Journal of Hydrology 584 (2020) 124754

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Journal of Hydrology
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Research papers

Eﬀects of large macropores on soil evaporation in salt marshes
Tingzhang Zhoua,⁎

, Pei Xin , Ling Li , D.A. Barry , Jirka Šimůnek

a,b

c

d

T

b

a

State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, China
Department of Environmental Sciences, University of California Riverside, Riverside, USA
School of Engineering, Westlake University, Hangzhou, ChinadLaboratoire de technologie écologique (ECOL), Institut d’ingénierie de l’environnement (IIE), Faculté de l’environnement naturel, architectural et construit (ENAC), Ecole
Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerlandbc

A R T I C LE I N FO

A B S T R A C T

This manuscript was handled by P. Kitanidis,
Editor-in-Chief, with the assistance of Martin
Thullner, Associate Editor

The occurrence of macropores in salt marsh sediments is a natural and ubiquitous phenomenon. Although they
are widely assumed to aﬀect pore-water ﬂow in salt marshes signiﬁcantly, the mechanisms involved and theirextent are not well understood. We conducted laboratory experiments and numerical simulations to examine theeﬀect of macropores on soil evaporation. Soil columns packed with either sand or clay and with or without
macropores were set up with watertables in the columns set at diﬀerent levels. A high potential evaporation rate
was induced by infrared light and a fan. The results showed that in the soil with a low saturated hydraulicconductivity (and thus a low water transport capacity), the macropore behaved as a preferential ﬂow path forgroundwater to recharge the surrounding soil during evaporation. The evaporated water originated largely fromthe macropore rather than the soil matrix, maintaining a high evaporation rate in comparison with a homogeneous soil. This eﬀect was more pronounced for sediments with lower hydraulic conductivities and shallower
watertables. These results improve our understanding of water ﬂow and evaporation in salt marshes with
continuous macropores between the soil surface and groundwater.

Keywords:
Salt marsh
Soil evaporation
Macropores
Preferential ﬂow
HYDRUS

1. Introduction
Salt marshes, a type of wetland with herbaceous vegetation(Fig. 1a), are found globally along shorelines (Pendleton et al., 2012).
As one of the most productive ecosystems, salt marshes maintaincoastal biodiversity and serve as essential habitats for many species ofintertidal fauna and ﬂora (Adams, 1963; Moﬀett et al., 2012; Morris,
1995; Wiegert and Freeman, 1990). They also play essential roles inshoreline protection, ﬁshery support, water quality improvement andcarbon sequestration (Pendleton et al., 2012). However, almost half ofglobal salt marshes were lost in the last century (Kennish, 2001),
mainly due to land reclamation and sea-level rise (Fagherazzi et al.,2012; Woodworth, 2010). Furthermore, existing marshes are threatened by severe degradation (Balke et al., 2016). Therefore, understanding the behavior of salt marshes subjected to various forcing factors is vital for protecting coastal eco-environments.
Salt marshes are commonly found in the upper part of intertidal
zones. Tidal creeks provide pathways for water, solute and energy
transport (Wilson and Gardner, 2006; Xin et al., 2011). The amounts ofwater and solute (e.g., salt) in a creek-marsh system are aﬀected bytides, evapotranspiration (including evaporation and plant root uptake)⁎and rainfall (Moﬀett et al., 2012; Morris, 1995; Xin et al., 2011, 2017)(Fig. 1a). Water exchange among surface water, groundwater, and theatmosphere aﬀect soil conditions (e.g., soil aeration) and solute transport (e.g., salt accumulation) in salt marshes (Xin et al., 2017). Understanding this exchange is crucial to answering two important scientiﬁc questions related to marsh ecology: plant zonation (i.e., plant
distribution in an organized fashion with distinct spatial patterns) andnutrient outwelling (i.e., marshes export nutrients to coastal water)(Chapman, 1960; Teal, 1962).
Recently, intensive studies were conducted to examine surface
water and groundwater interactions in creek-marsh systems subjected
to tides (Adams, 1963; Cao et al., 2012; Moﬀett et al., 2012; Shen et al.,2018; Ursino et al., 2004; Wilson and Gardner, 2006; Xiao et al., 2017;
Xin et al., 2013). They demonstrated a near-creek pore-water circulation, in which water inﬁltrates from the marsh platform during ﬂood
tides and seeps out of the creek bank and bottom during ebb tides (Caoet al., 2012; Wilson and Gardner, 2006; Xin et al., 2011). This circulation aﬀects solute exchange between the marsh and adjacent sea andregulates soil conditions that determine plant growth and marsh functions (Adams, 1963; Colmer and Flowers, 2008; Dacey and Howes,
1984; Marani et al., 2006; Mendelssohn et al., 1981; Wilson et al., 2015;

Corresponding author.
E-mail address: pei.xin@outlook.com (P. Xin).

https://doi.org/10.1016/j.jhydrol.2020.124754
Received 4 December 2019; Received in revised form 13 February 2020; Accepted 21 February 2020
Available online 24 February 20200022-1694/ © 2020 Elsevier B.V. All rights reserved.

