Soil & Tillage Research 219 (2022) 105355

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Soil & Tillage Research
journal homepage: www.elsevier.com/locate/still

Optimizing drip irrigation with alternate use of fresh and brackish waters
by analyzing salt stress: The experimental and simulation approaches
Yuehong Zhang a, Xianyue Li a, c, *, Jirí Šimůnek b, Haibin Shi a, Ning Chen a, Qi Hu aaCollege of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, Huhhot 010018, China
Department of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USAc
Collaborative Innovation Center for Integrated Management of Water Resources and Water Environment in the Inner Mongolia Reaches of the Yellow River, Hohhot
010018, Chinab

A R T I C L E I N F O

A B S T R A C T

Keywords:
Alternate drip irrigation
Duration and area of salt stress
HYDRUS (2D/3D)
Crop sensitivity function
Yield

Drip irrigation with alternate use of fresh and brackish waters is an excellent irrigation strategy to overcome salt
stress problems induced by brackish water. Understanding salt stresses in the soil profile and their relationshipwith crop growth is critical when optimizing irrigation strategies with alternate use of fresh and brackish waters.
The HYDRUS (2D/3D) model was calibrated and validated using experimental data collected in 2019 and 2020,
respectively. The calibrated model was then used to evaluate the effects of different irrigation strategies
involving the use of only freshwater (FW) or brackish water (BW), or the alternative use of brackish and fresh
waters (1B1F, one irrigation with brackish water and one with fresh water; 2B1F, two irrigations with brackish
water and one with fresh water; and 3B1F, three irrigations with brackish water and one with fresh water) on thesoil electrical conductivity (ECsw), salt stress, crop growth, and yield. In general, the model performed well, withthe average RMSE, M RE , and R2 for ECsw of 0.12 dS m− 1, 6.85%, and 0.97, respectively. The average ECsw in the
0–40 cm soil layer (the main root zone) showed large differences between different alternate irrigation strategies,
increasing by 55.9% between FW and BW. There was a strong linear relationship between salt stress (ECsw of
3.4–11.8 dS m− 1, SS) in the main corn root zone and salts introduced by irrigation water. The SS duration andarea increased with the frequency of irrigations with brackish water, while the opposite was observed for the
average desalination rate (DR) after irrigation during the entire growth stage. The SS duration was 90, 111, 115,
121, and 121 days for FW, 1B1F, 2B1F, 3B1F, and BW, respectively, while the corresponding values for theduration of high salt stress (ECsw of 7.6–11.8 dS m− 1, HSS) were 4, 24, 32, 37, and 46 days, respectively, and theaverage DRs were 4.3%, 2.4%, 1.9%, 1.7%, and 0.8%, respectively. Average fractions of the SS and HSS areas tothe main root zone area for BW increased by 5.3% and 26.6% compared with FW, respectively. When the HSS
duration increased by 42 days from the FW to BW scenarios, corn yield, plant height, stem diameter, and leaf
area index decreased by 8.8%, 10.4%, 17.7%, and 19.6%, respectively. Additionally, the quantitative relation­
ship between SS and corn yield, i.e., a sensitivity function of the crop yield to soil water and salt stresses, wasproposed based on HYDRUS (2D/3D) simulations. The 2B1F scenario can be recommended as the optimal
strategy for the studied region, using most brackish water with only a 4% decrease in the corn yield.

1. Introduction
Irrigation is critical to guarantee food security in most arid regions
and countries. However, a freshwater shortage is becoming a criticalproblem in most of these regions (Jennifer Clapp, 2017; Dinar et al.,2019; Shilpa et al., 2019). Generally, there is often an abundant amount(about 13.86 million km3 worldwide) of brackish water available, with aconductivity of 2–5 dS m− 1 (McMahon and Price, 2011; Rozema andFlowers, 2008). The evidence shows that freshwater can often be
securely replaced by brackish water in irrigated agriculture in mostwater-deficient countries, e.g., Israel (Kaner et al., 2017), Iran (Noshadi
et al., 2013), Spain (Aparicio et al., 2019), and Mexico (Ozturk et al.,2018), if appropriate irrigation management is used (Xie et al., 2017;
Hassanli et al., 2016; Cucci et al., 2019; Ahdab et al., 2020). In thewestern regions of China, due to severe drought, less rain, and an
extreme shortage of freshwater (Yuan et al., 2019), brackish water has* Corresponding author at: College of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, Huhhot 010018, China.
E-mail address: lixianyue80@126.com (X. Li).
https://doi.org/10.1016/j.still.2022.105355
Received 14 September 2021; Received in revised form 11 February 2022; Accepted 23 February 2022
Available online 28 February 2022
0167-1987/© 2022 Elsevier B.V. All rights reserved.

