Agricultural Water Management 226 (2019) 105788

Contents lists available at ScienceDirect

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

Evaluating the eﬀects of biodegradable ﬁlm mulching on soil water
dynamics in a drip-irrigated ﬁeld

T⁎Ning Chena, Xianyue Lia, , Jirí Šimůnekb, Haibin Shia, Zongjiang Dinga, Zunyuan PengaabCollege 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 LE I N FO

A B S T R A C T

Keywords:
Drip irrigation
Biodegradable ﬁlm mulching
Plastic ﬁlm mulching
Soil water
HYDRUS- 2D

Drip irrigation with plastic ﬁlm mulching (PM) is an important agricultural practice used to conserve soil waterand produce higher crop yields worldwide. However, the plastic ﬁlm is usually made from non-degradable
polyethylene materials, which may lead to the incorporation of plastic into soils and cause a series of negativeenvironmental eﬀects. As such, biodegradable ﬁlm mulching (BM) can represent a valuable alternative to PM. A
two-year experiment was carried out during the 2014 and 2015 seasons to evaluate the eﬀects of PM, BM, and noﬁlm mulching (NM) on the soil water balance in the corn ﬁeld. The two-dimensional numerical HYDRUS-2D
model was calibrated and validated using experimental data collected in 2014 and 2015, respectively, under BM,
PM, and NM and then used to evaluate soil water contents (SWC), water ﬂuxes, and soil water balances in twodimensional soil proﬁles. The results of numerical simulations were in good agreement with measurements, withthe mean relative errors for the calibration and validation periods for BM, PM and NM of 11.2%, 10.7%, and11.7%, respectively. The results showed that SWCs were similar in the BM and PM scenarios during early andmiddle crop growth periods. Signiﬁcant diﬀerences in SWCs were observed only after the average fraction of thedisintegrated area of the biodegradable ﬁlm was equal or higher than about 40% (during the late crop growth
period). The results also showed that mulching mainly aﬀects SWCs in the top 0–20 cm soil layer. There weresigniﬁcant diﬀerences in cumulative evaporation and water ﬂuxes between diﬀerent treatments, but an insigniﬁcant diﬀerence in root water uptake between PM and BM. Cumulative evaporation for BM increased by
30.5% compared to PM and decreased by 41.9% compared to NM. Corresponding cumulative water ﬂuxes at the
80 cm depth increased and decreased by 6.6% and 48.8%, respectively. Two-dimensional simulations of soil
water content distributions revealed that the “water stress” area in the soil proﬁle (with SWCs lower than readily
available water) before irrigation was 1.79 and 0.75 times larger for BM than for PM and NM, respectively. SWCs
in the surface soil layer of BM also increased after rainfall during the late crop growth period, which improved
the rainwater use eﬃciency by 8.2% compared to PM. Both experiments and simulations revealed that BM has
more comparable soil moisture dynamics to PM than to NM.

1. Introduction
Agricultural plastic ﬁlm mulching (PM) can reduce soil evaporation
by cutting oﬀ the vapor transport path between the soil surface and theatmosphere. It has signiﬁcant advantages compared to no mulching(NM), such as decreasing weed growth (Martín et al., 2017), conserving
soil moisture (Kader et al., 2017), stabilizing the daily range of soil
temperatures, increasing the water use eﬃciency (WUE), and improving crop yields (Taparauskiene and Miseckaite, 2014; Yu et al.,2018; Jia et al., 2018). As a result, plastic ﬁlms are widely used in crop
production systems due to their agronomic beneﬁts. For example, in
2012, more than sixty thousand ha of greenhouses in Spain used PM⁎(Department of Agriculture, Food and Environment of Spain, 2012),
and its use was increasing annually by 5.7% (Transparency Market
Research, 2016). In China, the world’s largest plastic ﬁlm user, theamount of applied plastic ﬁlm increased nearly ﬁve-fold from 3 × 105
tons to 14.7 × 105 from 1991 to 2017 (Gao et al., 2019). However,since plastic ﬁlms made of non-degradable polyethylene materials arevery hard to degrade in soil (Ammala et al., 2011; Steinmetz et al.,2016; Bläsing and Amelung, 2018), their use likely leads to a gradualincrease of plastic fragments in soil (Steinmetz et al., 2016; Liu et al.,2014), causing a series of negative environmental eﬀects. These include
lower soil porosity and air circulation, which aﬀects soil water movement and solute transport, as well as reduced crop root growth (Jiang

Corresponding author.
E-mail address: lixianyue80@126.com (X. Li).

https://doi.org/10.1016/j.agwat.2019.105788
Received 23 February 2019; Received in revised form 2 September 2019; Accepted 11 September 20190378-3774/ © 2019 Elsevier B.V. All rights reserved.

Agricultural Water Management 226 (2019) 105788

N. Chen, et al.

the 0–20 cm soil layer for PM and higher soil evaporation (also considering inﬁltration from rainfall inﬁltration) for BM. However, Saglam
et al. (2017) only focused on the accuracy between simulated andmeasured soil water contents and the two-dimensional water content
distributions in the root zone. They did not analyze water ﬂuxes fordiﬀerent boundaries, the soil water balance, or the water use eﬃciency.
They also did not quantify the contribution of shallow groundwater towater use that may reduce the volume of water required for irrigationin arid regions with shallow groundwater (Satchithanantham et al.,2014). Soil water contents, water ﬂuxes, soil water balances, and thecontribution of shallow groundwater should thus be further evaluatedunder BM, PM, and NM in the ﬁeld. Additionally, actual and cumulative
soil water ﬂuxes under diﬀerent fractions of the disintegrated area ofthe biodegradable ﬁlm (AFD) should also be evaluated.
The main objectives of this study thus are (i) to calibrate and validate the HYDRUS-2D model for a corn soil system under PM, BM, andNM, (ii) to evaluate soil water contents and water ﬂuxes under PM, BM,
and NM, and (iii) to compare soil water balances and two-dimensionalsoil water distributions in soil proﬁles under PM, BM, and NM.