Journal of Hydrology 584 (2020) 124754

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aeration and enhance salt transport in salt marshes, which may favor
plant growth (Colmer and Flowers, 2008; Marani et al., 2006;
Mendelssohn et al., 1981).
This study is motivated by a ﬁeld study on a salt marsh in the
Yancheng coast (32°43′−34°28′ N, 119°48′−121°15′ E), Jiangsu
Province, China. As the typical marsh soil stratigraphy (Carol et al.,2011; Dolphin et al., 1995; Gardner and Porter, 2001; Harvey et al.,
1987; Hughes et al., 1998; Perillo et al., 2005; Xin et al., 2009), thissite’s marsh sediment consists of a low-permeability clay layer overlying a high-permeability sandy-loam layer. The lower sandy-loam
layer is relatively well-connected to the creek nearby, and the watertable is controlled by the creek water level. Through the highly-conducted sandy loam layer, the position of the water level in the creek istransferred to the bottom of the clay layer. In this study, we focus only
on the surface clay layer and assume that macropores are continuous
openings connecting the soil surface with the groundwater from thelower layer. We further assume that evaporation directly from macropores can be neglected and that macropores enhance evaporation acrossthe soil surface by facilitating underground ﬂow towards the surface
soil (Fig. 1c).
Based on these assumptions, we conducted laboratory experiments
and numerical simulations (in which the vapor ﬂux from the macropore
is neglected) on soil evaporation with and without macropores. To
expand the results beyond laboratory conditions, a sensitivity analysis
with diﬀerent potential evaporation rates (1, 2, and 3 cm/d) and soil
textures (clay, silt loam, and sandy loam) was conducted. The results
address the following questions: (1) How do macropores aﬀect soil
evaporation rate, water ﬂow, and soil saturation distribution; (2) How
are these eﬀects altered by potential evaporation, and (3) How is thesoil evaporation aﬀected by macropores in combination with soil
properties (hydraulic conductivity and soil water retention curve).

Fig. 1. (a) A photograph of a creek in Chuandong, Jiangsu, China; (b) a photoof a crab; and (c) a schematic diagram of the creek-marsh system with macropores (a red rectangle indicates the system used in this study).

Xin et al., 2013).
By contrast, the eﬀects of evapotranspiration on salt marshes are
rarely studied and poorly understood (Xin et al., 2017). Evapotranspiration is a vital component of the global hydrologic cycle andaﬀects the water balance in various hydrological systems (Brutsaert,
1982). Almost 60% of the terrestrial precipitation returns to the atmosphere through evapotranspiration (Oki and Kanae, 2006). Around
25% of the total solar radiation on earth is consumed by evapotranspiration (Or et al., 2013; Trenberth et al., 2009). In salt marshes,
evapotranspiration aﬀects pore-water ﬂow and solute transport (Xinet al., 2017). For example, evapotranspiration takes freshwater away
but leaves salt behind, resulting in salt accumulation in the shallow soil
layer and plant rhizosphere that may inhibit plant growth (Adams,
1963; Morris, 1995; Shen et al., 2018). As evapotranspiration desaturates the soil pores during the marsh emergence, it could increase thewater inﬁltration during the tidal overtopping, resulting in enhanced
water and solute exchange between marsh sediment and surface water(Shen et al., 2018).
While understanding of soil evaporation at various scales and in
diﬀerent hydrological systems is well established, previous studies
predominately focused on homogeneous soils (Brutsaert, 1982;
Haghighi et al., 2013; Or et al., 2013; Penman, 1948). The eﬀects of soil
heterogeneity on evaporation are still poorly understood. Heterogeneity
can cause major diﬀerences in the duration of evaporation stages andtotal evaporation ﬂux. Lehmann and Or (2009) examined evaporationin heterogeneous soil consisting of coarse and ﬁne sands and found thatcapillarity drives the liquid from the coarse domain to the ﬁne one andsupplies evaporation from ﬁne sand. This prolongs the period of thehigh evaporation stage, which may increase the total evaporative ﬂux.
Assouline and Narkis (2017) found that diﬀerent irregular soil conﬁgurations could cause a shift in the development of the evaporation front
and the duration of diﬀerent evaporation stages.
It is unclear how to transfer existing knowledge of evaporation tohighly heterogeneous marsh sediments, which often include macropores such as crab burrows (Fig. 1a, b). For example, ﬁddler crabs (aspecies of marine crabs that makes large continuous openings in sediments) are found in a large portion of the global coastline (their global
distribution is given in Fig. S1, S refers to Supplementary Materials).
Their burrows are known to increase the sediment-water-atmosphereinterface and thus aﬀect water ﬂow in salt marshes. Xin et al. (2009)
and Xiao et al. (2019) found that macropores behave as preferential
pathways and increase the volume of tidally-driven water exchange
between marsh soil and tidal creek. Macropores can also improve soil