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Soil & Tillage Research 219 (2022) 105355

Fig. 1. Rainfall, irrigation, and potential evaporation (Ep) and transpiration (Tp) during the corn growth period in 2019 (left) and 2020 (right), respectively.

been used for large-scale agricultural irrigation in an area of about 3
million hectares (Hu et al., 2020; Wang and Qin, 2017; Chen et al.,2010).
However, the use of brackish water often leads to high soil salini­
zation (Zhang et al., 2021). Secondary soil salinization inevitably occurs
when irrigation with brackish water is applied continuously without anyother measures (Bakker et al., 2010; He et al., 2017; Aragüés et al.,2015), resulting in a decrease in soil productivity (Khan et al., 2004) andsustainability (Brouwer et al., 1985). When soil salinity exceeds acertain threshold (Ayers and Westcot, 1976), crop physiological drought
occurs (Li et al., 2015a, 2015b), resulting in a low crop yield (Ben
Ahmed et al., 2012; Yang et al., 2020). Therefore, excess brackish water
has usually been used to leach salts from the root zone, decrease soil
salinity, and prevent salt stress (SS) (Zeng et al., 2014; Libutti andMonteleone, 2012). However, it is impossible to thoroughly leach soil
salts from the soil profile by irrigation with brackish water. Soil salts
usually accumulate at the edge of the main root zone after irrigation
concludes, resulting in crop growth still being restrained by SS (Zhang
et al., 2019a, 2019b; Groenveld et al., 2013).
Drip irrigation under plastic film is used worldwide due to its ad­vantageous agronomic, water conservation, and economic aspects(Lazarovitch et al., 2009; Jia et al., 2020). However, since drip irrigation
applies water at single points in space, salts are frequently pushed to­wards the fringes of the wetting area (Chen et al., 2009) and accumulate
in the root zone. The conjunctive use of brackish water and freshwater
for irrigation, such as blending and alternate irrigation (Dudley et al.,2008; Kulkarni, 2011; Ghermandi et al., 2014), is an important way to
overcome SS problems caused by the exclusive use of brackish water(Rhoades, 1984; Kondash et al., 2020). Some studies have indicated thatthe alternate use of brackish water and freshwater is better thanblending, having high efficiency in decreasing soil salinity andincreasing crop yield (Bradford and Letey, 1992; Naresh et al., 1993a;
Minhas et al., 2007; Li et al., 2019). For instance, when freshwater wasused at the corn jointing stage, the desalination rate (DR) increased by
59% under brackish water drip irrigation (Zhang et al., 2021).
Furthermore, sunflower’s average yield and water productivity
increased by 8.2% and 7.6%, respectively, for alternate irrigation withbrackish and fresh waters compared to irrigation with freshwater only
(Xue and Ren, 2017).
Although soil desalination can be effectively improved by increasing
the amount or frequency of freshwater irrigation (Raij et al., 2016),
freshwater is often a very scarce resource in arid and semi-arid regions(Li et al., 2015). It is thus necessary to limit the use of freshwater as
much as possible without significantly reducing crop growth. Researchneeds to be carried out to reveal the relationship between irrigation, SS,and crop yield. At present, many field experiments have been conducted,
but they are very time-consuming, labor-intensive, and costly (Zhang
et al., 2018). Fortunately, physical and mathematical models canpartially replace experimental work (Liu et al., 2019; Wan et al., 2021; Liet al., 2018). Different irrigation strategies of conjunctive use of brackish
and fresh waters on soil water and salt movement have been successfully

evaluated by many models, such as SWAP (Kroes et al., 2000; Kumar
et al., 2015; Yuan et al., 2019), SALTMED (Karandish and Šimůnek,2019), and HYDRUS (Kanzari et al., 2018; Yang et al., 2019). There also
exist models (i.e., yield reduction models) that directly relate soil
salinity and crop yield (Ayers and Westcot, 1976; van Genuchten and
Hoffman, 1984; Qiu et al., 2017), and CropSyst (Stockle et al., 1994).
These SS crop sensitivity functions indicate that yield decreases (non)linearly with increased salinity and percent water depletion (Waller andYitayew, 2016) and, due to their simplicity and effectiveness, have beenstandard FAO tools (Doorenbos and Kassam, 1979; Allen et al., 1998).
Some comprehensive models, such as DSSAT (Garibay et al., 2019),
SIMDualKc (Anupoju and Kambhammettu, 2020), or AquaCrop(López-Urrea et al., 2020) even directly simulate soil salt dynamics andcrop yield. However, the above-discussed comprehensive models cannot
simulate the alternate use of brackish and fresh waters. Thus, numericalmodels need to distinguish between different irrigation water qualities
and simulate detailed distributions of soil water and salts in the soil
profile and their effects on root water uptake and crop yield.
HYDRUS (2D/3D) is likely the most popular model that can carry out
such simulations due to its comprehensive nature and flexibility in ac­commodating different types of boundary and initial conditions (Phogat
et al., 2012; Šimůnek et al., 2016; Mao et al., 2020). However, opti­
mizing an irrigation strategy of alternate use of brackish and fresh wa­
ters requires the knowledge of SS in the soil profile and the relationshipbetween SS and yield. Currently, studies evaluating alternate drip irri­
gation with brackish and fresh waters are rare, especially those jointly
applying HYDRUS (2D/3D) and water and SS crop sensitivity functions.
Hence, the main objectives of this study are: (1) to calibrate andvalidate HYDRUS (2D/3D) and a crop sensitivity function using
measured soil water contents, soil salinities, and corn yields, (2) toevaluate electrical conductivities and SS in the root zone under alternate
drip irrigation, (3) to analyze the effects of SS on crop growth and yieldunder alternate drip irrigation, and (4) to optimize an alternate drip
irrigation strategy.
2. Material and methods
2.1. Experimental site
The experiments were conducted in 2019 and 2020 at the Jiuzhuang
Water Saving Comprehensive Station in the middle of the Hetao Irri­gation District, in the Yellow River basin of Northwest China (40◦ 41′ N,
107◦ 18′ E). The region has a temperate continental monsoon climate
with annual sunlight of 3230 h and an average air temperature of 6.8 ◦ C.
Mean annual rainfall and open water surface evaporation are 137.6 and2332 mm, respectively. Total 2019 and 2020 rainfalls at the site during
the crop growth were 87.9 and 144.7 mm, respectively (Fig. 1). The
average clay, sand, and silt contents in the top 0–100 cm soil layer are
4.3%, 70.3%, and 25.4%, respectively. The soil type was classified as silt
loam (e.g., United States Department of Agriculture, 2010) with anaverage bulk density of 1.42 g cm− 3 and field capacity (θfc) of 0.41 cm3
2