et al., 2017), the release of phthalate acid esters and other toxins(Ramos et al., 2015), and lower crop yields.
A promising alternative to PM is biodegradable ﬁlm, mostly made ofpolysaccharides such as cellulose and starch, which can be directly
degraded to CO2 and H2O and that can retain the advantages andovercome the shortcomings of plastic ﬁlms (Kijchavengkul et al.,2008a,b; Gu et al., 2017). Moreno et al. (2016) reported that 200 days
after the incorporation of a biodegradable ﬁlm into the soil, the concentration of its residues was very small, similarly as associated environmental eﬀects. There is also evidence that biodegradable ﬁlm
mulching (BM) provides similar beneﬁts as PM (Costa et al., 2014), andin most studies there were no signiﬁcant diﬀerences in crop yields (Guet al., 2017; Moreno and Moreno, 2008; Costa et al., 2014; Ghimire
et al., 2018).
PM eﬀectively reduces soil evaporation and increases the soil watercontent in the upper soil layer by increasing the resistance to vapor ﬂow
between the soil surface and the atmosphere (Li et al., 2015; Saglam
et al., 2017). For example, soil evaporation declined by as much as 90%
when PM was used (Liu et al., 2013), and it decreased by almost 50%
under pine bark mulching (Anlauf et al., 2016). There is also clear
evidence that higher soil temperatures and soil water storage in thecrop root zone under BM can signiﬁcantly increase crop growth andcrop yield compared to no ﬁlm mulching (NM) (Gu et al., 2017; Yao
et al., 2017; Ghimire et al., 2018). However, while the eﬀects of PM and
BM on increasing soil temperature and soil water storage are similar
early after their application (Kasirajan and Ngouajio, 2012; Yao et al.,2017; Ghimire et al., 2018), they decrease signiﬁcantly after 150 days
due to the degradation of the biodegradable ﬁlm for BM (Gu et al.,2017). The use of diﬀerent mulching technologies leads to diﬀerent
values of soil evaporation, soil water storage, and water ﬂuxes. These
diﬀerences may be further enhanced by the presence of shallow
groundwater, especially in arid regions.
Since the establishment of a reasonable irrigation scheme requires a
precise knowledge of soil water dynamics, many ﬁeld experiments
would need to be conducted, which would be highly time-consuming
and costly. The alternative, low-cost approach would be to simulate theeﬀects of these factors using a mathematical model, which could also
provide additional details about soil water dynamics in the root zone. In
the past decades, several types of mathematical models, such as empirical, analytical, and numerical models, have been used to assess thesoil water pattern under drip irrigation (e.g., Philip, 1968; Warrick,
1974; Schwartzman and Zur, 1986; Cook et al., 2003; Šimůnek et al.,2016). Among these models, HYDRUS-2D has been widely used to simulate soil water movement for diﬀerent irrigation methods andmanagement strategies due to its ﬂexibility in accommodating diﬀerent
types of boundary conditions (e.g., Li et al., 2015; Chen et al., 2018;
Altaf et al., 2014; Šimůnek et al., 2016; Shahrokhnia and Sepaskhah,2018). HYDRUS-2D has been shown to provide a reasonable agreement
between simulated and measured soil water contents and solute concentrations in soil proﬁles with drip irrigation and PM (Selim et al.,2013; Li et al., 2015; Chen et al., 2014). Although most studies represented the mulching boundary as a zero ﬂux plane (Selim et al.,2013; Chen et al., 2018), several others regarded it as a time-variable
ﬂux boundary because a plastic ﬁlm damaged during crop growth allows some water to ﬂow through this boundary (Li et al., 2015; Liuet al., 2013).
The fraction of a disintegrated area is signiﬁcantly higher for abiodegradable ﬁlm than for a plastic ﬁlm during the middle and latecrop growth stages (Moreno et al., 2017). As a result, the original zeroﬂux plane of a mulching boundary vanishes, soil water starts ﬂowing up
from the soil to the atmosphere, and the soil water content in the upper
soil layer decreases compared to PM (Gu et al., 2017). Saglam et al.
(2017) simulated the soil water dynamics using HYDRUS-2D, representing a biodegradable ﬁlm boundary as a partially permeablesurface layer and a plastic ﬁlm boundary as an impermeable surface
layer. Their results showed that there were higher soil water contents in

2. Materials and methods
2.1. Field experiment
The experimental station is located in the western Hetao Irrigation
District, in the Yellow River basin of Northwest China (40°20′15′′N,107°1′45′′E), which is an area with intensive agricultural (mostly cornand sunﬂower) crop production. Mean annual precipitation in the area
is only 102.9 mm, but mean annual evaporation from a free water
surface is up to 2259 mm. The soil type is classiﬁed as sandy loam soil,
the mean bulk density is 1.53 g cm−3 within the top 100-cm soil layer,
the measured ﬁeld capacity θfc and the wilting point θWP are
0.32 cm3 cm-3 and 0.04 cm3 cm-3, respectively, and the groundwatertable was 83–246 cm deep in 2014 and 91–255 cm deep in 2015.
Additionally, the readily available soil water content is 0.19 cm3 cm-3
(Allen et al., 1998).
The experimental design was a complete random design comprisingof three replicates of three treatments, with plastic ﬁlm mulching (PM),biodegradable ﬁlm mulching (BM), and no ﬁlm mulching (NM). The
experimental crop was corn (Zea mays L.). One drip line (with a 30 cm
emitter spacing and an irrigation rate of 2.4 L per hour) was used fortwo corn rows with a 50 cm row spacing. One ﬁlm strip 80 cm wide wasused for two corn rows in order to reduce costs (Fig. 1). Seed corn wassown on April 20 and April 17 and harvested on September 26 and
September 25 in 2014 and 2015, respectively. Drip irrigation was applied at each plot with dimensions of 20 × 30 m. The amount of water
applied to each treatment was the same (8 times about 30 mm, i.e.,
240 mm total) and was measured using water meters (with a precisionof 0.001 m3). Each plot was fertilized with urea (50 kg ha−1),(NH4)2HPO4 (120 kg ha−1), and K2O (120 kg ha−1) as the base fertilizer. Urea (80 kg ha−1) was mixed with irrigation water and applied toeach plot in ﬁve-week intervals starting 30 days after the seedling stage.
2.2. Measurements and methods
Daily meteorological data including solar radiation, air temperature, relative humidity, air pressure, wind speed, and precipitation wereobtained from the automatic meteorological station (Onset Computer
Inc.; U30, Hobo, USA) installed about 500 m away from the experimentﬁeld (Li et al., 2017). Reference crop evapotranspiration (ET0) during
the 2014 and 2015 seasons was calculated using the Penman-Monteithequation (Allen et al., 1998). Potential crop evapotranspiration (ETp)was then calculated as ETp= Kc×ET0 (Fig. 2), where the Kc is the crop
coeﬃcient for corn, which has values of 0.7, 1.2 and 0.35 during theearly, middle, and late growth stages, respectively, based on the FAO56
recommendation (Allen et al., 1998).
2