2. Methodology
2.1. Laboratory experiment
Experiments were conducted in two cylindrical soil columns with aheight of 75 cm and an inner diameter of 30 cm (Fig. 2a, b). The ﬁrst
column was packed with soil without a macropore. In the second
column, a tube of the same length of the column was inserted in thecenter of the column to act as a macropore (Fig. 2c, d). The tube used inthis experiment was made from a metal frame (diameter 4 cm) covered
with a stainless mesh (aperture of around 0.1 mm). At the bottom of thecolumns, permeable porous stones were placed, and the columns werelinked to Marriott bottles, which can provide ﬁxed watertables. Bothsoil columns and Marriott bottles were placed on high-resolution, selflogging scales so that the soil evaporation rates could be determined byweight changes.
Eight sets of experiments with diﬀerent watertables in the soilcolumn were conducted. The watertables were located between 0 and
70 cm (in a 10-cm interval) below the soil surface. Before each experiment, the top in the Marriott bottles was adjusted, and the bottles
were connected with soil columns. At the beginning of each set of experiments, we adjusted the watertables in the Marriott bottle and left
the columns to evaporate. Evaporation was considered to reach asteady-state when the weight of the soil column became steady. The
columns were then left to evaporate for around additional 10 h and theevaporation rates were calculated as the averaged weight loss of the
Marriott bottles.
All the experiments were conducted under well-controlled indoor
conditions. An air conditioner was used to maintain a stable temperature and humidity. To shorten the period needed to reach steady-state
conditions, we set a high potential evaporation rate. The room temperature and humidity were set to 30 °C and 40%, respectively. For
each experimental column, an identical fan was used to generate a
steady airﬂow (around 3.8 m/s), with a channel installed on the top of2

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T. Zhou, et al.

Fig. 2. (a) A schematic diagram of the experimental setup; (b) a photograph of the experimental setup; (c) the tube used as a macropore in the experiments; (d) aphotograph of the top of the column with a macropore illuminated by a lamp.

bottom of the soil column to represent the watertable. For the caseswith a macropore, the part of the macropore below the watertable wastreated as a ﬁxed-head boundary and the part above the watertable as a
no-ﬂow boundary. Since the cross-section of the macropore was quite
small in comparison with the soil surface, the evaporation loss from themacropore was neglected in the model.
At the soil surface, an atmospheric boundary condition was used tosimulate evaporation from the soil. It is well known that actual soil
evaporation is determined by the atmospheric condition as well as thesoil moisture condition in the surface soil layer (Brutsaert, 1982;
Camillo, 1986; van de Griend and Owe, 1994). The former is also
termed as potential evaporation and was calculated using a widely used
evaporation model based on the measured vapor pressure diﬀerence
and an aerodynamic resistance during the experiments under steadystate conditions (Figs. S5 and S6) assuming thermodynamic equilibrium
between the liquid and gaseous phases (detailed calculations in
Supplementary Materials). With potential evaporation (ET0) determined, soil evaporation (i.e., actual evaporation, EP) was calculated
in HYDRUS by limiting the evaporative ﬂux using the following two
conditions:each column to keep airﬂow uniform. Above the soil columns, infrared
lamps (250 W, Philips) were installed for supplying heat to the soil
surface. Temperature and relative humidity close to the soil surface and12 cm above the soil surface were monitored by relative humidity
sensors (VP-4, Decagon), which were connected to data loggers. These
measured atmospheric conditions were used for calculating the potential evaporation rate (details in Supplementary Materials). Dielectricsoil moisture sensors (5TM, Decagon) were installed at 10, 25, 40, and55 cm below the soil surface to measure the liquid water saturation.
The moisture sensors were installed from outside of the soil column,
with their tips 5 cm away from the column center.