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Soil & Tillage Research 219 (2022) 105355

Fig. 2. A schematic showing the modeling domain with a dripper and plants in the experimental field, the cropping pattern, the boundary conditions (i.e., 0–0.8 cm,
0.8–30 cm, 30–50 cm were variable flux, no flux, and atmospheric boundary conditions, respectively) and the locations of observation nodes. I, II, III, and IV refer tofour regions used to calculate salt stress.

cm− 3. The salinity conditions were classified as low (Abrol et al., 1988).
The groundwater table depth was 1.3–2.4 m during the experiment,with an average electrical conductivity of 0.96 dS m− 1.

conductivities of fresh and brackish waters were 0.31 and 3.7 dS m− 1,
respectively. The irrigation quota was 22.5 mm per irrigation event, andthe total irrigation amount during the corn growth stage was 270 mm ineach treatment in both 2019 and 2020. Before sowing, based on the soil
analysis recommendation, basal fertilizer with 140 kg ha− 1 of dia­
mmonium phosphate ((NH4)2HPO4, N ≥ 18%), 140 kg ha− 1 of potas­
sium sulfate (K2SO4), and 50 kg ha− 1 of carbamide (CO(NH2)2,
N ≥ 32%) was applied. The solubilized topdressing fertilizer was
carbamide (CO(NH2)2, N ≥ 32%), which was applied during the elon­
gation 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. Experimental design and arrangement
Five irrigation strategies were considered: (a) irrigation with fresh­water only (FW), (b) alternating one irrigation with brackish water and
one with freshwater (1B1F), (c) two irrigations with brackish water and
one with freshwater (2B1F), d) three irrigations with brackish water and
one with freshwater (3B1F), and e) irrigation with brackish water only
(BW). All treatments had the same irrigation frequencies and amounts. A
detailed irrigation schedule is shown in Fig. 1. The experimental scheme
considered three repetitions per treatment, using a complete random­ized block design with 15 field plots. Each plot had a length of 20 m anda width of 7 m.
Corn seeds (Z mays L.) were sown on May 1 and May 4 and harvested
on September 17 and September 25 in 2019 and 2020, respectively. Drip
irrigation under plastic film was performed with a single line of drip tapewith an emitter discharge of 2.4 L h− 1. The plastic film with drip tape
was installed by a specialized machine. The average electrical2.3. Measurements and methods
2.3.1. Measurements
Meteorological data, including air temperature, air humidity, at­mospheric pressure, wind speed, and precipitation, were obtained froman automatic meteorological station (Onset Computer Inc.; U30, Hobo,
USA). Reference crop evapotranspiration (ET0) was calculated using thePenman-Monteith approach (Allen et al., 1998). Potential3

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Soil & Tillage Research 219 (2022) 105355

evapotranspiration (ETp) was obtained by multiplying ET0 with the crop
coefficient (Kc), which was obtained for corn from the FAO paper 56 (i.
e., 0.7, 1.2, and 0.35 during the early, middle, and late growth stages,
respectively) (Allen et al., 1998). Additionally, ETp was divided into
potential evaporation (Ep, mm day− 1) and transpiration (Tp, mm day− 1)
using Beer’s law (Campbell and Norman, 1989):)(
Tp = 1 − e− k⋅LAI ETp(1)
Ep = e− k⋅LAI ETp

desalination rate (DR, %) was calculated as a fraction of the ECsw dif­ference before and after irrigation and the ECsw before irrigation in thezone as follows:
DR =ECb − ECa
× 100
ECb

(3)

where ECb and ECa are ECsw before and after irrigation (dS m− 1) in thezone, respectively.