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the plant height, respectively (Li et al., 2015). The leaf area index (LAI)
was calculated based on the FAO method (Allen et al., 1988) (Fig. 3).
The groundwater table was measured using pressure transduces(Onset Computer Inc.; U20, Hobo, USA), which were installed in an
observation well 3 m deep with a 5 cm internal diameter. Data wereaveraged and recorded at one-day intervals, and stored in a datalogger,from which they were automatically downloaded.
The spatial distribution of corn roots was measured using root
samples collected from a soil transect (Li et al., 2017). Root samples
were scanned using a root system scan (Reagent Instru.; Perfection
4870photo, Epson, Japan) with the 450-dpi resolution on June 7, June
23, July 20, and August 22 in the 2014 season, as well as on June 12,
June 31, July 22, and August 25 in the 2015 season.
Since the fraction of the disintegrated area of the biodegradable ﬁlm(FD) varied in the direction perpendicular to the drip tape, themulching region was divided into three parts R1, R2, and R3 (Fig. 1).
The region around the dripper (R1, 0–10 cm) had maximum disintegration, while the region furthest from the dripper (R3, 25–40 cm)
had least disintegration (Fig. 4). The FD for three diﬀerent regions wascalculated using the following equation:

FD1 =

A1′
× 100%
A1

(1)

FD2 =

A2′
× 100%
A2

(2)

FD3 =

A3′
× 100%
A3(3)

where A1, A2, and A3 are total areas of ﬁlm mulching in diﬀerent regions (cm2) and A1′, A2′, and A3′ are total areas of the disintegrated
biodegradable ﬁlm in diﬀerent regions (cm2). These mulching anddisintegrated areas were measured every 10 days during the crop
growth season while remaining values were obtained by a linear interpolation. Since the widths of three diﬀerent regions are diﬀerent, theaverage fraction of the disintegrated area of the biodegradable ﬁlm(AFD) is calculated using the weighted average as follows:

Fig. 1. Cropping pattern, modeling domain, and boundary conditions for biodegradable ﬁlm mulching (BM), plastic ﬁlm mulching (PM), and no mulching(NM) scenarios.

n

AFD =

Two tube access probes (IMKO GmbH Inc.; IPH, TRIME-PICO,
Germany), based on time domain reﬂectometry for measuring soil
water content (SWC), were installed in each plot in the middle of two
rows of corn under the drip tubing (P1) and in the middle of the baresection (P2) (Fig. 1). The SWC measurements were taken once a week atsoil depths of 0–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm.
The leaf area meter (LI−COR Inc. Li-3000C, LI, USA) and a tape(the accuracy is about 0.1 cm) were used to measure the leaf area and1∑ FDi × wi
L i=1

(4)

where L is the width of the mulching area (cm) (set to be 40 cm) and wi
is the width of diﬀerent regions, which were 10, 15, and 15 cm for R1,
R2, and R3, respectively.
2.3. Model description
2.3.1. Main equations
The HYDRUS-2D model (Šimůnek et al., 2016) was used in thisFig. 2. Rainfall, evapotranspiration, irrigation, and cumulative water inﬂow (irrigation and rainfall) during the 2014 and 2015 growth periods.
3Agricultural Water Management 226 (2019) 105788

N. Chen, et al.

Fig. 3. Corn LAI (a, c) and height (b, d) in plots under plastic ﬁlm mulching (PM), biodegradable ﬁlm mulching (BM), and no ﬁlm mulching (NM) during the 2014 (a,b) and 2015 (c, d) seasons (DOY – Day Of Year).

Fig. 4. The fraction of a disintegrated area of the biodegradable ﬁlm during the 2014 (left) and 2015 (right) growth periods.

study to simulate soil water dynamics in a soil proﬁle under drip irrigation with three diﬀerent mulching treatments. Assuming homogeneous and isotropic soil, HYDRUS-2D uses the Galerkin ﬁnite element
method to numerically solve the governing Richards equation forvariably-saturated water ﬂow (Richards, 1931):

∂θ
∂ ⎡
∂h
∂h
∂K (h)=
K (h) ⎤ + ⎡K (h) ⎤ +− S (h)∂t∂x ⎣∂x ⎦ ⎣
∂z ⎦
∂z

m=1−

(6)

K (θ) = Ks Sel [1 − (1 − Se1/ m)m]2

(7)

Se =

2.3.2. Root water uptake
The root water uptake sink term S is computed according to theFeddes model (Feddes et al., 1978):S (h) = α (h) β (x , z ) Tp Lt

(10)
−1

where Tp is the potential transpiration rate (cm day ), Lt denotes thewidth of the soil surface associated with transpiration (cm), β(x,z) is thenormalized root density for any coordinate in the two − dimensionalsoil domain (cm-2), and α(h) is the root water uptake stress response
function (0 < α < 1). The water stress reduction function is given as
follows (Feddes et al., 1978):⎧ θr +

θ − θrθs − θr−3

where θs is the saturated water content (cm cm ), θr is the residual
water content (cm3 cm−3), α (cm-1), n (-), and m (-) are empiricalparameters, Ks is the saturated hydraulic conductivity (cm day-1), Se is
the eﬀective saturation (-), and l is the pore connectivity parameter (-).