2.2. Numerical model
To generalize the results from the experiments and obtain further
insights into water ﬂow and soil water saturation distributions, we also
conducted numerical simulations using HYDRUS (2D/3D) (Šimůnek
et al., 2011; Šimůnek et al., 2016). All simulations continued until
steady conditions were reached (deﬁned as when the evaporation rate
became constant). The soil hydraulic parameters of all materials used inthis study are listed in Table 1. The soil water retention curves were
measured in the laboratory and then ﬁtted using the van Genuchten(1980) model (Supplementary Materials and Fig. S2).
In this study, the 2D axisymmetric soil domains were discretizedusing unstructured triangular ﬁnite element meshes in an axisymmetriccoordinate system (Fig. S3). Although ﬂow in columns without amacropore is expected to be one-dimensional, in order to compare theresults from the columns with and without a macropore, we used thesame 2D model and the same radially-symmetric coordinate system tosimulate ﬂow in the both columns. In all simulations, the wall sideboundary was set to no ﬂow. The pressure head was speciﬁed at the∂hEP = −K (h) ⎛+ 1⎞ ⩽ ET0⎝ ∂z⎠ha ⩽ h ⩽ 0

Sand

Clay

Sandy loam

Silt loam

Clay-SKs (cm/d)
PorositySwres
α (m−1)
n

330.0
0.40
0.15
5
1.7

1.0
0.38
0.18
0.27
1.93

106.1
0.41
0.16
7.5
1.89

10.8
0.45
0.15
2
1.41

4.8
0.38
0.18
0.8
1.09

(2)
−1

where K (h) is the hydraulic conductivity [LT ] and ha is the limitingpressure head (cm) when the ﬁrst, optimal evaporation stage stops.
Note that ha is a function of the equilibrium between the soil moistureand the atmospheric water vapor, which can be derived using (De Vries,
1958; Philip and De Vries, 1957):

ha = −RTln(Hr )
Mg

(3)
−1

−1

K ), g is thewhere R is the universal gas constant (8.31 J mol
magnitude of gravitational acceleration (9.8 m s−2), M is the molecularweight of water (18 g mol−1), T is the absolute air temperature (°K),
and Hr is the relative humidity (set to 40% as maintained by the air
conditioner during the experiments).
To shorten the experimental period, we adopted a potential evaporation rate higher than those in natural systems. To generalize theresults and get a better insight into real systems, a sensitivity analysis
for diﬀerent potential evaporation rates, i.e., 1, 2, and 3 cm/d (Marani
et al., 2006), and diﬀerent soil textures was conducted. Clay, silt loam,
and sandy loam, i.e., three commonly found soil types in salt marshes,
were examined in this sensitivity analysis (Table 1 and Fig. S4).

Table 1
Soil hydraulic parameters (van Genuchten, 1980) for soils used in this study.
The ﬁrst two soil types, i.e., Sand and Clay, were used in the laboratory experimental columns, while the last three soil types, i.e., Sandy Loam, Silt Loam,and Clay-S, were used in the sensitivity analysis.
Soil type(1)

3

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Fig. 3. Measured and simulated (steady state) evaporation rates for the sandy
experimental columns with diﬀerent watertable depths.

3. Results
3.1. Results for the sandy columns
Measured and simulated evaporation rates for the sandy experimental columns with and without a macropore are shown in Fig. 3. In
general, evaporation rates can be divided into three groups dependingon the position of watertable:(1) When watertable depths (distance from the watertable to the soil
surface) were 0, 10, and 20 cm below the soil surface, the evaporation rates in both columns were largely similar and close to thepotential evaporation rate (around 6.2 cm/d). The simulation results were consistent with the experimental results. In addition, thepresence of the macropore had no apparent eﬀect on the evaporation rate.
(2) When watertable depths were 30, 40, and 50 cm below the soil
surface, the evaporation rates decreased below the potential evaporation rate as the watertable decreased, and correspondingly, thesoil became less saturated. Diﬀerences in evaporation rates between
the two cases (with/without a macropore) were minor, and there
were no diﬀerences in the simulation results.
(3) When watertable depths were 60 and 70 cm below the soil surface,the evaporation rates were around 1 cm/d. As the watertable declined to this depth, a dry soil layer with low water saturation was
detected during the experiment. Evaporation was expected to occur
mainly at low rates via water vapor diﬀusing through the dry soil
layer. Note that vapor diﬀusion was not considered in the numericalsimulations.