(2)

2.3.3. Crop sensitivity function to water and salt stress
The sensitivity function of the crop yield to soil water and salt
stresses was described by Doorenbos and Kassam (1979) and Allen et al.
(1998) as follows:)(
Ya = 1 − Ky (1 − Ks ) Ymax
(4)

where k is the extinction coefficient of corn (0.39), and LAI is the leaf
area index.
Soil water contents (SWC) and soil potentials were measured by TDR
probes (IMKO GmbH Inc.; IPH, TRIME-PICO, Germany) and tensiome­ters (Zhonghui Inc.; ZKNT-100, China), respectively, which were
installed under the drip tape and bare area (Fig. 2). SWCs were taken
once every 5–7 days at soil depths of 0–10, 10–30, 30–50, 50–70, and
70–100 cm, while soil potentials were measured only at depths of 90 and
110 cm. Soil sample measurements verified the TDR probe at periodic
intervals. The soil samplings were divided into two groups, one formeasuring SWCs and another for electrical conductivities of the satu­
ration extract (ECe).
ECe were measured once every two weeks at the same locations as
SWCs. ECe values were determined in a laboratory (Rhoades, 1993).
From each sample, approximately 300 g of soil was air-dried and satu­
rated with distilled water for 24 h. A vacuum system extracted soil so­lutions through a No. 1 Whitman paper filter in Buchner funnels.
Ultimately, ECe was measured by a conductivity meter (Shanghai Youke
Instrument and Meter Co., ltd.; DDS-307A, CHN). ECsw is assumed to be
twice as large as ECe (Ayers and Westcot, 1976). The titration method
was used to determine the soil salt concentrations (i.e., contents of Ca2+,
Mg2+, Cl–, SO, HCO, and CO) based on the specification of soil tests,
SL237–1999 (Ministry of Water Resources of the People’s Republic ofChina, 1999).
The position of the groundwater table and its electrical conductivity
were obtained using an automatic water level logger (Onset Computer
Inc.; U20L-01, Hobo, USA) and an automatic water conductivity datalogger (Onset Computer Inc.; U24–001, Hobo, USA), respectively,
installed in an observation well in the field.
Five corn plants from each plot were selected to measure the plantheight, stem diameter, leaf length, and leaf width once every 7–14 days.
The corn leaf area was calculated as 0.75 × leaf length × leaf width, andthe leaf area index (LAI) was calculated using the FAO method (Allenet al., 1998). Ten corn plants from each plot were randomly selected atharvest to measure the grain number per cob and the 100-grain drying
weight. The corn yield for each treatment was obtained from the meanvalue of three replicates (Chen et al., 2021a, 2021b; Mueller and Vyn,
2018).

where Ya is actual corn yield under SS, Ymax is the maximum corn yield
without SS, Ky is the crop sensitivity to water stress (1.25 according toAllen et al., 1998), and Ks is the combined salt and water stress coeffi­
cient, i.e., 0.82–0.78 (Doorenbos and Kassam, 1979; Allen et al., 1998).
2.4. HYDRUS (2D/3D)
2.4.1. Model description
The HYDRUS (2D/3D) model simulates the two-dimensional move­
ment of soil water and salts by solving the Richards (Richards, 1931) andconvection-dispersion equations, respectively. A detailed description
can be found in (Šimůnek et al., 2016).
Root water extraction was computed according to the Feddes model(Feddes et al., 1978; Šimůnek et al., 2018) adapted for two-dimensional
conditions and simultaneous water and salinity stresses:
S(h, hφ ) = α(h, hφ )β(x, z, t)Lt Tp(5)

where α(h, hϕ) is the water and osmotic stress function (-), β(x, z, t) is thenormalized spatial root water uptake distribution (cm− 2), Lt is the widthof the soil surface associated with transpiration (cm), and Tp is the po­tential transpiration rate (cm day− 1). The multiplication model was used
to account for combined water and salinity stresses. The root water
uptake reduction due to water stress was simulated using the piece-wiselinear function of Feddes et al. (1978), with default HYDRUS (2D/3D)
parameters for corn. The root water uptake reduction due to salinity
stress was simulated using the salinity threshold (ECt) and slope function(Maas, 1990). The salinity threshold and slope for corn were set to 3.4 dS
m− 1 and 12%, respectively (Karandish and Šimůnek, 2019).
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):⎧⎪⎨ θr + θs − θrn m h < 0[1 + |αh| ]θ(h) =(6)⎪⎩
θs h ≥ 0

2.3.2. Desalination rate, relative salt stress, and duration
The crop root zone was divided into four regions for exploring salt
stress (SS) and desalinization in more detail. The mulched (0–30 cm)
and non-mulched regions (30–50 cm) were distinguished in the hori­zontal direction, and the main (0–40 cm) and secondary (40–100 cm)
root zones were distinguished in the vertical direction (Fig. 2), resulting
in regions I (top left, 30 ×40 cm2), II (top right, 20 ×40 cm2), III (bot­tom left, 30 ×60 cm2), and Ⅳ (bottom right, 20 × 60 cm2).
In the study, SS was considered to be low, moderate, and high, withan average ECsw in the zone of 3.4–5.0 dS m− 1 (LSS), 5.0–7.6 dS m− 1
(MSS), and 7.6–11.8 dS m− 1 (HSS), which corresponds to about 10%,25%, and 50% corn yield reduction, respectively (Rhoades et al., 1989).
The SS area was calculated using Image J-1.6 (National Institutes of
Health, USA).
The SS duration was defined as the number of days when the average
ECsw in the zone exceeds 3.4 dS m− 1 during the growth period. The

⎡

⎛

⎞m ⎤2

1
θ − θr1⎟ ⎥⎢⎜K(h) = Ks ⋅Sle ⎣1 − ⎝1 − Sem ⎠ ⎦ , m = 1 − , Se =n
θ 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.
2.4.2. Initial and boundary conditions
A rectangular two-dimensional transport domain (50 cm wide and250 cm deep) was defined between the dripper and the middle of the4