(5)θs − θrh<0(1 + (α |h|)n)m⎨⎩ θs h ≥ 0(9)3

where θ is the volumetric water content of the soil (cm3 cm−3), t is time(day), x and z are the horizontal and vertical coordinates (cm), respectively, h denotes the soil water pressure head (cm), K(h) is theunsaturated hydraulic conductivity (cm day-1), and S is the root water
uptake sink term (day-1). Soil hydraulic properties are described using
the van Genuchten (1980) model as follows:

θ=

1
,n>1
n

⎧ h1 − h h2 < h ≤ h1
⎪ h1 − h2⎪
α (h) = 1h3 ≤ h ≤ h2⎨
⎪ h − h4 h ≤ h < h3⎪ h3 − h4 4
⎩

(8)
4

(11)

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N. Chen, et al.

Fig. 5. The measured average distribution of corn roots during DOY 110–169 (left) and DOY 170–270 (right) in 2014 and 2015.
Table 1
Soil textural properties (measured in the lab) and soil hydraulic parameters ((optimized by HYDRUS) for the top 0–250 cm soil layers.
Soil layers

Soil texture (%)θrBulk Density

(cm)

Sand

Silt

Clay

g/cm

0-30
30-100100-250

55.1
37.9
36.8

41.0
54.5
52.9

3.9
7.6
10.3

1.54
1.52
1.47

3

θs
3

(cm ·cm

−3

)

0.030
0.042
0.047

3

(cm ·cm

−3

)

0.406
0.415
0.409

α

n

Ks

(1/cm)(-)(cm·day−1)

0.01850.00580.0057

1.45
1.63
1.63

55.4
52.0
37.9

Table 2The error analysis of soil water contents for diﬀerent locations (P1, P2) and treatments (PM, BM, and NM).
Depth
(cm)

Error

2014 season (calibration)
PM

0-10

10-20

20-40

40-60

60-100R2
MRE (%)
RMSE(cm3 cm−3)
R2
MRE (%)
RMSE(cm3 cm−3)
R2
MRE (%)
RMSE(cm3 cm−3)
R2
MRE (%)
RMSE(cm3 cm−3)
R2
MRE (%)
RMSE(cm3 cm−3)

2015 season (validation)

BM

NM

PM

BM

NM

P1

P2

P1

P2

P1

P2

P1

P2

P1

P2

P1

P2

0.75
12.7
0.03

0.73
12.8
0.05

0.73
12.2
0.04

0.72
12.3
0.05

0.72
13.4
0.04

0.60
15.6
0.06

0.72
10.7
0.05

0.74
11.4
0.04

0.69
13.8
0.05

0.64
12.9
0.06

0.66
14.2
0.05

0.71
16.1
0.07

0.76
11.9
0.05

0.72
12.1
0.04

0.75
12.4
0.07

0.81
10.8
0.04

0.74
11.5
0.04

0.72
12.7
0.03

0.71
11.0
0.03

0.75
12.9
0.04

0.73
12.9
0.03

0.68
11.9
0.04

0.66
12.4
0.05

0.67
13.1
0.05

0.77
10.9
0.02

0.75
11.7
0.03

0.75
12.7
0.04

0.71
10.2
0.03

0.83
9.63
0.03

0.78
10.8
0.03

0.73
11.8
0.03

0.75
9.92
0.04

0.76
11.2
0.03

0.62
10.4
0.04

0.53
12.4
0.04

0.70
11.5
0.03

0.79
9.44
0.02

0.77
10.9
0.03

0.86
8.57
0.01

0.81
9.18
0.01

0.78
10.8
0.02

0.74
11.3
0.03

0.75
11.0
0.03

0.78
12.0
0.02

0.81
9.00
0.03

0.74
9.64
0.03

0.73
11.4
0.03

0.78
11.0
0.03

0.82
10.0
0.02

0.84
9.06
0.01

0.78
9.34
0.02

0.86
7.81
0.02

0.83
8.42
0.04

0.85
8.80
0.01

0.78
11.12
0.02

0.81
10.20
0.03

0.84
9.86
0.03

0.88
7.70
0.02

0.76
10.52
0.03

0.82
9.36
0.03

PM: plastic ﬁlm mulching; BM: biodegradable ﬁlm mulching; NM: no mulching; P1: the location under the emitter; P2: the location under the bare surface (Fig. 1).

where h1 is the root “anaerobic” point set to -15 cm, h2 and h3 are thepressure heads when optimal root water uptake starts (-30 cm) and ends(−600 cm), respectively, and h4 is the wilting point pressure head set to−8000 cm. The crop-speciﬁc values for corn were taken from the database of HYDRUS-2D (Šimůnek et al., 2016; Li et al., 2015).
The spatial root distribution was averaged during two time intervals(i.e., DOY [Day Of Year] 110–169, 170–270) to reduce the error of notconsidering a dynamic root growth (Fig. 5). We assumed that the root
length was equal to a relative value of 1.0 in the 10 × 10 cm squaredirectly beneath the corn stem and correspondingly adjusted the relative root length density values elsewhere (Šimůnek et al., 2016; Liuet al., 2013) in the HYDRUS-2D transport domain.
2.3.3. Evaporation and transpiration
In HYDRUS-2D, ETp (Fig. 2) needs to be divided into potential
evaporation Ep and potential transpiration Tp (Campbell and Norman,
1989), which can be done as follows:

ETp = Tp + Ep
5(12)

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N. Chen, et al.

