Simulated long term effects of different (3)
Field Crops Research 100 (2007) 311–319
www.elsevier.com/locate/fcr
Simulated long-term effects of different soil management regimes
on the water balance in the Loess Plateau, China
Shulan Zhang a,c,*, Elisabeth Simelton b, Lars Lo¨vdahl c, Harald Grip c, Deliang Chen b,d
b
a
College of Resources and Environmental Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, China
Department of Physical Geography, Earth Sciences Centre, Gothenburg University, Box 460, SE-405 30 Gothenburg, Sweden
c
Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea, Sweden
d
National Climate Centre, Beijing, China
Received 11 January 2006; received in revised form 18 August 2006; accepted 19 August 2006
Abstract
A soil management regime that improves water use efficiency (WUE) is urgently required to increase the sustainability of the winter wheatsummer fallow system in the Loess Plateau, China. However, the long-term partitioning of the water balance must be understood in order to
evaluate the viability of possible soil management regimes. Therefore, an ecosystem model (CoupModel) was used to explore the effects on
components of the water balance of five types of soil management regimes: conventional practice, wheat straw mulching, incorporation of high
organic matter contents, compaction, and use of a harvested fallow crop. Three variants of the fallow crop approach were also considered, in which
the crop was harvested 15, 30 and 45 days before sowing the wheat (designated Fallow-15d, Fallow-30d and Fallow-45d, respectively). Simulations
were used to identify the relative magnitude of soil evaporation, wheat transpiration and deep percolation and to elucidate the temporal variability
in these components for a selected location using climate records spanning 45 years. However, the soil management regime significantly influenced
the magnitude of every component of the water balance (in terms of minimum, maximum and mean values) over the long periods considered.
Consequently, wheat yield and WUE differed significantly among the simulated treatments. Mulching led to significantly lower soil evaporation,
higher transpiration, and more frequent and extensive deep percolation than other regimes, thereby improving fallow efficiency (percentage of
rainfall stored in the soil during the fallow period at the end of the fallow period), wheat yields and WUE. In contrast, soil compaction gave the
opposite results, leading to the most unfavourable partitioning of the water balance reflected in the lowest wheat yield and WUE values of all the
regimes. In 90% of the years no deep percolation occurred in the soil compaction simulations. Use of a fallow crop with optimal harvest timing (Fallow30d) improved partitioning of the water balance (decreased soil evaporation) and did not significantly reduce wheat yield compared with conventional
practice. High organic matter contents in the soil also had a positive influence on the water balance and improved wheat yield and WUE relative to
conventional practice. Therefore, mulching appears to be the best management practice for the winter wheat-summer fallow system in the Loess
Plateau, according to the simulations. Increasing soil organic matter may be the best option if mulching cannot be implemented. The ideal time for
harvesting a fallow crop for use as green manure or fodder appears to be ca. 30 days before sowing the winter wheat.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Long-term effects; Mulching; Soil compaction; Fallow crop; Deep percolation
1. Introduction
Water shortages pose the greatest threat to dryland farming
in semiarid areas. In China, 14.7 million ha of arable land can
be categorized as semiarid, mainly distributed in the Chinese
Loess Plateau, the world’s largest Loess Plateau (Li and Xiao,
* Corresponding author at: College of Resources and Environmental
Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, China.
Tel.: +86 29 87088120.
E-mail address: zhangshulan@nwsuaf.edu.cn (S. Zhang).
0378-4290/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.fcr.2006.08.006
1992). The main crop on a large part of the Loess Plateau is
winter wheat, conventionally cultivated with a single crop
being produced per year, followed by about three months
summer fallow. The fallow period falls in the rainy season, in
which water can be stored in the soil and used by the following
wheat crop. During the past 20 years both fertilizer applications
and wheat yields have increased, resulting in increased soilwater depletion (Huang et al., 2003). Consequently, soil water
is not being fully replenished during the fallow, and at sites
where a dry subsoil layer has formed the crop yield varies
strongly with rainfall during the growing season (Li, 2001). The
dry soil layer, where water content is close to the wilting point,
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S. Zhang et al. / Field Crops Research 100 (2007) 311–319
may be replenished with plant available water during the fallow
period if sufficient rain falls to penetrate the soil profile.
However, the conventional practice of keeping the soil bare
during the fallow period results in very low precipitation
storage efficiency or fallow efficiency (ratio of stored water to
rainfall during fallow) (Li and Xiao, 1992; Latta and O’Leary,
2003). Furthermore, the infiltration depth is shallower under
high fertilization than low fertilization conditions (Huang et al.,
2002). Therefore, the conventional practice with high
fertilization does not appear to be a sustainable management
option in the long-term.
One way to increase water storage is to retain crop residues
on the soil surface. Mulching can be an effective measure for
reducing soil evaporation and increasing water storage (Unger,
1978; Steiner, 1989; Baumhardt and Jones, 2002). Furthermore,
short-term studies have shown that mulching is beneficial for
water storage and crop yields in the Loess Plateau (Wang et al.,
2001; Huang et al., 2005; Zhang et al., 2006a). Zhang et al.
(2006a) also found that deep percolation occurred earlier and
more extensively under mulching than under conventional
practice, which should favour the recovery of the soil water
content of the dry sub-layer. However, mulching has not always
been shown to increase crop yields, and its effectiveness
depends on crop, soil and climate (Wicks et al., 1994; Gajri
et al., 1994). In addition, mulching has not been widely
practiced in China for two practical reasons: (1) the presence of
mulch reduces the quality of wheat sowing by standard
machines and (2) mulch material is also needed for animal food
stuff and/or fuel.
Another way to improve water storage is to change the
hydraulic properties of the soil in a way that increases rain
infiltration and decreases soil evaporation. Increasing the soil
organic matter content has been found to meet these objectives
(Unger and Stewart, 1974). Unfortunately, the most rapid and
convenient way to increase soil organic matter – applying
animal manure – cannot be implemented in this region in
practice, because such resources are very limited. Hence,
growing a crop during the fallow period, and ploughing it in as
green manure at an appropriate time before the next crop, might
be a more viable strategy to increase soil organic matter
contents and hence enhance fallow efficiency. However, various
short-term studies have shown somewhat inconsistent results.
After testing different crop rotation systems, Li et al. (2000)
reported that growing a fallow crop for forage does not greatly
influence the quantity of water stored in the soil for use by
subsequent winter wheat crops in the middle-west Loess
Plateau. Vigil and Nielsen (1998) found that wheat yields
following the application of green legume manure were lower
than those obtained with traditional summer fallow in a 2-year
study. In a 3-year study, use of a fallow crop as green manure
reduced the yield of the following winter wheat in a relatively
dry year compared with conventional practice, but did not
reduce the yield in a wet year (Zhang et al., 2006a). Therefore,
interpreting these direct measurements and assessing the value
of the examined management strategies is complicated by yearto-year variations in both the total amount and the seasonal
distribution of rainfall (Asseng et al., 2001).
In the Loess Plateau region there is an increasingly urgent
need not only to improve water use by the crops but also to
elucidate side-effects caused by modern mechanised operations
in the field. Soil compaction has been recognized as one of the
most serious factors promoting soil degradation in the world
(Oldeman et al., 1991). A laboratory study has shown that soil
compaction greatly affects the hydraulic properties of silt loam
soil from the Loess Plateau (Zhang et al., 2006b); saturated
hydraulic conductivity in compacted soil amounted to less than
15% of that in non-compacted soil. However, there is little
information on the effects of compaction on crop yields.
Due to the inter-annual climate variability, only long-term
analysis is likely to give clear indications of the risks associated
with management effects on crop yield and valid measures of
the water balance components involved (Keating et al., 2002).
Simulations are, therefore, powerful tools for extrapolating
short-term experimental results and for analyzing how different
measures are likely to influence production and the water
balance across the range of climatic conditions within a given
location. In the study presented here, simulations were applied
to identify the relative magnitude of the effects of different soil
management strategies on key water balance components (such
as transpiration, soil evaporation, and deep percolation). For
this purpose we explored the temporal variability in these
variables for the winter wheat zone in the Loess Plateau using
45 years of meteorological data. We also identified the impact
of the duration of fallow crop growth (forage or green manure)
on subsequent wheat yields. The overall purpose was to identify
a sustainable management strategy for agricultural production
in the Loess Plateau and similar regions.
