Introduction
From a
global perspective, arid and semiarid regions account for over 40% of the land
in the world. Dryland farming is the main type of farming in these regions and
plays an important role in food production (Farooq and Siddique 2017).
Water is the limiting factor in dryland farming. Sustainable development of
agriculture requires that water should be used effectively to produce more
yields (Blum
2009). In a typical dryland farming system, evapotranspiration (ET)
is often used to represent field water consumption (Ding et al. 2018). Crop growth requires a certain amount
of water. Generally, the higher the ET is, the higher the yield will be (Stewart and Lal 2018).
However, field water consumption is a complex process. ET can be simply divided
into two parts: soil evaporation (E) and plant transpiration (T). At the same
level of ET, the ratio of E and T can vary greatly (French and Schultz 1984). In
most cases, E is insensitive to yield, and T is the key to crop growth and
yield formation. Hence, in the field, crop yield under the same ET can change
substantially (Grassini et al. 2011; Edreira
et al. 2018), and T/ET can
also vary greatly (van Ittersum and Cassman 2013; Zhou et
al. 2016). Plotting yield against ET results in a scatter plot;
theoretically, regression analysis can be used to determine the best-fit
relationships between yield and ET. Since the data are spread out, the
coefficient of determination is low, and such a relationship does not explain
the complex effect of weather on growth and yield (Sadras
and McDonald 2012). In
that case, French and Schultz (1984) proposed
the concept of a boundary line, where an upper boundary line is fitted to
describe the relationship between yield and ET. Yield data lower than the
boundary line are considered to be limited by factors other than water (Sadras
and Angus 2006; Sadras and McDonald 2012). Such an
approach has been acknowledged by many researchers, and the boundary function
has been used in many regions and on many crops (Sadras and Angus 2006;
Grassini et al. 2009; Zwart et al. 2010). Using the
boundary line as a benchmark indicates whether water is used efficiently or not
at a certain level of water consumption.
The French & Schultz approach also provides a new
way to calculate the potential crop yield (attainable yield per unit of water
use) under water-limited conditions (French and Schultz 1984). Generally, the
yield obtained in the field is lower than the potential yield. Therefore, there
is a yield gap between the attainable yield and the actual yield. The yield gap
of a crop is defined as the difference between the yield under optimum
management practices (potential yield) and the average yield achieved (actual
yield) (van Ittersum and Cassman 2013). Yield gap analysis is a concept that has attracted
increasing attention in recent years (Hatfield and
Beres 2019). It supports decision making in
agricultural development and scientific research. The calculated yield gap
differs based on the yield potential. The potential yield from agricultural
systems can be defined at different levels, including light-limited potential
yield, light- and temperature-limited potential yield, climate-limited
potential yield and so on (Van Ittersum et al. 2013). In previous studies, different methods were applied to calculate the
yield potential and the yield gap. These models include field surveys (Affholder et al. 2013), modelling based on field experiments (Affholder et al. 2013; Meng et al. 2013; van den Berg and Singels
2013) and satellite data (Lobell 2013); among these, crop modelling is the
most popular method for yield gap analysis.
Boundary
line analysis provides a new method of calculating the potential yield. It also
presents a new way to calculate the yield gap. Hence, boundary line analysis
has previously been used in yield gap analysis (Wang et al. 2017; Hajjarpoor et
al. 2018; Lollato et al. 2019).
Yield gap analysis based on boundary function analysis calls for a large amount
of on-farm data to obtain representative results. The Loess Plateau is a
typical dryland farming region. Since 2009, in order to improve the yield and
water use efficiency in this region, our research team has carried out a series
of experiments on dryland winter wheat in Wenxi County in Shanxi Province,
China, and thus has collected a large amount of data. Our experiments have
included different kinds of field management practices, such as fertilization,
soil surface mulching, and tillage. Hence in this research, we intend to take
the Loess Plateau as a case study for using boundary line analysis to analyse
how the dryland winter wheat yield gap is influenced by field management.
Materials and Methods
Site
description
The study was conducted at the experimental station of Shanxi Agricultural University in Wenxi County, Shanxi Province, China (35°20’N, 111°17’E, and elevation 639 m), which is located in the south-eastern part of the Loess Plateau. The soil type at the experimental site is classified as silty clay loam. The basic soil properties in the region at 0–20 cm depth are shown in Table 1. In the drylands of this region, a single crop of winter wheat followed by a summer fallow period is the primary annual cropping system.
The
climate in Wenxi is a temperate continental monsoon climate, with a mean annual
temperature of 12.6°C. All experiments were conducted on drylands. The average
annual precipitation during 2009 and 2017 was 459 mm, 55% of which was
concentrated during the fallow period for winter wheat. In 2010–2011 and 2014–2015,
the proportion of precipitation during the fallow period accounted for as much
as 75 and 71% of the annual precipitation, respectively (Fig. 1). The
mean annual reference evapotranspiration (ET0) is 1840 mm, and the
mean annual sunshine duration is 2460 h.