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Soil & Tillage Research 219 (2022) 105355

Table 1
The root mean square errors (RMSE), mean relative errors (MRE), and coefficients of determination (R2) for soil water contents (SWC) and soil water electricalconductivities (ECsw) during 2019 (calibration) and 2020 (validation) under alternate drip irrigation strategies with fresh and brackish waters, i.e., scenarios FW, 1B1F,
2B1F, 3B1F, and BW.
Year

2019 (calibration)

Treatment
SWC
ECsw

RMSE (cm3 cm− 3)
MRE (%)
R2
RMSE (dS m− 1)
MRE (%)
R2

2020 (validation)

FW

1B1F

2B1F

3B1F

BW

FW

1B1F

2B1F

3B1F

BW

0.05
8.13
0.93
0.09
7.99
0.96

0.04
7.52
0.95
0.10
6.16
0.95

0.05
7.32
0.92
0.09
5.89
0.96

0.03
7.11
0.90
0.11
5.61
0.96

0.03
8.02
0.94
0.13
7.37
0.98

0.03
9.16
0.95
0.11
8.28
0.97

0.02
8.15
0.94
0.11
6.06
0.97

0.04
9.02
0.94
0.10
5.60
0.98

0.03
8.33
0.95
0.13
6.61
0.97

0.05
8.12
0.97
0.14
7.73
0.98

bare area (Fig. 2). The modeled domain was discretized using the nonuniform triangular finite element mesh generated by the HYDRUS(2D/3D) model. SWCs and ECsw (twice ECe) measured at the beginningof the experiment of each year were used as initial conditions. The top
and bottom SWCs were 0.19 and 0.35 cm3 cm− 3, respectively, and ECsw
were 3.14 and 3.2 dS m− 1, respectively. The no-flux and atmospheric (toapply precipitation/irrigation and potential evaporation Ep) boundary
conditions (BCs) were assigned to the top 0–30 and 30–50 cm bound­aries (Fig. 2), respectively, depending on whether plastic film mulchingat the soil surface was used or not. A 0.8-cm wide variable flux BC was
specified at the left corner of the soil surface to represent the dripper.
The potential transpiration (Tp) flux was specified to account for plantwater uptake. A variable head BC was used at the bottom boundary torepresent the position of the groundwater table. Left and right bound­aries were assigned a no-flux BC. Additionally, a third-type BC was used
to describe ECsw fluxes along the top and bottom boundaries.

1
n

MRE =

n
∑
|Si − Mi |
i=1

Mi

× 100

∑n
i=1 (Mi − M)(Si − S)̅√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
R2 = √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
∑n∑n
i=1 (Mi − M)
i=1 (Si − S)

(9)
(10)

where n is the number of data points, Si and Mi are simulated andmeasured values, respectively, and S and M are the mean values ofsimulated and measured values, respectively.
3. Results
3.1. Calibration and verification
Measured values of SWC and ECsw for different alternate irrigation
strategies at different locations during 2019 and 2020 were used formodel calibration and validation, respectively. Generally, HYDRUS (2D/3D) can well capture SWC and ECsw under alternate irrigation strategies(Table 1). The average RMSE, MRE, and R2 for SWC were 0.02 and0.05 cm3 cm− 3, 7.11% and 9.16%, and 0.90 and 0.95 during calibration(2019) and validation (2020) periods, respectively. Average RMSEs for
the 0–10,10–40, 40–60, and 60–100 cm soil layers were 0.13, 0.11,
0.09, and 0.07 dS m− 1 during the calibration period, and 0.14, 0.11,
0.11, and 0.09 dS m− 1 during the validation period, respectively. The
MREs of ECsw were 3.7–11.5% and 3.2–9.9% during the calibration andvalidation periods, respectively. The coefficients of determination R2 of
ECsw were 0.95–0.98 and 0.97–0.98 during the calibration and valida­tion periods, respectively. Additionally, visual inspection (Fig. 3)
showed that there was also strong consistency between simulated andmeasured ECsw for different alternate irrigation strategies.

2.4.3. Model parameters
Soil hydraulic parameters (i.e., θr, θs, α, n, and Ks) were obtainedusing the soil bulk density and percentage contents of sand, silt, and clay
using the Rosetta module of HYDRUS (2D/3D). These values of soil
hydraulic parameters were further manually calibrated by comparing
simulated and observed values of SWCs and ECsw (Zhang et al., 2021).
The longitudinal dispersivities for the two soil layers were 50 and 28 cm.
The transversal dispersivities for the two soil layers were 4 and 3 cm.
Since molecular diffusion can usually be neglected, the molecular
diffusion coefficient was set to zero (Radcliffe and Šimůnek, 2010).
2.5. Scenario simulation
The soil salinity (ECsw) for different drip irrigation strategies
involving the use of only freshwater (FW) or brackish water (BW), or thealternative use of brackish and fresh waters (1B1F, 2B1F, and 3B1F),
corresponding to the two-year field experiments, was simulated using
the HYDRUS (2D/3D) model. Additionally, ECsw and salt stress wereanalyzed using HYDRUS (2D/3D) for three additional scenarios(without corresponding experiments) involving alternating three irri­
gations with fresh water and one with brackish water (3F1B), two irri­
gations with fresh water and one with brackish water (2F1B), and one
irrigation with fresh water and one with brackish water (1F1B). Mean­
while, corn yields for all eight scenarios were predicted using the crop
yield sensitivity function to soil water and salt stresses.