2. Materials and methods
2.1. Model description
The CoupModel is a physically based one-dimensional
model to simulate fluxes of water, energy, carbon and nitrogen
in the soil–plant–atmosphere system, which incorporates both
the former SOIL (Jansson and Halldin, 1979) and SOIL-N
(Eckersten et al., 1998) models. A detailed description of the
model was given by Jansson and Karlberg (2004).
The model predicts water and heat flows between soil layers,
processes at the surface and the base of the profile including
runoff, infiltration, and deep percolation. In addition, evapotranspiration, surface energy balance, heat storage, soil frost
and snow dynamics are calculated. The water and heat flow are
described by Richard’s (1931) solution of Darcy’s law coupled
with Fourier’s law. Calculation of soil evaporation was based on
solving the soil surface energy balance while plant transpiration
and evaporation of intercepted water are based on the Penman–
Monteith equation (Monteith, 1965). Deep percolation will
appear in the simulations when water is transmitted beyond the
bottom layer of the soil profile (in our case at 2.4 m depth).
Plant growth, based on a radiation use efficiency approach, is
calculated as the potential yield which is proportional to the
amount of global radiation intercepted by the canopy, as
estimated using Beer’s law. Actual growth is the potential
313
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
growth reduced by linear response functions for unsuitable air
temperature, nitrogen availability and water stress. No
reduction from pests or nutrients deficiency (except nitrogen)
was included. In this simulation nitrogen is assumed to be
sufficient, thus plant growth was not reduced because of
nitrogen supply. The daily total assimilation is translocated to a
temporary storage pool, and then is partitioned to the roots,
leaves and stem. Grain development started according to air
temperature sum, and assimilates to grain were translocated
from leaf, stem and root, respectively.
2.2. Model calibration and long-term application
The model was first calibrated against a 3-year winter wheat
field experiment in the Loess Plateau by Zhang et al. (2006a)
including three different soil management regimes, which were
conventionally managed winter wheat-summer fallow, mulching (a conventionally managed, but unploughed treatment in
which air-dried, unchopped wheat straw (0.8 kg m2) was
evenly distributed over the soil surface and kept at relatively
constant levels), and winter wheat-summer fallow crop.
The long-term simulation was run for the same soil type as
the above field experiment during a 45-year period using daily
weather data at Luochuan (35.82N, 109.50E, 1159 m a.s.l.) as
driving variables. The long-term simulation was run for five
types of soil management regimes totalling seven treatments
(see Table 1). The crop and soil parameters applied were the
same for all treatments as the calibrated values found from the
field experiment, except those hydraulic properties, which were
derived from laboratory studies for soil compaction (Zhang
et al., 2006b) and high organic matter treatments (Zhang et al.,
2006c). In order to investigate the impact from climate
variability the soil properties of each treatment were kept
constant during the simulation period (from 20 September 1955
to 19 September 2000) and no nutrient limitation was
considered.
The simulation was started on 20 September 1955. In the
simulations, the date of sowing winter wheat (Triticum
aestivum L.) was fixed at 20 September for all years and
treatments, while the harvest date was calculated by the model
according to the temperature sum. The fallow crop, black bean
(Aphis fabae), was sown on 16 June every year. The annual
water balance was calculated from previous fallow period until
the end of the following wheat season. The first year (1955) the
fallow period was not included in the simulation. Therefore, the
annual water balance was calculated from summer in 1956 to
summer in 2000, totally for 44 years.
2.3. Analysis methods
2.3.1. Water balance
The field water balance can be written as
(1)
P ¼ E þ T þ D þ R þ DS þ Ei
where P is the precipitation, E the soil evaporation, T the crop
transpiration, R the surface runoff, D the deep percolation
below the root zone, DS the change in soil water storage and
Ei is evaporation from intercepted rainfall. In this study surface
runoff was zero because the topography was flat, and Ei was
neglected because it was quite constant and constituted a very
small proportion of the water balance compared with the other
terms (Zhang et al., 2006a). DS can be either positive or
negative. Therefore, the water balance was calculated as
(2)
P DS ¼ E þ T þ D
2.3.2. Water use efficiency
Water use efficiency (WUEg or WUEb) was defined as
WUE ¼
Y
ET
(3)
where WUEg or WUEb represents the water use efficiency for
the grain or biomass yield (kg m3), Y the grain or biomass
yield of the wheat, respectively, and ET is the evapotranspiration during the wheat season.
2.3.3. Water stress level
The wheat water stress level was expressed as
WSL ¼ 1
Ta
Tp
where WSL is the water stress level, Ta actual transpiration and
Tp is the potential transpiration. The higher the value of WSL
the more severe the water stresses.
Table 1
Description of the different treatments
Treatment
Fallow period (harvest–19 September)
Growing season (20 September–June)
Soil properties
Conventional management
Mulch
Fallow-15d
Fallow-30d
Fallow-45d
HOM (soil with high organic
matter content)
Ploughed bare soil
Wheat straw no summer ploughing
Black bean growing 16 June–5 September
Black bean growing 16 June–20 August
Black bean growing 16 June–5 August
Ploughed bare soil
Winter
Winter
Winter
Winter
Winter
Winter
Compacted
Bare soil
Winter wheat
Zhang et al. (2006d)
As conventional management
As conventional management
As conventional management
As conventional management
Zhang et al. (2006c); soil-OM
3%-units (0–10 cm depth) and
1.5%-unit (10–20 cm depth)
Zhang et al. (2006b); assumed to
increase bulk density by 20% down
to 45 cm depth
wheat
wheat + wheat straw
wheat
wheat
wheat
wheat
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
1
1
0
23
14
7
Maximum
3
9
4
1
2
2
3
b
a
b
c
bc
bc
bc
18
32
20
8
12
15
18
0
0
0
0
0
0
0
156
110
147
277
169
165
161
27
32
27
17
22
24
26
b
a
b
d
c
bc
b
35
45
36
30
35
35
35
15
22
15
10
3
9
11
16
17
16
23
29
22
17
12
7
2
Mean
Mean
Maximum
Minimum
CV
Mean
Maximum
Minimum
CV
Fallow crop transpiration
Wheat transpiration
9
13
9
6
11
10
9
58
43
56
68
47
53
57
84
76
85
90
81
82
86
b
f
bd
a
ce
de
bd
70
58
68
82
62
65
68
CV
Minimum
Maximum
% of total water allocated.
a
CV means coefficient of variation.
b
Different letters in the same column indicate differences that are significant at the P < 0.05 level (LSD).
The different soil management regimes resulted in different
magnitudes of soil evaporation, transpiration and deep
percolation (Table 2). Annual mean soil evaporation comprised
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
3.2. Simulated components of the water balance
b
In the present study the annual precipitation was calculated as
rainfall within the previous fallow period (from wheat harvest to
wheat sowing) plus precipitation during the following wheat
season. Over the 44 years, the annual precipitation varied from
351 to 819 mm, with an average of 568 mm (Fig. 1a). Fallow
rainfalls accounted for 39–85% (average 61%) of the annual
precipitation. Ten-year average annual precipitation was
604 mm in the 1960s, 544 mm in the 1970s, 607 mm in the
1980s and 538 mm in the 1990s. For about 10%, 25% and 20% of
the years the annual precipitation was 700, respectively. Fallow rainfalls were between 300 and
400 mm for nearly 50% of the years (Fig. 1b). Thus, annual
precipitation showed great variability; some years could be very
dry or very wet. The annual average temperature was 9.4 8C,
while the daily maximum and minimum temperatures were 28.2
and 16.4 8C, respectively (data not shown).
Mean
3.1. Climatic conditions
Deep percolation
3. Results
a
In the simulations, measured daily climatic data spanning 45
years (20 September 1955 to 19 September 2000) was used.
The measured variables were precipitation, air temperature,
relative air humidity, wind speed and sunshine hours, which
was used to estimate global radiation. Values were missing for
about 10% of the total number of records for the first variable,
and less than 0.1% for the other four. The data were provided by
the National Climate Centre, Beijing. The missing values were
replaced by zeroes for precipitation and interpolated for the
other variables in the simulations.