Experimental information
From 2009 to 2017, a series of experiments related to
improving yield and water use efficiency in Wenxi County in the southern area
of the Loess Plateau were conducted. Based on the treatments, experiments were
classified into four different management types: mulching,
tillage, sowing method and seeding rate. Descriptions of the experiments are
shown in Table 2.
Sampling and calculation
The
soil water content was measured before sowing and after harvest using a
gravimetrical method. A soil auger with a diameter of 5 cm was used to collect
soil samples in the centre of each plot. The sampling interval was 20 cm down
to a depth of 300 cm. Each soil sample was stored in an aluminium specimen box
to resist evaporation and oven-dried at 105°C for 24 h. The ration between the lost weight and dry
soil is soil water content. ET was calculated using Eq. 1:
(Eq. 1)
Where SW0 is the soil water
storage before sowing and SW1 is the soil water storage after
harvest. P is the precipitation during the wheat growth period, R is the soil
surface runoff, and D is the deep percolation. Since the field was flat and the
experimental plots were surrounded by ridges to prevent runoff, R was estimated
to be 0 in this research. The groundwater table was deeper than 50 m in the
research region, and no water percolated to the deep soil layer in our
experimental field. D also tended to be 0. Eq. 1 can be simplified to Eq. 2.
(Eq.
2)
At harvest time, a sample plot of 1 m2
in size in each experiment plot was sampled to determine the grain yield. The
water use efficiency was calculated using Eq. 3.
(Eq.
3)
Statistical analysis
Collected data were analysed using
analysis of variance technique and difference between treatments were compared
using LSD (least significant difference) test at P ≤ 0.05 using SAS 9.0. Graphs were constructed using
Microsoft Excel 2010 and sigmaPlot 12.0.
Table 1: Main soil properties before sowing in 2009
Soil properties |
Soil nutrients |
Contents |
Chemical properties |
Organic matter (g kg-1) |
11.88 |
Total nitrogen (g kg-1) |
0.61 |
|
Alkali-hydrolysis
nitrogen (mg kg-1) |
38.62 |
|
Available phosphorous
(mg kg-1) |
14.61 |
|
Available potassium (mg kg-1) |
238.16 |
|
pH |
8.08 |
|
Physical properties |
Bulk density (g cm-3) |
1.38 |
Soil porosity (%) |
45.35 |
|
Capillary porosity
(%) |
38.78 |
|
Field water holding capacity (v/v) (%) |
30.00 |
Table 2: Descriptions of the experiments from 2009 to 2017 in
Wenxi County
Management type |
Treatments |
Description |
Years |
Sample number |
Mulch |
M |
0.02 mm thick plastic film was applied during
the fallow period to prevent soil surface evaporation |
2009-2014 |
67 |
NM |
No plastic film was used |
2009-2014 |
29 |
|
Tillage |
NT |
The land was left untilled after harvest |
2010-2017 |
19 |
DP |
In the mid-July after a rainfall event, the
soil was ploughed to a depth of 25-30 cm with a tractor |
2009-2017 |
71 |
|
SS |
In the mid-July after a rainfall event, subsoiling to a depth of 30-40 cm was performed with a
tractor |
2009-2016 |
60 |
|
Sowing methods |
DS |
Drill sowing without plastic film mulch |
2011-2017 |
44 |
P-DS |
Drill sowing on the edges of plastic film
(Plastic film covered soil surface until anthesis) |
2011-2017 |
46 |
|
Seeding rate |
H |
112.5 to 120 kg ha-1 |
2012-2017 |
17 |
M |
90 to 105 kg ha-1 to 105 kg |
2012-2017 |
32 |
|
L |
60 to 75 kg ha-1 |
2012-2017 |
22 |
Note that sample number represents the
number of data points for a specific treatment.