Fig. 4. Measured and simulated saturations for the sandy experimental columns
with diﬀerent watertable depths: (a) 0 cm, (b) 10 cm, (c) 20 cm, (d) 30 cm, (e)40 cm, (f) 50 cm, (g) 60 cm, and (h) 70 cm). Red lines overlap blue lines. The
stand variances of the measured results were given by the horizontal bars.

Darcy ﬂux and soil saturation for diﬀerent watertable depths (Fig. 5,
note the central section across the macropore is shown). The evaporation rates and the saturation distributions were similar in both columns.
Above the watertable, saturations decreased with increasing elevation,
as expected. However, diﬀerences in water ﬂuxes between the twocolumns were apparent. While the Darcy velocities were uniformly
vertical both below and above the watertable in cases without a macropore, horizontal ﬂow occurred below the watertable and around themacropore in cases with a macropore. This was more signiﬁcant aroundthe watertable, where the Darcy ﬂux was the highest. Clearly, themacropore behaved as a preferential ﬂow path for groundwater to recharge the surrounding soil during evaporation, in which case theevaporated water originated from the macropore rather than the soil
matrix.
We further calculated horizontal ﬂuxes from the macropore to thesoil (Fig. 6). Consistent with the ﬂow ﬁeld discussed earlier, the horizontal ﬂux varied with elevation and the position of the watertable.
With the watertable at the soil surface, horizontal ﬂuxes started from
0 cm/d and increased monotonically as the elevation increased. In
contrast, for deeper watertables, the horizontal ﬂux increased ﬁrst andthen decreased around the watertable. For the cases with a macropore,
the evaporative ﬂux originates both from water in the macropore(called macropore water) and water ﬂowing upwards from the base of
the column through the soil proﬁle. We calculated the ratios between
ﬂuxes through the macropore and actual evaporation ﬂuxes (Table 2).
The ratios were very high (over 90%) when watertable depths were lessthan 40 cm. The ratios dropped rapidly for deeper watertables, e.g.,
from 79.7% for a 50-cm depth to 18.1% for a 70-cm depth, i.e., theeﬀect of the macropore on soil evaporation was more critical for the soil
with shallow watertables.

The saturation proﬁles in the sand columns are shown in Fig. 4. The
measured results show little diﬀerences for diﬀerent watertable depths
between 40 and 60 cm, while the lines representing the simulated results overlap. Due to the uncertainty in the experiments, the porositiesof the soil columns were not uniform. While the porosity was assumed
to be uniform in the numerical simulations, it was likely not uniform in
the laboratory soil column experiments due to packing. This non-uniformity may have resulted in diﬀerences between the simulated andmeasured results. According to the simulation, the macropore did notaﬀect saturation distributions in the sand columns, which was consistent with the evaporation rate results. The soil higher than the watertable remained saturated, and the soil proﬁle became drier when theposition was closer to the surface. For diﬀerent watertable depths, thesaturation at the soil surface decreased as the watertable was lowered
until the watertable reached 40 cm, when the surface saturation wasalmost equal to the residual saturation.
Based on the simulation results, we examined the distributions of4

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Fig. 5. Simulation results for the sandy experimental columns without (left) and with (right) a macropore for watertable depths of (a) 0 cm, (b) 20 cm, (c) 40 cm, and(d) 60 cm. Water saturation is shown in color, while the arrows represent the Darcy velocities (the magnitude is indicated by the arrow length). The white lines
indicate the position where the hydraulic head is equal to 0.

experimental data for clay columns with a macropore, due to uncertainty caused by soil packing (Fig. 7). The numerical model described the experimental data satisfactorily only with manually adjusted parameter values (i.e., the saturated hydraulic conductivity of
1.9 cm/d and the van Genuchten parameters α = 0.27 m−1 andn = 2.23).
In the clay columns without a macropore, diﬀerent watertable
depths did not produce diﬀerent evaporation rates, as in the sand columns. In the clay columns, the evaporation rates were low and changed
only slightly for diﬀerent watertable levels. Since the potential evaporation rate was much higher than the permeability of the soil, the low