3.2. Effects of alternate irrigation strategies on ECsw
There were apparent differences between ECsw in the 0–40 cm soil
layer (the main root zone) under different alternate irrigation strategies.
However, only small differences appeared in the 40–100 cm soil layer inthe late crop growth stage (Fig. 3). Also, ECsw fluctuations during theentire crop growth season were quite large in the 0–40 cm soil layers due
to irrigation or precipitation events, especially in the 0–10 cm soil layer.
In general, ECsw increased from sowing to harvest for all alternate irri­
gation strategies, but the differences were quite apparent among
different treatments. For example, the average ECsw in the 0–10 cm soil
layer increased by 0.042, 0.056, 0.060, 0.059, and 0.077 dS m− 1 day− 1
for the FW, 1B1F, 2B1F, 3B1F, and BW strategies in both years,
respectively. The average ECsw in the main root zone for FW, 1B1F,
2B1F, 3B1F was 27.7%, 13.3%, 8.7%, and 6.7% lower, respectively,
than for BW during both years. Additionally, ECsw in the main root zone
significantly decreased after irrigation with freshwater, while an
obvious decrease in ECsw only appeared during the late crop growthstage due to higher ECsw in the 0–40 cm soil layer after irrigation with

2.6. Model performance criteria
The agreement between simulated and measured values of eachtreatment was evaluated using the root mean square error (RMSE), themean relative error (MRE), and the coefficient of determination (R2)
(Gong et al., 2021; Chen et al., 2021a, 2021b):
[
]1/2
n
1∑
RMSE =(Si − Mi )2
(8)
n i=1
5

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Fig. 3. Simulated and measured soil water electrical conductivities (ECsw) under the dripper in the 0–10 (top), 10–40, 40–60, and 60–100 (bottom) cm soil depths in2019 (left) and 2020 (right) under alternate drip irrigation strategies with fresh and brackish waters, i.e., for FW, 1B1F, 2B1F, 3B1F, and BW scenarios.

2020. There was an apparent difference in SS (ECsw of 3.4–11.8 dS m− 1)
among different alternate irrigation strategies after the last irrigation
during 2020. SS increased in response to an increase in the frequency ofbrackish water irrigation (Fig. 4). Also, the SS duration in the 0–40 cm
soil layer (the main root zone) increased with an increase in brackish
water irrigation frequency, being 90, 111, 115, 121, and 121 days forFW, 1B1F, 2B1F, 3B1F, and BW, respectively (Fig. 5). Larger differences
appeared in high salt stress (ECsw of 7.6–11.8 dS m− 1, HSS) and middle
salt stress (ECsw of 5.0–7.6 dS m− 1, MSS). The HSS duration for 1B1F,

brackish water (Fig. 3). Average desalination rates (DR) increased withfrequency of freshwater irrigation, and average DR after 12 irrigation
events were 4.3%, 2.4%, 1.9%, 1.7%, and 0.8% for FW, 1B1F, 2B1F,
3B1F, and BW, respectively.
3.3. The effects of alternate irrigation strategies on salt stress
Only salt stress (SS) in the soil profile during the 2020 season was
analyzed in our study because of the high similarity between 2019 and
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Soil & Tillage Research 219 (2022) 105355

Fig. 4. Simulated two-dimensional distributions of ECsw on DAS 106 (before irrigation, top) and DAS 107 (after irrigation, bottom) in a field irrigated using different
alternate drip irrigation strategies with fresh and brackish waters, including (left to right) FW, 1B1F, 2B1F, 3B1F, and BW.

2B1F, 3B1F, and BW increased by 20, 28, 33, and 42 days compared toFW, respectively, and decreased by 5, 10, 12, and 20 days for MSS.
Additionally, the relationship between the HSS duration and irrigationwater salinity was y = 12.1x-3.7 (R2 =0.99).
SS mainly appeared in the main root zone (top 0–40 cm of themulched region (zone I) and the non-mulched region (zone II)). An
average fraction of the SS area to the main root zone area under FW,
1B1F, 2B1F, 3B1F, and BW was 39.0%, 44.2%, 45.2%, 46.0%, and47.9%, respectively, during the entire crop growth season (Fig. 5). In
zone I, a fraction of the HSS area increased with increasing brackish
water irrigation frequency by 1.2, 1.6, 1.7, and 3.5 for 1B1F, 2B1F,
3B1F, and BW compared with FW. In zone II, it increased by 1.0, 1.4,
1.5, and 2.2, respectively. The MSS area behaved similarly to the HSS
area; its fraction increased by 6.0%, 7.4%, 8.2%, and 10.1% for 1B1F,
2B1F, 3B1F, and BW compared with FW in zone I, respectively, and
1.1%, 1.4%, 2.1%, and 2.5% in zone II. Additionally, only low salt stress(ECsw of 3.4–5.0 dS m− 1, LSS) occurred in zone III (bottom 40–100 cm of
the mulched region) and Ⅳ (bottom 40–100 cm of the non-mulchedregion). Additionally, the relationship between the HSS area and irri­
gation water salinity was y = 0.11x + 0.06 (R2 =0.93).