M= Plastic film mulch; NM= No mulch; NT= No tillage; DP= Deep plough;
SS= Sub-soiling; DS= Drill sowing; P-DS= Drill sowing on the edges of plastic
film; H= High seeding rate; M= Mid seeding rate; L=
low seeding rate
Table 3: Yield, ET and WUE under
different management practices
Management type |
Practice |
ET
(mm) |
Yield
(kg ha-1) |
WUE
(kg ha-1 mm-1) |
N |
|||
Range |
Mean |
Range |
Mean |
Range |
Mean |
|||
Mulching |
M |
238-787 |
492 a |
2256-6908 |
4943 a |
5.43-15.82 |
10.51 a |
67 |
NM |
206-752 |
412 b |
1925-5816 |
4016 b |
6.16-14.53 |
10.27 a |
29 |
|
Tillage |
NT |
303-716 |
479 b |
2473-5719 |
4226 b |
5.8-11.88 |
9.06 b |
19 |
DP |
206-775 |
474 b |
2381-6628 |
4945 a |
5.97-15.82 |
11.28 a |
71 |
|
SS |
273-770 |
502 a |
1386-6382 |
4101 b |
2.59-13.61 |
8.25 b |
60 |
|
Sowing Method |
DS |
273-736 |
406 b |
1635-5576 |
3995 b |
3.84-16.64 |
10.12 a |
44 |
P-DS |
273-782 |
438 a |
1769-6807 |
4688 a |
4.58-18.40 |
10.91 a |
46 |
|
Seeding Rate |
H |
313-605 |
462 a |
1385-5360 |
3939 b |
2.59-15.65 |
8.95 b |
17 |
M |
312-596 |
428 b |
1803-6807 |
4418 a |
3.52-18.40 |
10.62 a |
32 |
|
L |
297-585 |
440 ab |
1515-5725 |
4236 a |
2.89-17.01 |
10.19 a |
22 |
Values of the same management type sharing
same letters differ non-significantly (P ≤
0.05)
N= Number of samples which represents the number of data points for a specific treatment; ET=
Evapotranspiration; WUE= Water use efficiency; M= Plastic film mulch; NM= No
mulch; NT= No tillage; DP= Deep plough; SS= Sub-soiling; DS= Drill sowing; P-DS=
Drill sowing on the edges of plastic film; H= High seeding rate; M= Mid seeding rate; L= low seeding rate
Results
Water
use, yield and WUE under different management practices
Since the experiments were carried out in different years
and both precipitation and temperature varied among the years, the ET, yield and WUE of the crops varied greatly (Table 3). Under
plastic film mulching, the mean ET was 492 mm, which is 80 mm higher than that
under no mulching. The wheat yield from mulching was 927 kg ha-1
higher than that from NM. Moreover, the lower limit and higher limit of yield
under M also increased. The WUE was calculated as the ratio of yield to ET. The
results showed that the WUE under M was to the same as that of NM.
Among the three tillage methods, SS resulted in the highest ET (502 mm), while the
average ET for NT and DP was almost the same (479 and 474 mm, respectively). The mean yield was the highest for DP among the tillage
methods. The WUE for NT, DP and SS was 9.06 kg ha-1, 11.28 kg
ha-1 and 8.25 kg ha-1, respectively.
Compared with DS, P-DS increased ET slightly (32 mm on average). The yield under P-DS was 4688 kg ha-1 on average, 692 kg
ha-1 higher than the 3995 kg ha-1 yield
under DS. The higher yield led to a higher WUE in
P-DS. The mean yield was highest at the
moderate seeding rate, with a value of 4418 kg ha-1. With the increase in seeding rate, the ET increased. A
higher ET did not lead to
higher yield. The WUE among the different seeding rates showed the opposite trend as yield, i.e., the WUE was highest
at the moderate seeding rate and lowest at the high seeding rate.
Fig. 1: Precipitation during wheat growth years from 2009 to
2017. The fallow period represents the time from the harvest of the previous
season to the sowing of the current season.
Fig. 2: Yield, ET
and WUE at different fertilizer application rates
Phosphatic fertilizer application rate is calculated as
the weight of P2O5 and nitrogen fertilizer application
rate is calculated as the weight of pure nitrogen
ET= Evapotranspiration; WUE= Water use efficiency
Wheat yield
first increased with increasing fertilizer input and reached a peak before
dropping again (Fig. 2).
The yield peaked around the application rates of 150 and 200 kg ha-1
for N and P, respectively. ET increased slightly as the N application rate
increased, but there was no evidence for the impact of P application on ET in
this research. With the increase in P application, WUE showed a similar trend
as yield, and the peak value for WUE also occurred at approximately 200 kg ha-1.
Boundary
analysis of the Y-ET relationship
The observed experimental data from 2009
to 2017 showed that during the experimental years, ET and yield varied greatly.
Even at the same level of ET, a broad range of wheat yields were obtained.
However, when all the data were plotted on a scatter diagram, a clear boundary
emerged (Fig. 3). Based on the
boundary function concept proposed by French and Schultz (French and Schultz 1984) and
the improved boundary function establishment method developed by Lin and Liu (2016), we
obtained the winter wheat boundary function for yield-ET at our research site.