3.2. Results for the clay columns
The results for the experiments with clay were more complex thanthose for sand. The particle size distribution for clay was wider(d 90 d10 = 12.96), suggesting a wider distribution of pore sizes. Soilheterogeneity might have increased due to the experimental setup, as it
was diﬃcult to control the soil packing. Indeed, in order to satisfactorily simulate the measured results for columns with clay soil without amacropore, the saturated hydraulic conductivity was set to 1.0 cm/dand the van Genuchten parameters α and n to 0.27 m−1 and 1.93, respectively (Table 1). However, these values failed to describe the5Journal of Hydrology 584 (2020) 124754

T. Zhou, et al.

Interestingly, actual evaporation for the watertable depth of 70 cm
was 1.7 cm/d, higher than that for the sand case (when it was less than
1 cm/d). At this level, the watertable was close to the soil column
bottom, and thus the impact of the macropore was minor. Instead, it
was the large capillary fringe of the clay and higher unsaturated hydraulic conductivity that led to the higher evaporation rate.
For the clay columns, the distributions of Darcy ﬂuxes and soil saturations for diﬀerent watertable levels diﬀered signiﬁcantly for scenarios without and with a macropore (Fig. 8, note the central section
across the macropore is shown). For the former, the one-dimensional
behavior is again apparent, similar to the sand cases. The ﬂuxes were
uniform and vertical. The soil saturation decreased with increasing
elevation in the unsaturated zone. In the near-surface area, neither
ﬂuxes nor soil saturations were signiﬁcantly aﬀected by the watertable
depth. However, the two-dimensional behavior occurred around themacropore. Overall, the watertable (deﬁned as the zero-pressure headline) decreased from the macropore to the soil-wall boundary. Accordingly, the local soil water saturation decreased, as shown in Fig. 8a.
When the watertable in the macropore was close to the soil surface, thearea far away from the macropore was unsaturated. As the watertable
depth increased from 0 to 20, 40, and 60 cm, the ﬂux in the near-soil
surface area decreased, which is consistent with the reduced evaporation rates. In this area, the soil saturation was also reduced as the watertable was lowered.
Overall trends and variations in the simulated horizontal ﬂuxes
through the macropore boundary for the clay columns were similar to
those for the sandy columns (compare Fig. 9 with Fig. 6). Fluxes similarly decreased with the lower groundwater level and reached theirpeaks around the watertable. Like the sand cases, the ratio of waterﬂowing from the macropore to the actual evaporation rate was signiﬁcant (more than 90%) when the watertable depths were less than
40 cm. It then dropped from around 80% for a 50-cm depth to 19.9%
for a 70-cm depth. These ratios for the clay cases were slightly higher
than those for the sand cases (Table 2).

Fig. 6. Simulated horizontal Darcy ﬂuxes at the macropore boundary of the
sandy columns with diﬀerent watertable depths.
Table 2
Ratios (in %) of the water ﬂux through the macropore to the actual evaporation
rate in experimental columns with diﬀerent watertable depths and soil types.
Soil Type

Sand
Clay

Watertable depth (cm)
0

10

20

30

40

50

60

70

99.3
99.3

98.9
99.2

98.1
98.1

95.8
96.0

90.5
90.5

79.7
80.8

53.0
59.3

18.1
20.0

3.3. Sensitivity analysis to potential evaporation
Overall, trends and variations of actual evaporation rates wereconsistent with experimental evaporation rates for the sand case, i.e.,
shallow watertables led to high evaporation rates. The eﬀects of amacropore on evaporation rates were more pronounced for soils with alower hydraulic conductivity, i.e., clay and silt loam. For sandy loam,
no apparent diﬀerences were caused by a macropore, and evaporation
rates for columns with and without a macropore overlapped (Fig. 10).
For silt loam, evaporation rates for columns with a macropore werelarger than for columns without a macropore for the same positions ofwater tables, e.g., for a watertable depth at 40 cm and ET0 of 1 cm/d(Fig. 10). The macropore eﬀect increased as the potential evaporation
rate increased. For ET0 = 3 cm/d, evaporation rates for columns with amacropore were larger than those for columns without a macropore forwatertable depths from 10 to 50 cm. As the potential evaporation rateincreased, the macropore played an increasingly important role in affecting the actual evaporation rates for cases with shallow watertables.
For both silt loam and sandy loam, actual evaporation rates for
scenarios without a macropore reached the potential evaporation rate
when the watertable was close to the soil surface, indicating that soils
were permeable and able to maintain a high evaporation rate. In contrast, clay, with the lowest hydraulic conductivity among all three soil
types, could not support a high evaporation rate without a macropore.
This result was demonstrated by the results with shallow watertables.
For a watertable close to the soil surface, evaporation rates were similar
for scenarios with and without a macropore for ET0 = 1 cm/d. As ET0
increased to 2 and 3 cm/d, scenarios with a macropore still reached thepotential evaporation rate, but for those without a macropore, actual
evaporation rates decreased to around 1 cm/d, which was the value ofthe soil saturated hydraulic conductivity.