Soil evaporation
2.4. Meteorological data
Treatment
Fig. 1. Observed (a) and accumulated probability (b) of annual (fallow + wheat
season) and fallow precipitation from 1957 to 2000.
Table 2
Simulated annual mean (fallow and wheat season) soil evaporation, deep percolation, wheat transpiration and fallow crop transpiration in relation to water balance defined by Eq. (2)
Minimum
49
57
70
CV
314
315
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
respectively. The proportions of the water balance accounted
for by soil evaporation were highest in the soil compaction
treatment (ranging from 68% to 90%) and lowest in the
mulching treatment (43–76%) during the simulation period.
Growing a fallow crop decreased the contribution of annual
mean soil evaporation by 2–8% compared with conventional
practice. The high organic matter treatment resulted in 2%
lower soil evaporation than that in the conventional treatment.
The contribution of wheat transpiration to the water balance
was highest under mulch and lowest in the compaction
treatment. The longer the period with a preceding fallow crop,
the lower the transpiration in the subsequent wheat season
compared with conventional practice, but the maximum
transpiration was the same (35%) and large differences were
found in minimum values (Table 2). The high organic matter
treatment gave similar mean transpiration values to the
conventional treatment, but the maximum value was higher
than that in the conventional treatment. The contributions of
deep percolation to the water balance differed significantly
among treatments; being highest under mulch (9%; ca. two- to
nine-fold higher than in the other treatments), and lowest in the
compaction treatment. Fallow crop treatments decreased deep
percolation to some extent compared with conventional
practice, except Fallow-45d, for which the proportions
(including maximum values) were the same.
3.3. Temporal variability of water balance components
Fig. 2. Simulated temporal variation of soil evaporation (E), transpiration (Tw
for wheat and Tf for fallow crop, respectively) and deep percolation (D) under
various soil management regimes from 1957 to 2000. From the top the
treatments are Fallow-45d (a), Fallow-30d (b), Fallow-15d (c), compaction
(d), mulch (e), high organic matter (HOM, f) and conventional (g).
58–82% of the water balance, regardless of the management
regime, with wheat transpiration the next largest term at 17–
32%. Fallow crop transpiration and deep percolation made up
the remainder of the balance, amounting to 2–12% and 1–9%,
The temporal variability in soil evaporation and wheat
transpiration in each treatment was relatively minor compared
to the temporal variability in deep percolation (Table 2 and
Fig. 2). For all treatments the coefficients of variation were
around 10% for soil evaporation, less than 30% for wheat
transpiration, but higher than 100% for deep percolation. The
accumulated probability showed that for 50% of the years deep
percolation amounted to less than about 30 mm under
mulching, but less than 5 mm for all other treatments
(Fig. 3). Growing a fallow crop affected the amount of deep
percolation to some extent, but did not substantially affect the
probability of its occurrence compared with conventional
practice. The amount of deep percolation significantly
Table 3
Long-term simulated fallow efficiencya under different soil management
regimes (%)
Fig. 3. Accumulated probability of deep percolation occurring under the
conventional, high organic matter (HOM), compaction, mulch, Fallow-15d,
Fallow-30d and Fallow-45d treatments.
Treatments
Mean
Maximum
Minimum
S.D.
Conventional
Mulch
HOMb
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
28
38a
30
19
19
23
27
46
57
48
46
46
46
47
3
10
1
15
29
12
5
11
11
11
11
16
13
12
bc
b
d
d
cd
bc
S.D. refers to standard deviation. Different letters in the same column indicate
differences that are significant at the P < 0.05 level (LSD).
a
Fallow efficiency expresses ratio of stored water to rainfall during fallow
period.
b
Soil with high organic matter content (see Table 1).
316
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
Table 4
Linear correlation coefficients for the relationship between fallow efficiency
and: rainfall throughout the fallow period, rainfall in the last month of the fallow
period and the rest of the time (n = 45)
Treatment
Whole fallow
The last month
Other time
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
0.73
0.66
0.75
0.57
0.70
0.73
0.71
0.59
0.59
0.56
0.71
0.61
0.58
0.56
0.40
0.31
0.43
0.14
0.34
0.40
0.39
Note: all coefficients are significant at the P < 0.01 level except the value of
0.14.
correlated with yearly precipitation, but was most strongly
correlated with fallow rainfall (R2 = 0.46 for conventional
practice, 0.28 and 0.63 for compaction and mulch, respectively). To generate deep percolation 405 mm rain was needed
under mulch conditions, compared to 504 and 589 mm for the
conventional and compaction treatments, respectively.
3.4. Fallow efficiency
The mean ratio of stored water to rainfall during the fallow
period (fallow efficiency) under various treatments ranged from
19% to 38%; the highest value was found in the mulch
treatment and the lowest values in the compaction and Fallow15d treatments (Table 3). In comparison with the conventional
treatment, growing a fallow crop decreased the fallow
efficiency by percentages ranging from 1% to 9%, depending
on the growing duration. High organic matter resulted in 2%
higher fallow efficiency than conventional practice. However,
all treatments except the mulching and high organic matter
treatments had similar maximum fallow efficiencies, but very
different negative minimum fallow efficiencies (ranging from
29% to 3%).
Table 5
Long-term simulated wheat yield (kg m2) and water use efficiency (WUE)
(kg m3) under various treatments
Treatment
Grain
S.D. Biomass S.D. WUEg
S.D. WUEb
S.D.
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
0.39
0.44
0.40
0.28
0.35
0.37
0.39
0.09
0.09
0.09
0.09
0.12
0.11
0.10
0.25
0.24
0.26
0.20
0.31
0.28
0.25
0.39
0.36
0.40
0.33
0.60
0.49
0.40
b
a
b
d
c
bc
b
0.84
0.98
0.86
0.59
0.74
0.79
0.83
b
a
b
d
c
bc
b
0.19
0.19
0.19
0.18
0.26
0.24
0.21
1.26
1.41
1.27
0.97
1.09
1.17
1.24
b
a
b
d
c
bc
b
2.68
3.09
2.73
2.00
2.29
2.46
2.62
b
a
b
e
d
cd
bc
S.D. refers to the standard deviation. Different letters in the same column
indicate differences that are significant at the P < 0.05 level (LSD).
Fallow efficiency significantly correlated with fallow
rainfall and rainfall distribution during the fallow period
(Table 4). The lowest correlation coefficient between fallow
efficiency and rainfall throughout the fallow period was for the
compaction treatment (0.57), followed by mulching (0.66) and
the Fallow-15d treatment (0.70); the other treatments had
similar correlation coefficients, all >0.7. However, rainfall in
the last month of the fallow period was also highly correlated
with fallow efficiency, with correlation coefficients of 0.71 for
the compaction treatment, and 0.5–0.6 for all of the other
treatments.
3.5. Water stress probability
The simulations indicated that the wheat was subjected to
some degree of water stress (WSL 0.05) during all years
(Fig. 4). During 50% of the years the water stress level was less
than or equal to 0.19 for the mulch treatment, and 0.29, 0.30,
0.33, 0.34, 0.36 and 0.45 for the high organic matter,
conventional, Fallow-45d, Fallow-30d, Fallow-15d and compaction treatments, respectively. The treatments could be
separated into three groups according to the water stress levels
associated with them: slight stress (the mulching); severe stress
(compaction); and moderate stress (the other treatments). In the
moderate stress group Fallow-15d generated the most severe
water stress.
3.6. Wheat yield and water use efficiency
Fig. 4. Accumulated probability of water stress occurring under the conventional, mulch, high organic matter (HOM), compaction, Fallow-15d, Fallow30d and Fallow-45d treatments.
