for T2, and drip irrigation for
T1, T3, and T0), DAS 68 (after irrigation), DAS 87 (before surface irrigation for T3, and drip irrigation for T1, T2, and T0) and DAS 89 (after irrigation). Dashed blue
squares indicate the application of LFSI. The red contours delineate the HSS areas (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article).

9

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Agricultural Water Management 244 (2021) 106601

Table 5
Soil water and salt balance components in 0–100 cm for treatments T0, T1, T2, and T3 during 2017 and 2018. ∆W is the change in soil water storage between the end
and beginning of the season (mm), P is precipitation (mm), F is the cumulative bottom water flux (at a depth of 100 cm; positive out of the region) (mm), I is the total∑
irrigation quantity (mm), ET is total evapotranspiration during the growth stage, w is the total water balance error (mm), Si is the initial soil salt storage (t ha− 1), Se is
the final soil salt storage (DAS 125) (t ha− 1), Sg is the amount of salts migrated from groundwater into the soil profile (at a depth of 100 cm) (t ha− 1), Sa is the amount of∑
the salts added with irrigation water (t ha− 1), Sl is the amount of salts leached below 100 cm (t ha− 1), and s is the total salt balance error (t ha− 1), Corn yield is the
− 1
− 1
− 1
mean value of corn yield for three replicates (kg ha ), WUE is water use efficiency (kg ha mm ). Different letters in the same column indicate a significant
difference (P < 0.05) among treatments.
Year
2017

2018

Salt balance (t ha− 1)

Water balance (mm)
T0
T1
T2
T3
T0
T1
T2
T3

∆W

P

F

I

ET

∑
w

–52.1
–21.5
–14.0
–5.1
–50.7
–34.1
–14.5
–12.4

36.6
36.6
36.6
36.6
82.2
82.2
82.2
82.2

–154.3–141.1–138.3–132.6–120.3
–96.0
–95.8
–95.7

180.0
292.5
292.5
292.5
180.0
292.5
292.5
292.5

420.9
493.2
479.4
468.7
430.5
501.7
487.5
481.4

2.1
–1.5
2.0
–1.9
2.7
3.1
–2.5
1.4

Si

Se

Sg

Sa

Sl

∑
s

18.4
18.4
18.4
18.4
19.2
19.2
19.2
19.2

25.0
22.4
20.7
19.0
27.5
22.6
22.5
19.6

12.8
11.7
11.5
11.0
14.2
11.4
11.4
11.4

4.6
4.6
4.6
4.6
4.6
4.6
4.6
4.6

10.5
12.2
13.6
15.5
10.3
12.5
12.5
15.8

0.3
0.1
0.2
–0.5
0.2
0.1
0.2
–0.2

Corn yield (kg ha− 1)

WUE (kg ha− 1 mm− 1)

9300a11049b10054c
9365
9378a
11077b10095c
9465a

22.1
22.4
21.0
20.0
21.8
22.1
20.7
19.7

freshwater with a salinity of 0.76 d S− 1 (Fig. 5).
In areas with shallow groundwater, saline groundwater can moveinto the soil due to the capillary action, and the salts then remain in theroot zone. The risk of secondary salinization is practically unavoidable
when there is no human intervention (Satchithanantham et al., 2014). It
was found in this study that when brackish water was the only source for
irrigation (T0), 8.6 t ha− 2 of salts migrated from groundwater into thesoil during the crop growth period (65% of total soil salts). Forkutsa
et al. (2009) found that salinization was inevitable in shallow ground­water areas unless underground drainage conditions were improved.
Shallow groundwater areas with high salinity can thus easily produce
physiological drought (Xu et al., 2019). Leaching using freshwater irri­
gation is then needed to move soil salts out of the root zone. For
example, in our study, freshwater irrigation leached 12.4, 13.1, and15.7 t ha− 2 of salts out of the root zone on DAS 41, 66, and 89,
respectively.