the SS area and the SS and HSS duration increased by 5.2%, 21, and 28
days, respectively (Fig. 6a, e, i, m).
4. Discussion
4.1. Soil salt and its stress dynamics under different irrigation strategies
Irrigation with brackish water usually temporarily reduces soil
salinity and salt stress (SS) in saline soils (Zhang et al., 2019a, 2019b).
However, SS inevitably reappears under continuous irrigation withbrackish water (Holthusen et al., 2012). The conjunctive use of brackish
and fresh waters for irrigation can effectively reduce or avoid SS(Rhoades, 1984). The salt accumulation after irrigation is mainly related
to irrigation water salinity and irrigation amount. The Food and Agri­culture Organization of the United Nations (FAO) proposed a leachingstrategy based on accumulated scientific knowledge. The FAO manual
defined the minimum leaching fraction as the amount of water, in excess
of consumptive use, necessary to leach accumulated soil salts below theroot zone (Ayers and Westcot, 1976). However, a large amount of irri­
gation water would be required for the proposed leaching strategy. This
strategy would not only waste freshwater resources, but would also not
be able to remove salt stress in time. In fact, a relatively small amount offreshwater can relieve SS, e.g., alternate or blending irrigation strategies
with brackish and fresh waters can effectively control soil salinity andshorten the SS duration (Hassanli and Ebrahimian, 2016; Ramos et al.,2012).
In this study, the average ECsw, a fraction of the SS area to the main
root zone area, and the SS duration decreased by 9.7%, 3.7, and 10 days,
respectively, under an alternate irrigation strategy 1B1F compared withBW (Figs. 3 and 4). However, the alternate (1− 1) irrigation strategy stillcannot save enough freshwater in a water-shortage area. The frequency
of using brackish water for irrigation is thus frequently increased,
inevitably leading to an increase in the soil salinity. For example, theaverage soil salinity in the root zone increased by 45.5% when brackish
water was used during the sixth-leaf to tasseling stages, while freshwater
was used during the remaining growth stages, resulting in significantly
inhibited crop growth (Huang et al., 2019). In this study, average soil
salinity increased by 31.9% when the irrigation frequency with brackish
water increased three times (3B1F).
Therefore, it is necessary to optimize alternate irrigation strategies to

3.4. Response of corn growth and yield to salt stress
The corn height, stem diameter, LAI, and yield decreased with anincrease in the average fraction of the SS area (Fig. 6). When this fraction
increased by 5.2%, 6.1%, 7.0%, and 8.8% for 1B1F, 2B1F, 3B1F, and BW
compared with FW, the corn yield decreased by 2.4%, 3.8%, 4.9%, and18.8%, respectively. Additionally, the corn height, stem diameter, and
LAI decreased by 10.4%, 17.7%, and 19.6%, respectively, for BW
compared to FW. When the average fraction of the HSS area increased by
2.9% compared with FW, the corn height, stem diameter, LAI, and cornyield decreased by 38.5 cm, 0.59 cm, 1.0, and 2.3 t ha− 1, respectively.
Meanwhile, the SS duration also significantly affected corn growth andyield. When the SS duration increased by one day, the corn yield, height,
stem diameter, and leaf area decreased on average by 0.07 t ha− 1,
1.24 cm, 0.02 cm, and 0.03, respectively (Fig. 6). However, when the
HSS duration increased by one day, the corn yield, height, stem diam­eter, and leaf area decreased by 0.05 t ha− 1, 0.92 cm, 0.01 cm, and 0.02,
respectively. Moreover, the corn yield for 2B1F only decreased by 3.8%
compared with that for FW, yet the corresponding average fraction of7

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Soil & Tillage Research 219 (2022) 105355

Fig. 5. Fractions of salt stress regions with low (3.4–5.0 dS m− 1), middle (5.0–7.6 dS m− 1), and high (7.6–11.8 dS m− 1) salt stress in (from top to bottom) zone I (top0–40 cm of the mulched region), zone II (top 0–40 cm of the non-mulched region), zone III (bottom 40–100 cm of the mulched region), and zone Ⅳ (bottom
40–100 cm of the non-mulched region) under alternate drip irrigation strategies with fresh and brackish waters, including (from left to right) FW, 1B1F, 2B1F, 3B1F,
and BW.

reduce the amount of freshwater and decrease SS. In this study, HYDRUS(2D/3D) simulations showed that an increase in the frequency ofbrackish water irrigation resulted in a linear increase in the average
ECsw, SS area, and SS duration (Fig. 7). However, the desalination rate(DR) decreased with an increase in the brackish water irrigation fre­quency. For low, moderate, and high SS levels, the SS duration for 1F1B
was shortened by 7, 2, and 2 days compared to 1B1F, while the averagefraction of the SS area to the main root zone area was reduced by 2.8%,
2.5%, and 14.3%, respectively. When no fresh water was used for irri­
gation (BW), the HSS (7.6–11.8 dS m− 1) duration in the main root zone
during the growth stage reached 20 days (Fig. 7c).