The simulated average annual wheat biomass yield ranged
from 0.59 to 0.98 kg m2, regardless of treatment effects, and
biomass water use efficiency ranged from 2.00 to 3.09 kg m3
(Table 5). Mulching increased both the grain and biomass
yields, and consequently resulted in higher water use efficiency
than conventional practice. Conversely, the compaction
treatment greatly decreased the wheat yield and resulted in
the lowest WUE values. The treatment in which the fallow crop
was grown for the shortest time (Fallow-45d) gave the same
wheat yield as the conventional treatment, while wheat yields
were reduced under the other two fallow crop treatments, and
the WUE values followed the same trends. The high organic
matter treatment slightly increased wheat yield and WUE.
The accumulated probability of the treatments to increase
wheat biomass yields relative to the conventional practice is
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
Fig. 5. Accumulated probability of a difference in yield increase between
various soil management regimes and conventional practice. The soil management regimes were: high organic matter (HOM), compaction, mulching,
Fallow-15d, Fallow-30d and Fallow-45d.
shown in Fig. 5. Mulching increased the yields in 95% of the
years, and in 50% of the year yields increased by about 20%.
The high organic matter treatment increased the yields in more
than 85% of the years, although by less than 5% in 50% of them.
The effects of the fallow crop treatments differed according to
harvest time. In 50% of the years yields were decreased by
about 10% in the Fallow-15d treatment, less than 3% in the
Fallow-30d treatment, and zero in the Fallow-45d treatment.
The probability that yield would be reduced by growing a
fallow crop was highest when the fallow crop period was
longest (i.e. higher for the Fallow-15d treatment than for the
Fallow-30d and Fallow 45-d treatments). However, soil
compaction was the only treatment for which yields were
reduced in all years, relative to the conventional practice, and
the reductions were substantial—amounting to about 30% in
half of the years.
4. Discussion
The soil management regime strongly influenced the
magnitude of the water balance components. Although this
study considered seven types of management, two had extreme
effects, namely mulch and compaction. Mulching decreased
soil evaporation, increased transpiration and deep percolation,
leading to increased wheat yields and WUE. In contrast,
compaction caused significantly higher soil evaporation, which
led to lower yields and WUE compared with the other
treatments.
Soil evaporation depended on the rates and frequency of
rainfall, atmospheric demand, soil moisture and during periods
with the degree of crop cover (LAI). Generally, soil evaporation
proceeds through two stages (Ritchie, 1972); the amounts of
water evaporated in the first and second stages being
determined by atmospheric evaporative demand and soil
hydraulic properties, respectively. Mulch significantly reduces
317
soil evaporation in the first stage (Steiner, 1989; Ji and Unger,
2001). In the present study, fallow rainfall accounted for more
than 60% of the yearly precipitation, hence the soil evaporation
was often in the first stage. During the wheat season soil
evaporation was often in the second stage because the surface
layer was very dry due to water uptake by the crop and the lower
rates of precipitation during that season. Therefore, the ratios of
soil evaporation to potential evaporation were similar between
mulching and conventional practice (data not shown). On an
annual basis mulching reduced soil evaporation by 12%, on
average, compared with the conventional practice. Consequently, the fallow efficiency was significantly improved; more
water was available for wheat transpiration and (thus) both
wheat yield and WUE increased. The potential of mulching to
increase soil organic matter, which has lower unsaturated
conductivity than the other soil components (Zhang et al.,
2006c), should further decrease soil evaporation, as shown by
the data for the high organic matter treatment (Table 2). In
contrast, soil compaction adversely affects the pore size
distribution and increases unsaturated hydraulic conductivity,
thereby enhancing soil evaporation in both the first and second
stages (Tamari, 1994). In our simulations this resulted in higher
maximum and minimum evaporation relative to the conventional treatment, causing the wheat to be subjected to more
water stress, and thus reduced yields. This conflicts somewhat
with the results of a short-term field study of heavy textured
soils by Radford et al. (2000), who found that soil compaction
reduced wheat emergence, but not the yield. In the present longterm simulation, alleviation of soil compaction by shrinking
and swelling cycles between rainfall events was not considered.
Furthermore, under field conditions field operations can loosen
the topsoil and water stress may be less severe than what the
simulation suggested. Therefore, results of the present
simulation might somewhat overestimate the side-effects of
soil compaction. On the other hand our simulations did not
account for eventual reduced root development due to soil
compaction.
The simulations indicate that the value of using fallow crops
depends on the duration of fallow crop growth. The outcome of
the Fallow-45d treatment was similar to that of conventional
practice for all of the water balance, fallow efficiency, wheat
yield and WUE terms. Although the Fallow-30d treatment
resulted in significantly lower soil evaporation and WUEb, the
fallow crop transpiration was higher than in Fallow-45d. The
Fallow-15d treatment significantly decreased the fallow
efficiency, more during a dry year (minimum value) and less
during a wet year (maximum value) (Table 3). This adverse
effect was not fully compensated by a reduction in soil
evaporation during the fallow crop period and, consequently,
less water was available for wheat transpiration (Table 2).
Consequently, wheat yield and WUE were lower than for
conventional practice. The negative effects of long fallow crop
periods are in agreement with the findings of Vigil and Nielsen
(1998).
Deep percolation is a crucial component of the water balance
in the Loess Plateau region. Short-term investigations have
found a dry subsurface-layer in the soil profile of agricultural
318
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
land (Huang et al., 2002) and in a long-term (15-year)
experiment it was found that less soil water replenishment
occurred in plots subjected to high current levels of fertilization
than in unfertilized control plots during fallow periods (Huang
et al., 2003). These results imply that conventional practice
reduces the frequency and extent of deep percolation. Our longterm simulations indicate that during the 1960s, 1970s and
1980s, the mean annual deep percolation amounted to 26, 33
and 25 mm, respectively, under conventional practice. However, in the last 10 years (1991–2000) annual mean deep
percolation amounted to only about 1 mm. Mulching nearly
doubled the probability of deep percolation occurring
compared to conventional practice in our simulations
(Fig. 3), and approximately tripled the quantities of water
involved (Table 2). This is because mulch reduces soil
evaporation by changing the surface energy balance (Horton
et al., 1996), favouring rainfall infiltration (Baumhardt and
Lascano, 1996), transporting more water to deeper soil layers
and possibly recharging groundwater. Moreover, deep percolation has the potential to maintain higher crop yields by
buffering against a following drought year. For example, high
rainfall in 1989 generated deep percolation which maintained
the yield in 1990 when the precipitation was low. Hence, one
advantage of mulching is that it could help conserve water in
irrigated areas. Furthermore, mulching reduced the amount of
precipitation required to generate deep percolation compared to
the conventional and compacted treatments (especially the
latter). Therefore, soils compaction, by the use of heavy
machinery for example, can be detrimental to a sustainable
hydrological cycle.
The huge temporal variability in deep percolation in
dryland cropping systems (Fig. 2) indicated that use of longterm data is important for estimating water balance
components, either in simulations or by empirical measurements. Even with mulching, which resulted in the most
frequent deep percolation, there was still no deep percolation
in 30% of the years. In such cases short-term experiments can
easily give biased estimates of long-term water balance
performance because of the likelihood that the period
considered will be unrepresentatively wet or dry. Furthermore,
in this study we assumed a condition with no nutrient
(nitrogen) limitation. It should be similar to the conditions in
the study by Huang et al. (2003) where high levels of
fertilization were used and a dry soil layer was formed. The
interaction between nutrient and water is an important issue,
but out of the scope of this paper.
5. Conclusions
In the long-term, mulching improved the partitioning of the
water balance between different components compared with
conventional practice and increased both the winter wheat yield
and WUE. Thus, mulching in the winter wheat-summer fallow
system could be a sustainable management strategy in the
Loess Plateau, China. In contrast, soil compaction generated
the most unproductive water balance. Considering all of the
factors involved, the duration of the bare fallow should not be
less than 30 days (Fallow-30d or Fallow-45d) to maximise the
benefits of producing green manure or fodder without
interfering significantly with the wheat yield.
Acknowledgements
We thank John Blackwell for linguistic improvements. This
study was part of a joint project between the Swedish
University of Agricultural Sciences and Northwest A&F
University of Agriculture and Forestry in China, funded by
the Swedish International Cooperation Development Agency
(INEC-KTS/453/01) and project from Sida/SAREC (SWE2002-038).