Fig. 7. Measured and simulated steady state evaporation rates for the clay
experimental columns with diﬀerent watertable depths. The solid red lines represent simulation with the same soil properties, with and without a macropore. The dashed red line represents the simulation using the adjusted soil
properties (Ks = 1.9 cm/d, n = 2.23) that ﬁt well the experimental results.

hydraulic conductivity of the clay prevented actual evaporation fromreaching the potential evaporation even for shallow watertables (0 and
20 cm). Also, the zero-pressure head line in the soil (i.e., depth where
the pressure head was equal to zero) was signiﬁcantly lower than thecorresponding applied pressure head at the bottom of the column. In
contrast, two diﬀerent evaporation scenarios were observed for thecases with a macropore. Evaporation rates were almost constant andclose to the potential evaporation rate when the watertable was close to
the soil surface (0 and 10 cm). After the watertable dropped to 20 cm,
evaporation rates decreased and continued to do so as the watertable
was lowered further.
6

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Fig. 8. Simulation results for the clay experimental columns without (left) and with (right) a macropore for watertable depths of (a) 0 cm, (b) 20 cm, (c) 40 cm, and(d) 60 cm. The saturation is shown in color, while the arrows represent the Darcy velocities (the magnitude is indicated by the arrow length). White lines indicate theposition where the hydraulic head is equal to 0.

Ratios of water ﬂuxes through the macropore to the actual evaporation rate for cases considered in the sensitivity analysis are demonstrated in Fig. 11. Overall trends and values for these cases are
similar to the experimental cases, being over 90% when the watertable
depths are less than 40 cm and dropping rapidly from 80.7% for a 50cm watertable depth to about 20% for a 70-cm watertable depth. Different potential evaporation rates and soil properties did not showmuch inﬂuence on the ratios of water ﬂuxes from a macropore to theactual evaporation rate. These results indicate that the watertable depth
signiﬁcantly aﬀects the water bypass caused by the macropore.

4. Discussion
While this study focused on water ﬂow in cylindrical soil columns, it
should help to increase our understanding of the eﬀects of macroporeson water ﬂow and solute transport in real salt marshes. Macropores did
not aﬀect soil evaporation markedly in high-permeability soils such as
sandy loam. However, low-permeability silt loams and clays were unable to deliver enough water to the soil surface to maintain a highevaporation rate. Under such conditions, continuous macropores incontact with groundwater behave as preferential ﬂow paths, providing
water for evaporation.
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T. Zhou, et al.

saturation in shallow soil layers. This is expected to lower soil aerationand oxygen availability in marsh soils, which could aﬀect plant root
respiration and thus marsh plant growth (Colmer and Flowers, 2008;
Dacey and Howes, 1984; Mendelssohn et al., 1981; Silvestri et al.,2005). However, in macropores, there is no capillary-induced upwardﬂow, which would decrease soil saturation of near-surface soil layers insoils next to macropores with deeper watertables. Under such conditions, evaporation would be reduced, similarly as associated water exchange. Natural marshes are periodically inundated by tidal water. Forless saturated soils, tidal inﬁltration would increase, which would leadto enhanced water exchange in shallow marsh soils.
Our results show that the contribution of water from the macropore
to the evaporation ﬂux is signiﬁcant compared with that which comesfrom the watertable and passes entirely through the soil before evaporation. With denser distributions of macropores, most of the evaporated water would bypasses the soil matrix and has only a little contact
with soil grains. This is expected to aﬀect the fate of chemicals transported through marsh soils. For example, chemicals in deep soil layers
would be unlikely to be transported to the shallow soil layers due to
weak upward ﬂow. Furthermore, interactions between the deep soil
layer and creek water would be weakened.
Guimond et al. (2019) recently found that the presence of macropores reduces carbon sequestration in salt marshes. This is consistent
with Xin et al. (2009) and Xiao et al. (2019) who indicated that macropores likely enhance lateral water exchange between marsh soil andcreek water, which may increase a lateral loss of dissolved carbon.
These same authors also suggested that macropores would decrease soil
water saturation of the shallow soil layer and thus improve local soil
aeration conditions. This would favor carbon oxidation and lead to afurther vertical carbon loss. Our results indicate that while macroporesenhance evaporation from soil columns with a ﬁxed watertable, the soil

Fig. 9. Simulated horizontal Darcy ﬂuxes at the macropore boundary for theclay columns with diﬀerent watertable depths.