Hoffman, 1977; Munns et al., 2006; Estrada et al., 2013). When average
ECsw in the root zone reaches 5 dS m− 1, the corn yield is reduced by 10%,and no grain harvest is obtained when ECsw is higher than 20 dS m− 1
during the entire crop growing season (Ayers and Westcot, 1976). The
magnitude of the SS area and its duration in the root zone during thecrop growing season directly influence crop yield. For example, when
the soil was leached by surface irrigation with freshwater during thecorn elongation stage, the HSS duration was reduced by about half, andthe corn yield increased by 18.5% compared with no leaching treatment(Zhang et al., 2021). Similar results were obtained in this study when thehigher frequency of freshwater irrigations was accompanied by thelower SS area and the shorter SS duration. For instance, the average SS
area and duration for 1B1F decreased by 11.0% and 8.3% compared toBW, respectively (Fig. 7a, b, c).
It is necessary to develop a relationship between crop yield and SS or
irrigation water salinity to use brackish water for irrigation more

4.2. Effect of soil salinity on crop yield
Root water uptake stress occurs when soil salinity reaches a criticalthreshold due to the very high osmotic potential in the soil (Maas and8

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Soil & Tillage Research 219 (2022) 105355

Fig. 6. The corn yield (a, e, i, m), height (b, f, j, n), stem diameter (c, g, k, o), and leaf area index (d, h, l, p) as a function of an average fraction of the salt stress area
to the main root zone area (A) and its duration (B) under salt stress (3.4–11.8 dS m− 1, the first and third row) and high salt stress (7.6–11.8 dS m− 1, the second andfourth row) (averaged over 2019 and 2020). The FW, 1B1F, 2B1F, 3B1F, and BW strategies are shown in each figure from left to right.

efficiently (Sepaskhah et al., 2006). However, the effects of irrigation
with different alternate uses of fresh and brackish waters on crop yield
are still not clear. In the study, the crop sensitivity functions, i.e., therelationships between crop yield and water and salt stresses, were ob­tained based on the combination of experimental results and HYDRUS(2D/3D) simulations (Waller and Yitayew, 2016). Generally, when thefrequency of irrigations with brackish water increased, the SS area andduration increased, and the yield decreased (Fig. 8). Average values ofECsw for 3F1B, 2F1B, 1F1B, 1B1F, 2B1F, 3B1F, and BW during the
growth period were 6.1, 6.2, 6.4, 6.5, 6.7, 6.8, and 7.2 dS m− 1, which
was about 0.4, 0.5, 0.7, 0.8, 1.0, 1.1, and 1.5 dS m− 1 higher than for FW,
respectively, while the yield decreased by 0.2, 0.4, 0.8, 1.1, 1.3, 1.5, and
2.4 t ha− 1, respectively. For the 1F1B alternate irrigation strategy, theaverage ECsw was 6.4 dS m− 1 during the growth period, the averagefraction of the SS area to the main root zone area was 120.4%, and theyield was reduced by 0.7 t ha− 1 compared with FW. The relationship9

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Soil & Tillage Research 219 (2022) 105355

Fig. 7. Simulated fractions of the salt stress area to the main root area and salt stress duration for SS levels of 3.4–11.8 (a), 5.0–11.8 (b), and 7.6–11.8 dS m− 1 (c).
Simulated average desalination rates (DR) and average ECsw (d). Reported values are for the soil layer of 0–40 and 0–100 cm and for scenarios with different alternate
drip irrigation strategies with fresh and brackish water, including FW, 3F1B, 2F1B, 1F1B, 1B1F, 2B1F, 3B1F, and BW.

Fig. 8. Simulated corn yield as a function of the simulated durations of salt stress (left) and a fraction of the salt stress area to the main root zone area (right) for
different alternate drip irrigation strategies with fresh and brackish waters, including FW, 3F1B, 2F1B, 1F1B, 1B1F, 2B1F, 3B1F, and BW averaged over 2019 and2020 seasons.

between the SS fraction and yield in both years was y = − 0.06x + 19.8
(R2 =0.88), and the relationship between the SS duration and yield was
y = − 0.07x + 19.5 (R2 =0.78).

5. Conclusions
Both experiments and the numerical model were used to evaluate theeffects of different alternate drip irrigation strategies with fresh andbrackish waters on soil salt dynamics. The results showed that simulated
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Soil & Tillage Research 219 (2022) 105355

ECsw was in good agreement with measurements in all irrigation sce­narios. An apparent difference in salt accumulation could be found in
the 0–40 cm soil layer at the end of the growth stage under different
alternate irrigation strategies. The higher the irrigation frequency withbrackish water, the greater the average ECsw, the longer duration of salt
stress (SS), and the larger the SS area. Both field experiments and nu­
merical simulations showed that crop yield decreased linearly with anincrease in the SS duration and area. The scenario with two irrigations
with brackish water and one with fresh water represented the optimal
strategy for the studied region, using the most brackish water with theleast significant yield decrease.

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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 (2021YFC3201202), the Trans­
formation Projects of Scientific and Technological Achievements ofInner Mongolia (2021CG0022), the National Natural Science Founda­tion of China (52079064, 51969024, and 51469022), and the Inner
Mongolia Autonomous Region Graduate Education Innovation Plan
(SZ2020081).
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