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www.elsevier.com/locate/fcr
Simulated long-term effects of different soil management regimes
on the water balance in the Loess Plateau, China
Shulan Zhang a,c,*, Elisabeth Simelton b, Lars Lo¨vdahl c, Harald Grip c, Deliang Chen b,d
b
a
College of Resources and Environmental Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, China
Department of Physical Geography, Earth Sciences Centre, Gothenburg University, Box 460, SE-405 30 Gothenburg, Sweden
c
Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea, Sweden
d
National Climate Centre, Beijing, China
Received 11 January 2006; received in revised form 18 August 2006; accepted 19 August 2006
Abstract
A soil management regime that improves water use efficiency (WUE) is urgently required to increase the sustainability of the winter wheatsummer fallow system in the Loess Plateau, China. However, the long-term partitioning of the water balance must be understood in order to
evaluate the viability of possible soil management regimes. Therefore, an ecosystem model (CoupModel) was used to explore the effects on
components of the water balance of five types of soil management regimes: conventional practice, wheat straw mulching, incorporation of high
organic matter contents, compaction, and use of a harvested fallow crop. Three variants of the fallow crop approach were also considered, in which
the crop was harvested 15, 30 and 45 days before sowing the wheat (designated Fallow-15d, Fallow-30d and Fallow-45d, respectively). Simulations
were used to identify the relative magnitude of soil evaporation, wheat transpiration and deep percolation and to elucidate the temporal variability
in these components for a selected location using climate records spanning 45 years. However, the soil management regime significantly influenced
the magnitude of every component of the water balance (in terms of minimum, maximum and mean values) over the long periods considered.
Consequently, wheat yield and WUE differed significantly among the simulated treatments. Mulching led to significantly lower soil evaporation,
higher transpiration, and more frequent and extensive deep percolation than other regimes, thereby improving fallow efficiency (percentage of
rainfall stored in the soil during the fallow period at the end of the fallow period), wheat yields and WUE. In contrast, soil compaction gave the
opposite results, leading to the most unfavourable partitioning of the water balance reflected in the lowest wheat yield and WUE values of all the
regimes. In 90% of the years no deep percolation occurred in the soil compaction simulations. Use of a fallow crop with optimal harvest timing (Fallow30d) improved partitioning of the water balance (decreased soil evaporation) and did not significantly reduce wheat yield compared with conventional
practice. High organic matter contents in the soil also had a positive influence on the water balance and improved wheat yield and WUE relative to
conventional practice. Therefore, mulching appears to be the best management practice for the winter wheat-summer fallow system in the Loess
Plateau, according to the simulations. Increasing soil organic matter may be the best option if mulching cannot be implemented. The ideal time for
harvesting a fallow crop for use as green manure or fodder appears to be ca. 30 days before sowing the winter wheat.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Long-term effects; Mulching; Soil compaction; Fallow crop; Deep percolation
1. Introduction
Water shortages pose the greatest threat to dryland farming
in semiarid areas. In China, 14.7 million ha of arable land can
be categorized as semiarid, mainly distributed in the Chinese
Loess Plateau, the world’s largest Loess Plateau (Li and Xiao,
* Corresponding author at: College of Resources and Environmental
Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, China.
Tel.: +86 29 87088120.
E-mail address: zhangshulan@nwsuaf.edu.cn (S. Zhang).
0378-4290/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.fcr.2006.08.006
1992). The main crop on a large part of the Loess Plateau is
winter wheat, conventionally cultivated with a single crop
being produced per year, followed by about three months
summer fallow. The fallow period falls in the rainy season, in
which water can be stored in the soil and used by the following
wheat crop. During the past 20 years both fertilizer applications
and wheat yields have increased, resulting in increased soilwater depletion (Huang et al., 2003). Consequently, soil water
is not being fully replenished during the fallow, and at sites
where a dry subsoil layer has formed the crop yield varies
strongly with rainfall during the growing season (Li, 2001). The
dry soil layer, where water content is close to the wilting point,
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S. Zhang et al. / Field Crops Research 100 (2007) 311–319
may be replenished with plant available water during the fallow
period if sufficient rain falls to penetrate the soil profile.
However, the conventional practice of keeping the soil bare
during the fallow period results in very low precipitation
storage efficiency or fallow efficiency (ratio of stored water to
rainfall during fallow) (Li and Xiao, 1992; Latta and O’Leary,
2003). Furthermore, the infiltration depth is shallower under
high fertilization than low fertilization conditions (Huang et al.,
2002). Therefore, the conventional practice with high
fertilization does not appear to be a sustainable management
option in the long-term.
One way to increase water storage is to retain crop residues
on the soil surface. Mulching can be an effective measure for
reducing soil evaporation and increasing water storage (Unger,
1978; Steiner, 1989; Baumhardt and Jones, 2002). Furthermore,
short-term studies have shown that mulching is beneficial for
water storage and crop yields in the Loess Plateau (Wang et al.,
2001; Huang et al., 2005; Zhang et al., 2006a). Zhang et al.
(2006a) also found that deep percolation occurred earlier and
more extensively under mulching than under conventional
practice, which should favour the recovery of the soil water
content of the dry sub-layer. However, mulching has not always
been shown to increase crop yields, and its effectiveness
depends on crop, soil and climate (Wicks et al., 1994; Gajri
et al., 1994). In addition, mulching has not been widely
practiced in China for two practical reasons: (1) the presence of
mulch reduces the quality of wheat sowing by standard
machines and (2) mulch material is also needed for animal food
stuff and/or fuel.
Another way to improve water storage is to change the
hydraulic properties of the soil in a way that increases rain
infiltration and decreases soil evaporation. Increasing the soil
organic matter content has been found to meet these objectives
(Unger and Stewart, 1974). Unfortunately, the most rapid and
convenient way to increase soil organic matter – applying
animal manure – cannot be implemented in this region in
practice, because such resources are very limited. Hence,
growing a crop during the fallow period, and ploughing it in as
green manure at an appropriate time before the next crop, might
be a more viable strategy to increase soil organic matter
contents and hence enhance fallow efficiency. However, various
short-term studies have shown somewhat inconsistent results.
After testing different crop rotation systems, Li et al. (2000)
reported that growing a fallow crop for forage does not greatly
influence the quantity of water stored in the soil for use by
subsequent winter wheat crops in the middle-west Loess
Plateau. Vigil and Nielsen (1998) found that wheat yields
following the application of green legume manure were lower
than those obtained with traditional summer fallow in a 2-year
study. In a 3-year study, use of a fallow crop as green manure
reduced the yield of the following winter wheat in a relatively
dry year compared with conventional practice, but did not
reduce the yield in a wet year (Zhang et al., 2006a). Therefore,
interpreting these direct measurements and assessing the value
of the examined management strategies is complicated by yearto-year variations in both the total amount and the seasonal
distribution of rainfall (Asseng et al., 2001).
In the Loess Plateau region there is an increasingly urgent
need not only to improve water use by the crops but also to
elucidate side-effects caused by modern mechanised operations
in the field. Soil compaction has been recognized as one of the
most serious factors promoting soil degradation in the world
(Oldeman et al., 1991). A laboratory study has shown that soil
compaction greatly affects the hydraulic properties of silt loam
soil from the Loess Plateau (Zhang et al., 2006b); saturated
hydraulic conductivity in compacted soil amounted to less than
15% of that in non-compacted soil. However, there is little
information on the effects of compaction on crop yields.
Due to the inter-annual climate variability, only long-term
analysis is likely to give clear indications of the risks associated
with management effects on crop yield and valid measures of
the water balance components involved (Keating et al., 2002).
Simulations are, therefore, powerful tools for extrapolating
short-term experimental results and for analyzing how different
measures are likely to influence production and the water
balance across the range of climatic conditions within a given
location. In the study presented here, simulations were applied
to identify the relative magnitude of the effects of different soil
management strategies on key water balance components (such
as transpiration, soil evaporation, and deep percolation). For
this purpose we explored the temporal variability in these
variables for the winter wheat zone in the Loess Plateau using
45 years of meteorological data. We also identified the impact
of the duration of fallow crop growth (forage or green manure)
on subsequent wheat yields. The overall purpose was to identify
a sustainable management strategy for agricultural production
in the Loess Plateau and similar regions.