amounts of leached salts in the T1, T2, and T3 treatments were 12.4,
13.1, and 15.7 t ha− 1, which were 1.2, 1.3, and 1.5 times higher than in
T0. Since T0 utilized more saline groundwater, more salts were trans­
ported into the soil profile from groundwater in this treatment than in
the other treatments (on average 13.5, 11.6, 11.4, and 11.2 t ha− 1 in T0,
T1, T2, and T3 in both years, respectively). Additionally, Se (the final soil
salt storage) in T0 was higher than those in the LFSI treatments, and LFSI
applied later produced lower Se. The average (over two years) Se was
26.2, 22.5, 21.6, and 19.3 t ha− 1 in T0, T1, T2, and T3, respectively.
4. Discussion
4.1. The necessity of leaching fields irrigated with brackish water in areas
with shallow groundwater
Irrigation with brackish water (i.e., with salt concentrations of
2.8–8.2 dS m− 1) inevitably leads to salt accumulation in the soil profile.
The salt distribution depends on the irrigation method. For example,
salts are usually accumulated in the shallow wetted soil layer under drip
irrigation (Selim et al., 2012; Goldberg et al., 1976). In this study,
although ECe in the surface soil decreased after the application of drip
irrigation with brackish water, the accumulated salts were not leached
out of the root zone (0–100 cm) due to a low drip irrigation volume.
These salts were only transported sideways to bare areas and into thedeeper root zone. As a result, some portions of the corn root systemremained under salt stress. In contrast, salts are mostly distributed below
the 80 cm depth under surface irrigation, mainly due to the larger irri­
gation quota of each application (FAO29, 1976; Sinha and Singh, 1976).
When the soil salt concentrations are higher than that of brackish water,
an area with reduced salinity (a desalinization zone) will be temporarily
formed in the wetted soil layer after brackish water irrigation. For
example, in this study, the region with reduced salinity occurs in the top
40 cm (Fig. 6). The salts are pushed out of the wetted soil and accu­
mulate around the wetting front (Zhang et al., 2019). However, accu­
mulated salts can still move back into the crop root zone due to croptranspiration and soil evaporation (Groenveld et al., 2013). In this study,
the soil salt concentration in the root zone gradually increased with time
when no freshwater irrigation was used. For example, the average salt
concentration on DAS 41, DAS 66, DAS 89 was 4.8, 7.4, and 9.6 dS m− 1
in the fan-shaped region with a radius of about 40 cm, respectively(Fig. 6s, u, w). As a result, salinity stress inevitably appeared underbrackish water drip irrigation. It is necessary to leach the salts out of thecrop root zone with freshwater in order to keep salinity below thethreshold salinity level and not to restrict crop growth (Zhang et al.,2019). Usually, soil salts are leached out of the crop root zone bypercolation. In this study, 55 mm of water with 19.3–39.1 mg cm− 2 ofsalts were leached below the 40 cm depth by irrigation of 112.5 mm of

4.2. Effects of the timing of leaching on soil salinity in a field irrigatedwith brackish water
Usually, leaching using freshwater is carried out only at the end of
the growing season or at the beginning of the next growing season (Penget al., 2016). However, in such a case, salts can accumulate in the rootzone during the growing season so that crop growth may still be undersalinity stress in the later growth stages (Minhas and Gupta, 1993a,
1993b). Therefore, alternative irrigation with salt and freshwater isoften applied to control soil salinity during the season (Chauhan et al.,2007; Xu et al., 2014), although more freshwater is then needed.
Leaching using irrigation with fresh water during critical growth periods
can not only decrease soil salinity but may also reduce the amount of
required water for irrigation (Bradford and Letey, 1992). Alternate
irrigation has thus become an important technology used in conjunction
with brackish water irrigation (Minhas et al., 2007).
We define the desalination rate as DR = (ECe(t1)–ECe(t2))/ECe(t1) ×100, where t1 and t2 are times before and after leaching, respectively.
Since salts accumulate in the soil with time, earlier leaching using
freshwater irrigation leads to a lower desalination rate and higher cornyield (due to low salt accumulation). In comparison, later leaching isaccompanied by more prolonged salt stress on crop growth after theelongation stage (Naresh et al., 1993a, 1993b). For example, in thisstudy, the desalination rate for the 0–40 cm soil horizon was 66%, 76%,
and 77% (Fig. 7), while the number of days with “high salt stress” (HSS)
was 56, 69, and 70 when leaching was applied during the corn elon­
gation (T1), tasseling (T2), and grain-filling (T3) stages, respectively(Fig. 4). However, the highest corn yield and WUE of 11,063 kg ha− 1
and 22.3 kg ha− 1 mm− 1 (an average for two years), respectively, were
found for T1, which represented an increase of 9.8%, 17.5%, 18.5% and10

Y. Zhang et al.

Agricultural Water Management 244 (2021) 106601

Fig. 7. The simulated average ECe (dS m− 1), the duration of "high salt stress" (HSS) (day) (left), the desalination rate (%), and the cumulative salt flux (t ha− 1) at a
depth of 40 (top) and 100 cm (bottom) (right) for scenarios with a field drip-irrigated with brackish water and different timings of LFSI (at 6, 19, 31, 36, 39, 41, 43,
48, 55, 66, 79, 89, 98, and 125 days (symbols) after sowing) during 2018 seasons.