Fig. 6. Simulated SWCs at the P1 position in the 0–10 (top), 10–20 (middle), and 20–80 (bottom) cm soil layers in 2014 (left) and 2015 (right) for plastic ﬁlm
mulching (PM), biodegradable ﬁlm mulching (BM), and no ﬁlm mulching (NM). θfc – ﬁeld capacity, θRAW – readily available water.

Tp = (1 − e−k ⋅ LAI ) ETp(13)

Ep = ETp − Tp

(14)

Ed = Ep × FD − R(15)where Ed is the surface ﬂux (mm d−1), Ep is potential evaporation (mmd−1), and R is the rainfall rate (mm d−1).
In the PM scenario, the mulching soil surface (0–40 cm) was assigned the "No-Flow" boundary condition. In both of these scenarios(BM and PM), the bare section of the soil surface (P2) was assignedthe"Atmospheric" boundary condition. In the NM scenario, the entire
soil surface (0–60 cm) was assigned the "Atmospheric" boundary condition.
The initial conditions were set using water contents measured before sowing of corn and interpolating linearly between locations withmeasured values. The top and bottom water contents were 0.20 and
0.35 cm3 cm−3, respectively. The horizontal distribution of the initialwater content was assumed to be uniform.

where k is the extinction coeﬃcient of the crop canopy and LAI is theleaf area index (Fig. 3).
2.3.4. Initial and boundary conditions
The depth of the modeling region was set to 250 cm since thegroundwater table ﬂuctuated within the range of 87–251 cm. The timevariable pressure head boundary condition was speciﬁed at the bottomof the ﬂow domain. The left and right vertical sides of the ﬂow domain
were assigned the no-ﬂow boundary condition. The top boundary (i.e.,the soil surface) reﬂected three diﬀerent mulching scenarios. In the BM
scenario, the mulching soil surface (0–40 cm) was divided into threeparts (Fig. 1) to represent an uneven disintegration of BM: the soil
surface up to 10 cm away from the emitter (R1) was assigned the "TimeVariable Flux 1″ boundary condition, the 10–25 cm soil surface (R2)
was assigned the "Time-Variable Flux 2″ boundary condition, and the
25–40 cm soil surface (R3) was assigned the "Time-Variable Flux 3″boundary condition. Assuming rainfall was not evaporated, the surface
ﬂux (i.e., the diﬀerence between potential evaporation and rainfall) is
adjusted depending on the rate of disintegration of the surface mulch indiﬀerent degradation regions (i.e., R1, R2, and R3) as follows:

2.3.5. Soil hydraulic parameters and emitter ﬂow
Soil hydraulic parameters for diﬀerent soil layers were ﬁrst estimated using the Rosetta software package (Schaap et al., 2001) and its
neural-network based pedotransfer functions from percentages of sand,silt, and clay and the soil bulk density (Table 1). The α, n and Ks
parameters were be further calibrated using the inverse approach in
HYDRUS-2D (Lazarovitch et al., 2007).
The emitter ﬂux in HYDRUS-2D was calculated using the following
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evaluated using the mean relative error (MRE), the root mean squareerror (RMSE), and the coeﬃcient of determination (R2) as follows:
n

MRE =

1
S −M
∑ i M i × 100%
n i=1
i

(17)

n

1
∑ (Si − Mi)2
n i=1

RMSE =(18)n

R2 = 1 −∑i = 1 (Si − Mi )2n¯ )2∑ (Mi − M
i=1

(19)

where Si and Mi are simulated and measured values, respectively, and n
is the number of data points.
3. Results
3.1. Model calibration and validation
The values of R2, RMSE, and MRE for volumetric soil water contents(SWCs) are shown in Table 2 for the BM, PM, and NM scenarios for both
calibration (2014) and validation (2015) periods. The average R2 forthe calibration and validation periods for the BM, PM, and NM scenarios were 0.76, 0.75, and 0.73, respectively; the average RMSE for thecalibration and validation periods for the BM, PM, and NM scenarios
were 0.03, 0.03, and 0.04 cm3 cm−3, respectively; and the average MRE
for the calibration and validation periods for the BM, PM, and NM
scenarios were 11.2%, 10.7%, and 11.7%, respectively. The model accuracy for BM was lower than for PM, and it decreased on average by
1.3%, 9.5%, and 4.7% for R2, RMSE, and MRE, respectively. However,
the error measures showed that, in general, there was a high consistency between measured and simulated SWCs for BM, PM and NM
during both calibration and validation periods that meet the requirements of soil water simulations. Since SWCs in the 0–20 cm soil layer
were more dynamic and greatly aﬀected by irrigation and precipitation,modeling errors increased and the model accuracy decreased in this soil
layer (and even more in the 0–10 cm soil layer) compared to the subsoil
layers. The average R2, RMSE, and MRE values were 0.71,
0.05 cm3 cm−3, and 12.65%, respectively, for the top 0–20 cm soil
layer, and 0.78, 0.03 cm3 cm−3, and 10.27%, respectively, for the
20–100 cm soil layer. It can thus be concluded that the HYDRUS-2D
model performed well in simulating SWCs in the root zone of the soil
proﬁle under all mulching scenarios.
3.2. Comparison of SWCs between diﬀerent ﬁlm mulching scenarios
SWCs at the P1 position were selected for further analysis as they
were more dynamic and more aﬀected by irrigation and mulching thanat the P2 position. The entire crop growing season was divided into
three time periods to diﬀerentiate the eﬀects of diﬀerent degrees of
disintegration of the biodegradable ﬁlm: an early period with no difference between BM and PM (DOY 110–165 in 2014 and 113–176 in2015 year), a middle period with a small diﬀerence (DOY 166–234 in2014 and 177–238 in 2015), and a late period with a signiﬁcant difference (DOY 235–270 in 2014 and 239–270 in 2015). There were
insigniﬁcant diﬀerences in measured SWCs between BM and PM during
the early period without ﬁlm disintegration (Fig. 6). SWCs in the
0–20 cm soil layer were by about 0.9 and 0.6% lower under BM thanunder PM during the middle period in 2014 and 2015, respectively.
However, they were 2.6 and 5.2% higher, respectively, than under NM.
During the late period with signiﬁcant diﬀerences between BM and PM,
SWCs decreased by 7.8 and 6.2% under BM compared to PM during
2014 and 2015, respectively, while they increased by 12.5 and 15.3%,
respectively, compared to NM. These signiﬁcant diﬀerences in SWCs
were caused by signiﬁcant disintegration of the biodegradable ﬁlm,
which disintegrated by 39.7 and 37.1% in 2014 and 2015, respectively,