2. Materials and methods
2.1. Model description
The CoupModel is a physically based one-dimensional
model to simulate fluxes of water, energy, carbon and nitrogen
in the soil–plant–atmosphere system, which incorporates both
the former SOIL (Jansson and Halldin, 1979) and SOIL-N
(Eckersten et al., 1998) models. A detailed description of the
model was given by Jansson and Karlberg (2004).
The model predicts water and heat flows between soil layers,
processes at the surface and the base of the profile including
runoff, infiltration, and deep percolation. In addition, evapotranspiration, surface energy balance, heat storage, soil frost
and snow dynamics are calculated. The water and heat flow are
described by Richard’s (1931) solution of Darcy’s law coupled
with Fourier’s law. Calculation of soil evaporation was based on
solving the soil surface energy balance while plant transpiration
and evaporation of intercepted water are based on the Penman–
Monteith equation (Monteith, 1965). Deep percolation will
appear in the simulations when water is transmitted beyond the
bottom layer of the soil profile (in our case at 2.4 m depth).
Plant growth, based on a radiation use efficiency approach, is
calculated as the potential yield which is proportional to the
amount of global radiation intercepted by the canopy, as
estimated using Beer’s law. Actual growth is the potential
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S. Zhang et al. / Field Crops Research 100 (2007) 311–319
growth reduced by linear response functions for unsuitable air
temperature, nitrogen availability and water stress. No
reduction from pests or nutrients deficiency (except nitrogen)
was included. In this simulation nitrogen is assumed to be
sufficient, thus plant growth was not reduced because of
nitrogen supply. The daily total assimilation is translocated to a
temporary storage pool, and then is partitioned to the roots,
leaves and stem. Grain development started according to air
temperature sum, and assimilates to grain were translocated
from leaf, stem and root, respectively.
2.2. Model calibration and long-term application
The model was first calibrated against a 3-year winter wheat
field experiment in the Loess Plateau by Zhang et al. (2006a)
including three different soil management regimes, which were
conventionally managed winter wheat-summer fallow, mulching (a conventionally managed, but unploughed treatment in
which air-dried, unchopped wheat straw (0.8 kg m2) was
evenly distributed over the soil surface and kept at relatively
constant levels), and winter wheat-summer fallow crop.
The long-term simulation was run for the same soil type as
the above field experiment during a 45-year period using daily
weather data at Luochuan (35.82N, 109.50E, 1159 m a.s.l.) as
driving variables. The long-term simulation was run for five
types of soil management regimes totalling seven treatments
(see Table 1). The crop and soil parameters applied were the
same for all treatments as the calibrated values found from the
field experiment, except those hydraulic properties, which were
derived from laboratory studies for soil compaction (Zhang
et al., 2006b) and high organic matter treatments (Zhang et al.,
2006c). In order to investigate the impact from climate
variability the soil properties of each treatment were kept
constant during the simulation period (from 20 September 1955
to 19 September 2000) and no nutrient limitation was
considered.
The simulation was started on 20 September 1955. In the
simulations, the date of sowing winter wheat (Triticum
aestivum L.) was fixed at 20 September for all years and
treatments, while the harvest date was calculated by the model
according to the temperature sum. The fallow crop, black bean
(Aphis fabae), was sown on 16 June every year. The annual
water balance was calculated from previous fallow period until
the end of the following wheat season. The first year (1955) the
fallow period was not included in the simulation. Therefore, the
annual water balance was calculated from summer in 1956 to
summer in 2000, totally for 44 years.
2.3. Analysis methods
2.3.1. Water balance
The field water balance can be written as
(1)
P ¼ E þ T þ D þ R þ DS þ Ei
where P is the precipitation, E the soil evaporation, T the crop
transpiration, R the surface runoff, D the deep percolation
below the root zone, DS the change in soil water storage and
Ei is evaporation from intercepted rainfall. In this study surface
runoff was zero because the topography was flat, and Ei was
neglected because it was quite constant and constituted a very
small proportion of the water balance compared with the other
terms (Zhang et al., 2006a). DS can be either positive or
negative. Therefore, the water balance was calculated as
(2)
P DS ¼ E þ T þ D
2.3.2. Water use efficiency
Water use efficiency (WUEg or WUEb) was defined as
WUE ¼
Y
ET
(3)
where WUEg or WUEb represents the water use efficiency for
the grain or biomass yield (kg m3), Y the grain or biomass
yield of the wheat, respectively, and ET is the evapotranspiration during the wheat season.
2.3.3. Water stress level
The wheat water stress level was expressed as
WSL ¼ 1
Ta
Tp
where WSL is the water stress level, Ta actual transpiration and
Tp is the potential transpiration. The higher the value of WSL
the more severe the water stresses.
Table 1
Description of the different treatments
Treatment
Fallow period (harvest–19 September)
Growing season (20 September–June)
Soil properties
Conventional management
Mulch
Fallow-15d
Fallow-30d
Fallow-45d
HOM (soil with high organic
matter content)
Ploughed bare soil
Wheat straw no summer ploughing
Black bean growing 16 June–5 September
Black bean growing 16 June–20 August
Black bean growing 16 June–5 August
Ploughed bare soil
Winter
Winter
Winter
Winter
Winter
Winter
Compacted
Bare soil
Winter wheat
Zhang et al. (2006d)
As conventional management
As conventional management
As conventional management
As conventional management
Zhang et al. (2006c); soil-OM
3%-units (0–10 cm depth) and
1.5%-unit (10–20 cm depth)
Zhang et al. (2006b); assumed to
increase bulk density by 20% down
to 45 cm depth
wheat
wheat + wheat straw
wheat
wheat
wheat
wheat
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
1
1
0
23
14
7
Maximum
3
9
4
1
2
2
3
b
a
b
c
bc
bc
bc
18
32
20
8
12
15
18
0
0
0
0
0
0
0
156
110
147
277
169
165
161
27
32
27
17
22
24
26
b
a
b
d
c
bc
b
35
45
36
30
35
35
35
15
22
15
10
3
9
11
16
17
16
23
29
22
17
12
7
2
Mean
Mean
Maximum
Minimum
CV
Mean
Maximum
Minimum
CV
Fallow crop transpiration
Wheat transpiration
9
13
9
6
11
10
9
58
43
56
68
47
53
57
84
76
85
90
81
82
86
b
f
bd
a
ce
de
bd
70
58
68
82
62
65
68
CV
Minimum
Maximum
% of total water allocated.
a
CV means coefficient of variation.
b
Different letters in the same column indicate differences that are significant at the P < 0.05 level (LSD).
The different soil management regimes resulted in different
magnitudes of soil evaporation, transpiration and deep
percolation (Table 2). Annual mean soil evaporation comprised
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
3.2. Simulated components of the water balance
b
In the present study the annual precipitation was calculated as
rainfall within the previous fallow period (from wheat harvest to
wheat sowing) plus precipitation during the following wheat
season. Over the 44 years, the annual precipitation varied from
351 to 819 mm, with an average of 568 mm (Fig. 1a). Fallow
rainfalls accounted for 39–85% (average 61%) of the annual
precipitation. Ten-year average annual precipitation was
604 mm in the 1960s, 544 mm in the 1970s, 607 mm in the
1980s and 538 mm in the 1990s. For about 10%, 25% and 20% of
the years the annual precipitation was 700, respectively. Fallow rainfalls were between 300 and
400 mm for nearly 50% of the years (Fig. 1b). Thus, annual
precipitation showed great variability; some years could be very
dry or very wet. The annual average temperature was 9.4 8C,
while the daily maximum and minimum temperatures were 28.2
and 16.4 8C, respectively (data not shown).
Mean
3.1. Climatic conditions
Deep percolation
3. Results
a
In the simulations, measured daily climatic data spanning 45
years (20 September 1955 to 19 September 2000) was used.
The measured variables were precipitation, air temperature,
relative air humidity, wind speed and sunshine hours, which
was used to estimate global radiation. Values were missing for
about 10% of the total number of records for the first variable,
and less than 0.1% for the other four. The data were provided by
the National Climate Centre, Beijing. The missing values were
replaced by zeroes for precipitation and interpolated for the
other variables in the simulations.