Soil layering is commonly found in natural marshes: a low-permeability clay layer overlying a high-permeability sandy-loam layer (Caoet al., 2012; Dolphin et al., 1995; Xiao et al., 2017; Xin et al., 2012).
Enhanced evaporation would intensify water exchange between creek
water and marsh soil. While creek water in natural systems is commonly saline and evaporation would lead to soil salinization (Xin et al.,2017), we used freshwater in this study to minimize this eﬀect.
For soils with shallow watertables, macropores increased soil

Fig. 10. The eﬀects of potential evaporation (ET0) and soil type on actual evaporation for soils with and without a macropore. The ﬁrst, second, and third columns
show the simulation results for clay, silt loam, and sandy loam, respectively. The potential evaporation rates are the same for each row and given in the ﬁgure titles.
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T. Zhou, et al.

Fig. 11. Ratios (in %) of the water ﬂux through the macropore to the actual evaporation rate for diﬀerent potential evaporation rates (ET0 = 1–3 cm/d), groundwaterlevels (0–70 cm), and soil types (clay, silt loam, and sandy loam) considered in the sensitivity analysis.

While the present study has produced insights into steady-stateevaporation in salt marshes, it focused on steady-state conditions in
idealized soil columns with steady watertables. Further investigations
should be conducted to conﬁrm the results from this study in real
marshes, in which tides, rainfall, and evaporation would lead to dynamic watertables. Macropores are distributed randomly and vary indiameter and depth. Furthermore, the density and size of macropores
likely vary spatially and are related to creek networks. Notwithstanding
these diﬀerences, the present study highlights the importance of macropores on soil evaporation, and their potential eﬀects on surface waterand groundwater interactions and solute transport/reaction in salt
marshes.

water saturation in the upper soil layer is increased. This would lead to
reduced oxygen availability and inhibited carbon oxidation. This isinconsistent with previous studies (Xin et al., 2009; Xiao et al., 2019).
In natural salt marshes, evaporation would lower a local watertable,
which would improve soil aeration of the shallow soil layer. The eﬀects
of macropores on carbon sequestration thus depend on how they synergistically regulate lateral water ﬂow, the position of the watertable,
and associated aeration conditions.
Natural marshes are aﬀected by the combination of evaporation,
rainfall and tidal ﬂuctuations. Watertables are dynamic, particularly innear-creek zones (Wilson and Gardner, 2006; Xin et al., 2011). Our
results indicate that when watertables are at diﬀerent depths, the effects of macropores on soil evaporation are not consistent. Macroporesare expected to speed up soil evaporation in the clay marsh withshallow watertables that often occur after tidal inundation or rainfall
events. However, the near-surface soil is hard to desaturate as macropores allow water to be easily transported from the lower soil proﬁle. As
roots of marsh plants are distributed in the shallow soil layer, this more
rapid evaporation would not reduce the period of oxygen deﬁciency,
which would not favor plant growth.

Tingzhang Zhou: Methodology, Formal analysis, Data curation,
Writing - original draft. Pei Xin: Conceptualization, Writing - review &editing, Supervision. Ling Li: Methodology, Formal analysis,
Validation. D.A. Barry: Methodology, Formal analysis, Validation.
Jirka Šimůnek: Methodology, Writing - review & editing.

5. Conclusions

Declaration of Competing Interest

Based on laboratory experiments and numerical simulations, we
examined the eﬀects of large macropores on soil evaporation in soilcolumns. From the results, the following conclusions can be drawn:

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.

(1) The macropore increased evaporation rates in low-permeability
soils with shallow watertables. The macropore did not signiﬁcantly
aﬀect soil evaporation in high permeability soils (e.g., sand andsandy loam).
(2) The presence of the macropore altered water ﬂow in the soil compared to that without the macropore. The macropore behaved as apreferential ﬂow path, delivering groundwater to the shallow soil,
which resulted in higher soil saturation in the near-surface area.
(3) Water ﬂow from the macropore contributed signiﬁcantly to theactual evaporation rate, and this proportion was reduced as thewatertable declined. Vertical ﬂow through the soil proﬁle (i.e., independent of the macropore) was reduced by the presence of amacropore.

Acknowledgment

CRediT authorship contribution statement

This work was supported by the National Natural Science
Foundation of China (51579077).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jhydrol.2020.124754.
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