Fig. 7. Cumulative soil water ﬂuxes at depths of 20 (a, b) and 80 (c, d) cm,
cumulative surface water ﬂuxes (e, f), and cumulative root water uptake (g, h)
in 2014 (left) and 2015 (right) under plastic ﬁlm mulching (PM), biodegradable
ﬁlm mulching (BM), and no ﬁlm mulching (NM).

equation (Skaggs et al., 2004):

q=

Q
L × 2πr(16)

where q is the emitter ﬂux (15.9 cm h−1), Q is the emitter discharge(2.4 L h−1), L is the distance between emitters (30 cm), and r is theradius of the emitter (0.8 cm).
2.3.6. Evaluation of model performance
The agreement between simulated and measured values was
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Table 3
Simulated cumulative water balance components under PM, BM, and NM during 2014 and 2015 growth period.
Year

2014

2015

Treatment

Irrigationmm

Rainfallmm

Root water uptakemm

Evaporationmm

Drainage (capillary rise)mm

Change in soil water storage
mm

PM
BM
NM
PM
BM
NM

270
270
270
270
270
270

61.8
61.8
61.8
40.0
40.0
40.0

392.4
385.6
370.9
386.3
382.9
366.7

55.8
72.3
126.9
58.7
77.1
130.3

−52.3
−55.6
−89.7
−64.1
−68.5
−94.9

−67.1
−69.7
−75.4
−71.9
−76.7
−84.6

PM: plastic ﬁlm mulching; BM: biodegradable plastic ﬁlm mulching; NM: no ﬁlm mulching.

was only about 6.6% higher than for PM because larger disintegrationof the biodegradable ﬁlm occurred only at the end of the crop growing
period. Average upward groundwater ﬂuxes for PM, BM, and NM accounted for 13.0, 13.5, and 18.6% of water losses (i.e., root water uptake and evaporation), respectively. While there was only a small difference in root water uptake between PM and BM, there was asigniﬁcant diﬀerence between NM and the other two mulching treatments. The diﬀerence between the maximum (PM) and minimum (NM)root water uptake was 20.5 mm. While the change in the soil water
storage at the end of the season increased with the degree of the active
mulching area (Table 3), there was only a small diﬀerence between BM
and PM. As a whole, water balance components were not signiﬁcantly
diﬀerent between BM and PM, indicating that the biodegradable ﬁlm
can be used to replace the plastic ﬁlm in semi-arid regions (e.g., in the
Hetao irrigation district).

during the late period. Overall, there were statistically signiﬁcant differences in SWCs between PM and BM during about 36 and 32 days of2014 and 2015, respectively. Overall, while there was no statisticallysigniﬁcant diﬀerence between average SWCs under BM and PM during
the entire growing period, they were both signiﬁcantly higher thanunder NM. The average SWCs in the 0–20 cm soil layer during 2 years
were 0.254, 0.253, and 0.245 cm3 cm−3 under PM, BM, and NM, respectively. It is apparent that mulching mainly aﬀects SWCs in the
0–20 cm soil layer (Fig. 6). As discussed above, SWCs under BM were
almost the same as under PM until a signiﬁcant disintegration of thebiodegradable ﬁlm occurred. There was a statistically insigniﬁcantdiﬀerence between SWCs under BM and PM throughout the growing
period, but both SWCs were statistically signiﬁcantly higher than underNM.
3.3. Simulated soil water ﬂuxes

3.5. Eﬀects of irrigation and rainfall on two-dimensional soil water content
distributions

Since roots were predominantly present in the 0–20 cm surface soil
layer (the maximum rooting depth was about 80 cm) (Fig. 5), root
water uptake occurred mainly from this soil layer and signiﬁcantly affected water ﬂuxes from/to deeper soil layers. The average cumulativeﬂux (positive upward) at the 20-cm depth (CF20) during 2014 and2015 under BM was 1.72 times larger than under PM and 1.31 times
smaller than under NM (P < 0.05), respectively (Fig. 7a and b). The
average CF20 was 12.43 mm higher under BM than under PM. The
average cumulative ﬂux at the 80-cm depth (CF80) increased 6.62%
under BM compared to PM. However, the maximum cumulative ﬂux of
92.3 mm occurred under NM and was 58.6% and 48.8% higher thanunder PM and BM, respectively (Fig. 7c and d). Overall, the CF20 and
CF80 ﬂuxes were mainly upward, representing a capillary rise fromgroundwater. The CF20 ﬂux accounted for 30.0, 32.8, and 41.2% of the
CF80 ﬂux under PM, BM, and NM, respectively.
As the biodegradable ﬁlm disintegrated, evaporation signiﬁcantly
increased (Fig. 7e and f). The average cumulative surface ﬂux during
the growing seasons of 2014 and 2015 for PM, BM, and NM were 41.0,
54.9, and 106.6 mm, respectively. The average cumulative surface ﬂux
for BM was 33.8% higher than for PM and 48.5% lower than for NM.
Average root water uptake in 2014 and 2015 (Fig. 7g and h) under PM
and BM were not signiﬁcantly diﬀerent (P > 0.05), but increased by
5.57 and 4.19% compared to NM, respectively.