Soil evaporation
2.4. Meteorological data
Treatment
Fig. 1. Observed (a) and accumulated probability (b) of annual (fallow + wheat
season) and fallow precipitation from 1957 to 2000.
Table 2
Simulated annual mean (fallow and wheat season) soil evaporation, deep percolation, wheat transpiration and fallow crop transpiration in relation to water balance defined by Eq. (2)
Minimum
49
57
70
CV
314
315
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
respectively. The proportions of the water balance accounted
for by soil evaporation were highest in the soil compaction
treatment (ranging from 68% to 90%) and lowest in the
mulching treatment (43–76%) during the simulation period.
Growing a fallow crop decreased the contribution of annual
mean soil evaporation by 2–8% compared with conventional
practice. The high organic matter treatment resulted in 2%
lower soil evaporation than that in the conventional treatment.
The contribution of wheat transpiration to the water balance
was highest under mulch and lowest in the compaction
treatment. The longer the period with a preceding fallow crop,
the lower the transpiration in the subsequent wheat season
compared with conventional practice, but the maximum
transpiration was the same (35%) and large differences were
found in minimum values (Table 2). The high organic matter
treatment gave similar mean transpiration values to the
conventional treatment, but the maximum value was higher
than that in the conventional treatment. The contributions of
deep percolation to the water balance differed significantly
among treatments; being highest under mulch (9%; ca. two- to
nine-fold higher than in the other treatments), and lowest in the
compaction treatment. Fallow crop treatments decreased deep
percolation to some extent compared with conventional
practice, except Fallow-45d, for which the proportions
(including maximum values) were the same.
3.3. Temporal variability of water balance components
Fig. 2. Simulated temporal variation of soil evaporation (E), transpiration (Tw
for wheat and Tf for fallow crop, respectively) and deep percolation (D) under
various soil management regimes from 1957 to 2000. From the top the
treatments are Fallow-45d (a), Fallow-30d (b), Fallow-15d (c), compaction
(d), mulch (e), high organic matter (HOM, f) and conventional (g).
58–82% of the water balance, regardless of the management
regime, with wheat transpiration the next largest term at 17–
32%. Fallow crop transpiration and deep percolation made up
the remainder of the balance, amounting to 2–12% and 1–9%,
The temporal variability in soil evaporation and wheat
transpiration in each treatment was relatively minor compared
to the temporal variability in deep percolation (Table 2 and
Fig. 2). For all treatments the coefficients of variation were
around 10% for soil evaporation, less than 30% for wheat
transpiration, but higher than 100% for deep percolation. The
accumulated probability showed that for 50% of the years deep
percolation amounted to less than about 30 mm under
mulching, but less than 5 mm for all other treatments
(Fig. 3). Growing a fallow crop affected the amount of deep
percolation to some extent, but did not substantially affect the
probability of its occurrence compared with conventional
practice. The amount of deep percolation significantly
Table 3
Long-term simulated fallow efficiencya under different soil management
regimes (%)
Fig. 3. Accumulated probability of deep percolation occurring under the
conventional, high organic matter (HOM), compaction, mulch, Fallow-15d,
Fallow-30d and Fallow-45d treatments.
Treatments
Mean
Maximum
Minimum
S.D.
Conventional
Mulch
HOMb
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
28
38a
30
19
19
23
27
46
57
48
46
46
46
47
3
10
1
15
29
12
5
11
11
11
11
16
13
12
bc
b
d
d
cd
bc
S.D. refers to standard deviation. Different letters in the same column indicate
differences that are significant at the P < 0.05 level (LSD).
a
Fallow efficiency expresses ratio of stored water to rainfall during fallow
period.
b
Soil with high organic matter content (see Table 1).
316
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
Table 4
Linear correlation coefficients for the relationship between fallow efficiency
and: rainfall throughout the fallow period, rainfall in the last month of the fallow
period and the rest of the time (n = 45)
Treatment
Whole fallow
The last month
Other time
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
0.73
0.66
0.75
0.57
0.70
0.73
0.71
0.59
0.59
0.56
0.71
0.61
0.58
0.56
0.40
0.31
0.43
0.14
0.34
0.40
0.39
Note: all coefficients are significant at the P < 0.01 level except the value of
0.14.
correlated with yearly precipitation, but was most strongly
correlated with fallow rainfall (R2 = 0.46 for conventional
practice, 0.28 and 0.63 for compaction and mulch, respectively). To generate deep percolation 405 mm rain was needed
under mulch conditions, compared to 504 and 589 mm for the
conventional and compaction treatments, respectively.
3.4. Fallow efficiency
The mean ratio of stored water to rainfall during the fallow
period (fallow efficiency) under various treatments ranged from
19% to 38%; the highest value was found in the mulch
treatment and the lowest values in the compaction and Fallow15d treatments (Table 3). In comparison with the conventional
treatment, growing a fallow crop decreased the fallow
efficiency by percentages ranging from 1% to 9%, depending
on the growing duration. High organic matter resulted in 2%
higher fallow efficiency than conventional practice. However,
all treatments except the mulching and high organic matter
treatments had similar maximum fallow efficiencies, but very
different negative minimum fallow efficiencies (ranging from
29% to 3%).
Table 5
Long-term simulated wheat yield (kg m2) and water use efficiency (WUE)
(kg m3) under various treatments
Treatment
Grain
S.D. Biomass S.D. WUEg
S.D. WUEb
S.D.
Conventional
Mulch
HOM
Compaction
Fallow-15d
Fallow-30d
Fallow-45d
0.39
0.44
0.40
0.28
0.35
0.37
0.39
0.09
0.09
0.09
0.09
0.12
0.11
0.10
0.25
0.24
0.26
0.20
0.31
0.28
0.25
0.39
0.36
0.40
0.33
0.60
0.49
0.40
b
a
b
d
c
bc
b
0.84
0.98
0.86
0.59
0.74
0.79
0.83
b
a
b
d
c
bc
b
0.19
0.19
0.19
0.18
0.26
0.24
0.21
1.26
1.41
1.27
0.97
1.09
1.17
1.24
b
a
b
d
c
bc
b
2.68
3.09
2.73
2.00
2.29
2.46
2.62
b
a
b
e
d
cd
bc
S.D. refers to the standard deviation. Different letters in the same column
indicate differences that are significant at the P < 0.05 level (LSD).
Fallow efficiency significantly correlated with fallow
rainfall and rainfall distribution during the fallow period
(Table 4). The lowest correlation coefficient between fallow
efficiency and rainfall throughout the fallow period was for the
compaction treatment (0.57), followed by mulching (0.66) and
the Fallow-15d treatment (0.70); the other treatments had
similar correlation coefficients, all >0.7. However, rainfall in
the last month of the fallow period was also highly correlated
with fallow efficiency, with correlation coefficients of 0.71 for
the compaction treatment, and 0.5–0.6 for all of the other
treatments.
3.5. Water stress probability
The simulations indicated that the wheat was subjected to
some degree of water stress (WSL 0.05) during all years
(Fig. 4). During 50% of the years the water stress level was less
than or equal to 0.19 for the mulch treatment, and 0.29, 0.30,
0.33, 0.34, 0.36 and 0.45 for the high organic matter,
conventional, Fallow-45d, Fallow-30d, Fallow-15d and compaction treatments, respectively. The treatments could be
separated into three groups according to the water stress levels
associated with them: slight stress (the mulching); severe stress
(compaction); and moderate stress (the other treatments). In the
moderate stress group Fallow-15d generated the most severe
water stress.
3.6. Wheat yield and water use efficiency
Fig. 4. Accumulated probability of water stress occurring under the conventional, mulch, high organic matter (HOM), compaction, Fallow-15d, Fallow30d and Fallow-45d treatments.