6.7%, 12.1%, 1.4% compared with T2, T3, and T0, respectively. Hence,
the optimal leaching time is crucial for improving the desalination rateand corn yield, and for reducing the duration of salt stress.
The statistical tests showed that there was a high consistency be­
tween measured and simulated values for various treatments during
both calibration and validation periods that meets the requirements ofsoil water and salinity simulations (R2 > 0.88). Hence, numerical sim­
ulations were carried out for scenarios with leaching at 6, 19, 31, 36, 39,
41, 43, 48, 55, 66, 79, 89, 98, and 125 days after sowing (DAS), while
keeping all other conditions the same as above. The results showed thatthe average ECe was not significantly affected by the timing of leaching,
except when no leaching was applied. The average ECe for all leachingtreatments in the 0–40 cm soil layer was 6.2 dS m− 1, which wassignificantly lower than in the T0 scenario with the average ECe of
7.8 dS m− 1. The corresponding values for the 0–100 cm soil layer were
5.5 dS m− 1 (leaching treatments) and 6.9 dS m− 1 (T0) (Fig. 7a,c).
Based on the T1, T2, and T3 scenarios, it was found that earlier
leaching resulted in a shorter duration of HSS. Simulations discussed inthis section showed that the shortest duration of HSS was obtained whenleaching was applied on DAS 41 (Fig. 4). When leaching was applied
either before or after 41 DAS, the duration of HSS always increased. The
duration of HSS for leaching at DAS 41 decreased by 32% and 50% in the
0–40 cm and 0–100 cm soil layers, respectively, compared with noleaching (T0) (Fig. 7a, c). Later leaching always resulted in higher
desalination rates and higher cumulative salt fluxes at both depths of 40
and 100 cm. When leaching was applied after DAS 41, the desalination
rate increased, but the growth would be reduced after DAS 66 (Fig. 7b,d). The optimal irrigation time for leaching is thus between DAS 41-66
when considering both the duration of HSS and the desalination rate.

5. Conclusions
The soil salt dynamics in experimental plots drip-irrigated withbrackish water and LFSI in the corn elongation (T1), tasseling (T2), or
grain-filling (T3) stages, or not leached (T0) was evaluated bothexperimentally and numerically with HYDRUS (2D/3D). The model was
successfully calibrated and validated using observed soil salt concen­
trations from 2017 and 2018, respectively. The accuracy for the vali­
dation period meets standard requirements with RMSE, MAE, MRE, andR2 of 0.02–0.13 mg cm− 3, 0.0465–0.0801 mg cm− 3, 8.7–11.1%, and
0.88–0.92, respectively. The electrical conductivity of a saturated
extract (ECe) in the soil dramatically decreased after LFSI in bothshallow (0–40 cm) and deep (40–100 cm) soil layers. The earlier LFSI
was applied, the shorter duration of “high salt stress” (ECe higher than
5.9 dS m− 1, HSS). The duration of HSS was 42, 56, 57, and 82 days in the
T1, T2, T3, and T0 scenarios, respectively. However, the later LFSI isapplied, the more salts are leached from the root zone. The desaliniza­tion rate was 297%, 410%, and 431% for T1, T2, and T3, respectively.
While significantly more salts could be leached below the 100 cm depth
when LFSI was applied later, the differences in salt leaching below the
40 cm depth were much smaller. Cumulative salt fluxes at the 40 cm
depth for T1, T2, and T3 increased by 36.7%, 66.7%, and 80% comparedto that for T0. The two-dimensional ECe distributions showed that the“salt stress” area in the soil profile after LFSI in the T0 scenario was 4%,
48%, and 92% higher than in the T1, T2, and T3 scenarios, respectively.
In the T0 scenario, the soil salts from the shallow soil layer were onlytransported to deeper soil zone. The HSS area in the soil profile
completely disappeared after LFSI. Additionally, later leaching waspreceded by more salts being accumulated in the soil profile before
leaching. Although later leaching improved the desalination rate, the11

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Agricultural Water Management 244 (2021) 106601

duration of the salt stress was longer. The optimal time for LFSI,
determined based on scenario simulations with HYDRUS (2D/3D), wasabout 41 days after sowing.

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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
This research was jointly supported by the National Key Researchand Development Program of China (2016YFC0400205), National
Natural Science Foundation of China (51969024, 51669020, and51539005), Local Science and Technology Development Fund Projects
Guided by the Central Government (2020ZY0023), and the Major Sci­
ence and Technology Projects of Inner Mongolia (zdzx2018059).
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