Since the eﬀects of irrigation and rainfall on soil water distributions
were similar during 2014 and 2015, only the results for 2014 will be
discussed below. Two-dimensional soil water content distributions in
the root zone (0–80 cm) for PM, BM, and NM on DOY 188 (i.e., before
irrigation), 190 (i.e., one day after irrigation), and 192 (i.e., one day
after rainfall) of 2014 are shown in Fig. 8. SWC proﬁles for diﬀerent
mulching scenarios are diﬀerent mainly in the top 0–30 cm soil layer,
especially in the 0–20 cm layer (Fig. 8). The maximum diﬀerence in
SWCs occurred before irrigation (Fig. 8), when the “water stress” area,i.e., locations with SWCs lower than readily available water(θRAW = 0.19 cm3 cm−3 in this study) (Allen et al., 1998), was about
304, 544, and 729 cm2 for PM, BM, and NM, respectively. The water
stress area was about 1.79 and 0.75 times larger for BM than for PM andNM, respectively. In addition, θRAW was reached at depths of 18.3, 27.4,
and 29.5 cm for PM, BM, and NM, respectively. SWCs increased in the
0–20 cm soil layer 1 day after irrigation, and the “wetting area” (i.e.,
SWCs equal to or larger than ﬁeld capacity) was 120.9, 112.8, and 64.0
cm2 for PM, BM, and NM, respectively. Diﬀerences in wetting areas ofdiﬀerent scenarios were caused mainly by diﬀerences in pre-irrigation
saturations.
Because rainwater could inﬁltrate directly into the soil in the NM
scenario and at locations of ﬁlm disintegration in the BM scenario, the
SWC in these two scenarios were higher than in the PM scenario one
day after rainfall (i.e., DOY 192) (Fig. 8). The SWC in the 0–20 cm soil
layer three days after rainfall increased by 12.0% and 8.1% for BM
compared to PM in 2014 and 2015, respectively (Fig. 6). The BM scenario can thus increase rainwater use eﬃciency (RUE) compared to PM.
For example, RUE for BM improved on average by 8.2% compared to
PM due to the disintegration of the ﬁlm. In short, there was no SWC
deﬁcit for BM compared to PM and NM (Fig. 8) because of decreasing
evaporation and increasing RUE.

3.4. Eﬀects of diﬀerent ﬁlm mulching on soil water balance components
Simulated soil water balance components for the root zone
(0–80 cm depth) for all mulching scenarios are shown in Table 3.
Mulching signiﬁcantly aﬀected evaporation. Average evaporation
during 2 years for BM increased by 30.5% (17.5 mm) compared to PM
but decreased by 41.9% (53.9 mm) compared to NM. Diﬀerent types ofmulching signiﬁcantly decreased soil evaporation, and consequently
also aﬀected (i.e., lowered) capillary rise from shallow groundwater(Fig. 7). Average capillary rise (i.e., negative drainage) for NM increased by 58.6 and 48.8% compared to PM and BM during two
growing seasons (Table 3). However, upward groundwater ﬂux for BM
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Fig. 8. Two-dimension soil water distributions under plastic ﬁlm mulching (PM) (left), biodegradable ﬁlm mulching (BM) (middle), and no ﬁlm mulching (NM)(right) in the 0–80 cm soil layer at DOY 188 d (top), 190 d (middle), and 192 d (bottom).

9

Agricultural Water Management 226 (2019) 105788

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Fig. 9. Average SWCs (white, gray, and black bars) in the
top 0–20 cm soil layer during the entire growing season(left) and during the late crop growth period (right) for
plastic ﬁlm mulching (PM), biodegradable ﬁlm mulching(BM), and no ﬁlm mulching (NM) treatments, respectively.
Values for BM are shown for regions with low, medium,and high mulch disintegration with AFDs of 38.1, 54.4, and70.7%, respectively. Also shown (the right axis) are differences in SWCs (MRE, %; lines) between diﬀerent treatments (e.g., PM-BM, PM-NM, and BM-NM). Note that vertical scales are diﬀerent in left and right ﬁgures.

treatment (Fig. 9). However, SWCs for the BM treatment under the
medium and high AFD decreased by 3.5% and 6.3% compared to PM,
respectively. The SWC decreased linearly with increasing AFD (y=0.0071x+0.2616). In the late crop growth period, SWCs for low,
medium, and high AFD were lower by 6.5, 100 and 13.5% than for PM,
respectively (Fig. 9).

Table 4
Water balance components for low, medium, and high (38.1%, 54.4%, and70.7%) average fractions of the disintegrated biodegradable ﬁlm (AFD).
Component
AFD