The simulated average annual wheat biomass yield ranged
from 0.59 to 0.98 kg m2, regardless of treatment effects, and
biomass water use efficiency ranged from 2.00 to 3.09 kg m3
(Table 5). Mulching increased both the grain and biomass
yields, and consequently resulted in higher water use efficiency
than conventional practice. Conversely, the compaction
treatment greatly decreased the wheat yield and resulted in
the lowest WUE values. The treatment in which the fallow crop
was grown for the shortest time (Fallow-45d) gave the same
wheat yield as the conventional treatment, while wheat yields
were reduced under the other two fallow crop treatments, and
the WUE values followed the same trends. The high organic
matter treatment slightly increased wheat yield and WUE.
The accumulated probability of the treatments to increase
wheat biomass yields relative to the conventional practice is
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
Fig. 5. Accumulated probability of a difference in yield increase between
various soil management regimes and conventional practice. The soil management regimes were: high organic matter (HOM), compaction, mulching,
Fallow-15d, Fallow-30d and Fallow-45d.
shown in Fig. 5. Mulching increased the yields in 95% of the
years, and in 50% of the year yields increased by about 20%.
The high organic matter treatment increased the yields in more
than 85% of the years, although by less than 5% in 50% of them.
The effects of the fallow crop treatments differed according to
harvest time. In 50% of the years yields were decreased by
about 10% in the Fallow-15d treatment, less than 3% in the
Fallow-30d treatment, and zero in the Fallow-45d treatment.
The probability that yield would be reduced by growing a
fallow crop was highest when the fallow crop period was
longest (i.e. higher for the Fallow-15d treatment than for the
Fallow-30d and Fallow 45-d treatments). However, soil
compaction was the only treatment for which yields were
reduced in all years, relative to the conventional practice, and
the reductions were substantial—amounting to about 30% in
half of the years.
4. Discussion
The soil management regime strongly influenced the
magnitude of the water balance components. Although this
study considered seven types of management, two had extreme
effects, namely mulch and compaction. Mulching decreased
soil evaporation, increased transpiration and deep percolation,
leading to increased wheat yields and WUE. In contrast,
compaction caused significantly higher soil evaporation, which
led to lower yields and WUE compared with the other
treatments.
Soil evaporation depended on the rates and frequency of
rainfall, atmospheric demand, soil moisture and during periods
with the degree of crop cover (LAI). Generally, soil evaporation
proceeds through two stages (Ritchie, 1972); the amounts of
water evaporated in the first and second stages being
determined by atmospheric evaporative demand and soil
hydraulic properties, respectively. Mulch significantly reduces
317
soil evaporation in the first stage (Steiner, 1989; Ji and Unger,
2001). In the present study, fallow rainfall accounted for more
than 60% of the yearly precipitation, hence the soil evaporation
was often in the first stage. During the wheat season soil
evaporation was often in the second stage because the surface
layer was very dry due to water uptake by the crop and the lower
rates of precipitation during that season. Therefore, the ratios of
soil evaporation to potential evaporation were similar between
mulching and conventional practice (data not shown). On an
annual basis mulching reduced soil evaporation by 12%, on
average, compared with the conventional practice. Consequently, the fallow efficiency was significantly improved; more
water was available for wheat transpiration and (thus) both
wheat yield and WUE increased. The potential of mulching to
increase soil organic matter, which has lower unsaturated
conductivity than the other soil components (Zhang et al.,
2006c), should further decrease soil evaporation, as shown by
the data for the high organic matter treatment (Table 2). In
contrast, soil compaction adversely affects the pore size
distribution and increases unsaturated hydraulic conductivity,
thereby enhancing soil evaporation in both the first and second
stages (Tamari, 1994). In our simulations this resulted in higher
maximum and minimum evaporation relative to the conventional treatment, causing the wheat to be subjected to more
water stress, and thus reduced yields. This conflicts somewhat
with the results of a short-term field study of heavy textured
soils by Radford et al. (2000), who found that soil compaction
reduced wheat emergence, but not the yield. In the present longterm simulation, alleviation of soil compaction by shrinking
and swelling cycles between rainfall events was not considered.
Furthermore, under field conditions field operations can loosen
the topsoil and water stress may be less severe than what the
simulation suggested. Therefore, results of the present
simulation might somewhat overestimate the side-effects of
soil compaction. On the other hand our simulations did not
account for eventual reduced root development due to soil
compaction.
The simulations indicate that the value of using fallow crops
depends on the duration of fallow crop growth. The outcome of
the Fallow-45d treatment was similar to that of conventional
practice for all of the water balance, fallow efficiency, wheat
yield and WUE terms. Although the Fallow-30d treatment
resulted in significantly lower soil evaporation and WUEb, the
fallow crop transpiration was higher than in Fallow-45d. The
Fallow-15d treatment significantly decreased the fallow
efficiency, more during a dry year (minimum value) and less
during a wet year (maximum value) (Table 3). This adverse
effect was not fully compensated by a reduction in soil
evaporation during the fallow crop period and, consequently,
less water was available for wheat transpiration (Table 2).
Consequently, wheat yield and WUE were lower than for
conventional practice. The negative effects of long fallow crop
periods are in agreement with the findings of Vigil and Nielsen
(1998).
Deep percolation is a crucial component of the water balance
in the Loess Plateau region. Short-term investigations have
found a dry subsurface-layer in the soil profile of agricultural
318
S. Zhang et al. / Field Crops Research 100 (2007) 311–319
land (Huang et al., 2002) and in a long-term (15-year)
experiment it was found that less soil water replenishment
occurred in plots subjected to high current levels of fertilization
than in unfertilized control plots during fallow periods (Huang
et al., 2003). These results imply that conventional practice
reduces the frequency and extent of deep percolation. Our longterm simulations indicate that during the 1960s, 1970s and
1980s, the mean annual deep percolation amounted to 26, 33
and 25 mm, respectively, under conventional practice. However, in the last 10 years (1991–2000) annual mean deep
percolation amounted to only about 1 mm. Mulching nearly
doubled the probability of deep percolation occurring
compared to conventional practice in our simulations
(Fig. 3), and approximately tripled the quantities of water
involved (Table 2). This is because mulch reduces soil
evaporation by changing the surface energy balance (Horton
et al., 1996), favouring rainfall infiltration (Baumhardt and
Lascano, 1996), transporting more water to deeper soil layers
and possibly recharging groundwater. Moreover, deep percolation has the potential to maintain higher crop yields by
buffering against a following drought year. For example, high
rainfall in 1989 generated deep percolation which maintained
the yield in 1990 when the precipitation was low. Hence, one
advantage of mulching is that it could help conserve water in
irrigated areas. Furthermore, mulching reduced the amount of
precipitation required to generate deep percolation compared to
the conventional and compacted treatments (especially the
latter). Therefore, soils compaction, by the use of heavy
machinery for example, can be detrimental to a sustainable
hydrological cycle.
The huge temporal variability in deep percolation in
dryland cropping systems (Fig. 2) indicated that use of longterm data is important for estimating water balance
components, either in simulations or by empirical measurements. Even with mulching, which resulted in the most
frequent deep percolation, there was still no deep percolation
in 30% of the years. In such cases short-term experiments can
easily give biased estimates of long-term water balance
performance because of the likelihood that the period
considered will be unrepresentatively wet or dry. Furthermore,
in this study we assumed a condition with no nutrient
(nitrogen) limitation. It should be similar to the conditions in
the study by Huang et al. (2003) where high levels of
fertilization were used and a dry soil layer was formed. The
interaction between nutrient and water is an important issue,
but out of the scope of this paper.
5. Conclusions
In the long-term, mulching improved the partitioning of the
water balance between different components compared with
conventional practice and increased both the winter wheat yield
and WUE. Thus, mulching in the winter wheat-summer fallow
system could be a sustainable management strategy in the
Loess Plateau, China. In contrast, soil compaction generated
the most unproductive water balance. Considering all of the
factors involved, the duration of the bare fallow should not be
less than 30 days (Fallow-30d or Fallow-45d) to maximise the
benefits of producing green manure or fodder without
interfering significantly with the wheat yield.
Acknowledgements
We thank John Blackwell for linguistic improvements. This
study was part of a joint project between the Swedish
University of Agricultural Sciences and Northwest A&F
University of Agriculture and Forestry in China, funded by
the Swedish International Cooperation Development Agency
(INEC-KTS/453/01) and project from Sida/SAREC (SWE2002-038).
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