Drainagemm

Evaporationmm

Root water uptakemm

Low
Medium
High

−53.7
−55.6
−70.8

62.3
74.7
92.4

389.7
385.6
381.2

4.2. Evaluation of cumulative soil water ﬂuxes
The extent of mulching has a negative relationship with evaporation(Fig. 7), i.e., evaporation from the BM system is higher than from thePM system, and the NM system has signiﬁcantly higher evaporation
than the mulching scenarios (BM and PM) (Han et al., 2015). Soil water
losses are reduced by mulching especially in semi-arid regions withintensive evaporation (Zhang et al., 2018). In addition, diﬀerent irrigation patterns and mulching materials also aﬀect soil evaporation. For
example, evaporation from soils with large frequent irrigations issmaller than from soils with small frequent irrigations (He et al., 2018).
In addition, Dlamini et al. (2017) found that in semiarid areas mulchingwith sand, similarly as with PM and BM, can also signiﬁcantly reduce
evaporation in comparison to no mulching (Table 3).
Corn roots (about 66–70%) were mainly distributed in the shallow
soil layer (0–30 cm), which was the main crop root water uptake layer(Li et al., 2017). This layer can be easily aﬀected by evaporation, irrigation, and rainfall. He et al. (2018) reported that seasonal corn root
water uptake ranged from 348 to 394 mm under diﬀerent soil waterconditions, which was similar to this study (Table 4). Due to theshallow groundwater table in the Hetao irrigation district, groundwater
contributed signiﬁcantly to crop root water uptake (e.g., Forkutsa et al.,2009; Liu et al., 2016; Ayars et al., 2006). Forkutsa et al. (2009) indicated that about 79–106 mm of crop root water uptake came fromgroundwater under the sandy loam soil. Liu et al. (2016) indicated thatthe groundwater contribution reached about 20–40% of crop water
consumption. Ayars et al. (2006) reported that groundwater contributed about 0–29% in the silt loam soil, while in this study it only
contributed about 13–19%.
The upward groundwater ﬂux and evaporation were directly related
to AFD (Table 4). The groundwater ﬂux for BM under high AFD was
70.8 mm, which was 1.32 and 1.27 times higher than under low andmedium AFD, respectively. Evaporation under high AFD increased by
30.1 and 17.7 mm compared to under low and medium AFD, respectively. The groundwater ﬂux and evaporation increased by 1.05 mm
and 2.46 mm, respectively, when 1 cm2 of plastic ﬁlm disintegrated.
The contribution of groundwater increased with AFD according to
y = 0.16x+24.43 (where x is AFD in cm2 and y is the groundwater
uptake amount in mm). However, root water uptake was only slightly
aﬀected by AFD. Root water uptake values decreased only slightly from
low to medium and high AFDs. The maximum diﬀerence in cumulativeroot water uptake was reached between low and high AFD and was only
8.5 mm.

4. Discussion
4.1. The eﬀects of BM on soil water
The average fraction of the disintegrated biodegradable ﬁlm (AFD)
reached 39.7% and 37.1% during the later stages of the crop growing
season in 2014 and 2015, respectively, which was much higher than forthe plastic ﬁlm. These results are similar to those of Saglam et al.(2017)
and explain diﬀerences in the spatial and temporal distributions of
SWCs between BM and PM. There were no signiﬁcant diﬀerences in
SWC proﬁles between BM and PM (Fig. 6) early in the season before thebiodegradable ﬁlm started disintegrating, Signiﬁcant diﬀerences onlyoccurred late in the growing season when AFD reached 38.4%(Fig. 4). SWCs for BM were usually lower than for PM (Sun et al., 2018),
especially when irrigation and rainfall events did not occur 6–7 days
prior (Khan et al., 2016). In this study, SWCs for BM were signiﬁcantly
lower than for PM during some irrigation intervals (Fig. 6), which issimilar to the results of Khan et al. (2016). Furthermore, not only did
BM eﬀectively decrease evaporation losses compared to NM, but it also
produce higher SWCs in the root zone after rainfalls during the disintegration season (Fig. 7, h). As the mulched area decreased with increasing AFD during the late stages of the growing season for BM,
rainwater could more easily inﬁltrate into the soil, increase SWCs, andeﬀectively improve RUE compared to PM, which had little ﬁlm disintegration (Saglam et al., 2017; Sun et al., 2018). There were obvious
diﬀerences in SWCs between BM and PM because of BM disintegration,
especially in the 0–20 cm soil layer and in time periods after irrigation
or rainfall (Figs. 7 and 8). The main reason was evaporation and soil
water loss from the surface soil layer. The eﬀect of mulching on SWCs
was usually only found in the top 0–20 cm soil layer (Fig. 6), which was
similar to the results of Selim et al. (2013). Gao et al. (2014) also indicated that signiﬁcant diﬀerences in SWCs occurred only in the top
0–20 cm soil layer between mulching and no mulching locations andthat mulching could increase SWCs by 15% compared to no mulching.
As discussed above, diﬀerences in the active (not disintegrated)mulching area of the biodegradable ﬁlm caused diﬀerent SWC distributions in the soil proﬁle (similarly as in Saglam et al., 2017). The
averaged measured fraction of the disintegrated area of the biodegradable ﬁlm (AFD) at the very last day of the season was 54.4%, while
the low and high AFD values were either lower or higher by 30%
compared to the mean AFD value. The SWC in the 0–20 cm soil layer ofthe BM treatment with low AFD was almost the same as for the PM
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5. Conclusions

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Numerical modeling based on HYDRUS-2D was used in this study toevaluate the eﬀects of BM on SWCs and water ﬂuxes. The model was
ﬁrst calibrated and validated using experimental data from two seasonsof 2014 and 2015. In general, there was good agreement between the
results of numerical simulations and measurements. The beneﬁts of BM
were similar to those of PM. Both mulching scenarios (BM and PM)
functioned similarly during the early growing season before ﬁlm disintegration. Signiﬁcant diﬀerences occurred only in the late growing
season when AFD was equal to or higher than 38.4%. Additionally,
diﬀerences in SWCs were only found in the top 0–20 cm soil layer.
Average cumulative evaporation and average cumulative ﬂuxes at
depths of 20 and 80 cm were signiﬁcantly diﬀerent for diﬀerent
mulching scenarios, increasing from PM to BM, and further to NM(P < 0.05). Two-dimensional soil water content distributions showedthat before irrigation the “water stress” area in the surface soil layer ofthe BM soil was larger than in the PM soil and signiﬁcantly lower thanin the NM soil. SWCs in the root zone after rainfall would increase in the
BM system during the disintegration season and eﬀectively improve
RUE. The eﬀects of ﬁlm disintegration on SWCs and water ﬂuxes wereeﬀectively simulated using HYDRUS-2D. BM is recommended as a viable option to replace PM in regions with shallow groundwater.
Acknowledgements
This research was jointly supported by the National Key Researchand Development Program of China (2016YFC0400205), National
Natural Science Foundation of China (51669020, 51469022, and51539005), and the Natural Sciences Foundation of Inner Mongolia(2016JQ06). Additionally, we are sincerely appreciate the supportionof Inner Mongolia autonomous region graduate education innovation
plan subsidize project and the careful and precise reviews by the
anonymous reviewers, editors.
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