When ET is below 393 mm, the water-limited potential yield can be calculated as
yield = 23.6 (ET-104.5). When ET is above 393 mm, the potential yield reached a
plateau at 6807 kg ha-1
and no longer changed with the increase in ET. The boundary line also shows
that when ET < 393 mm, the potential yield and WUE increase as ET increases;
when ET > 393 mm, the potential remains stable, while the potential WUE
decreases as ET increases. The potential WUE (WUEmax, dashed line in
Fig. 3) shows that
when ET<393 mm, WUEmax increases as ET increases, while when
ET>393 mm, WUEmax decreases as ET increases. When ET>393 mm, the
dashed line is a hyperbolic curve with a slope equal to the slope of the
boundary line (23.6).
Yield
gap analysis
The gap analysis showed that even
though 393 mm is a threshold for water use in this dryland farming area, the
yield gap was lower for the lower water
consumption group than for the higher water use
group (Fig. 4). For the mulching method, the yield gaps for M and NM were 1249 and 1280 kg ha-1, respectively, which means that the plastic film mulching
did not have a significant influence on the yield gap. Similar results were found with the sowing method. The yield gaps for DS and P-DS were 1971 and 1511 kg ha-1, respectively. The yield gap among different tillage methods varied greatly. DP showed the lowest
yield, with a value of 935 on average. The mean yield gaps for NT and SS were similar, with values of 2027 kg ha-1 and 2426 kg
ha-1, respectively.
Compared with those under NT and DP, the yield gap of SS varied greatly,
ranging from 330 kg ha-1 to 5326 kg ha-1.
Fig. 3: Yield-ET relationship within a boundary framework
(the solid line represents the upper boundary of yield, and the dashed line
represents the corresponding WUE of the upper boundary). ET is
evapotranspiration, WUE is water use efficiency
Fig. 4: Yield gap under different
management methods
The dots to the
right of each box represent the distribution of the data
M= Plastic film mulch; NM= No mulch; NT= No
tillage; DP= Deep plough; SS= Sub-soiling; DS= Drill sowing; P-DS= Drill sowing
on the edges of plastic film
Fig. 5: Yield gap under different
fertilizer application rates
Phosphatic fertilizer application rate is calculated as
the weight of P2O5 and nitrogen fertilizer application
rate is calculated as the weight of pure nitrogen
To analyse the impact of fertilization on the yield gap,
we further analysed the yield gap under different nitrogen and phosphate
application rates (Fig. 5). The results showed that with
the increase in the N fertilization application rate, the yield gap showed a
decreasing trend and reached its lowest value of 928 kg ha-1 at 180
kg ha-1, after which it increased dramatically to 1651 kg ha-1
when the N application rate increased to 210 kg ha-1. A similar
trend was found for P application. The yield gap dropped from 2441 kg ha-1
to 1062 kg ha-1 and reached its lowest value when the P application
rate increased from 0 to 150 kg ha-1. When the P application rate
rose from 150 to 375 kg ha-1, the yield gap increased.
Discussion
In this research, we used field
experimental data from a dryland farming system and drew an upper boundary in
the yield-ET plot based on the French & Schultz framework as improved by
Lin and Liu (2016). This upper boundary represents the on-farm potential
yield under water-limited conditions. Generally, given a specific ET, the
actual yield is lower than the potential yield. The gap between these two yield
values is the yield gap. Such a
boundary approach was used widely in the agricultural research.
In 2013, Zhang et al. (2013) collected the experimental yield and water use data
for winter wheat on the Loess Plateau. In that research, Zhang suggested that
the boundary function yield = 22 (ET-60) from Sadras and Angus (2006)
reflects the situation on the Loess Plateau. In this research, the function of
the boundary line was yield = 23.6 (ET-104.5), which is similar to those
results. However, differences still exist. Our research showed a higher slope
and a higher intercept. A higher slope indicates higher potential yield in this
region, and a higher ET intercept means that more water is needed to obtain the
yield. In Sadras' research, the boundary line represents the conditions in 4
mega-environments. In this study, all the data were obtained from a single site
that represents the precise conditions in Wenxi County on the southern Loess
Plateau, where the annual precipitation is 459 mm. Lower precipitations usually
leads to high potential evapotranspiration (ET0). A high ET0
means that water is prone to loss due to soil evaporation; hence, the ET
intercept is higher than that in other regions. The difference between our
results and those of most previous boundary analysis studies is that the
boundary line in this study reaches a plateau (6807 kg ha-1 when ET ≥
393 mm). This means that in dryland farming, crop yield does not continuously
increase with the increase in ET. When < 393 mm, water is
the limiting factor that prevents the achievement of maximum yield.
Appropriate field management practices help to make full use of the limited
water and obtain the maximum yield under a specific water input level. When ET ≥ 393 mm, water is no longer the limiting factor for
the maximum yield. During this stage, effective methods should be
applied to reduce soil evaporation and hence reduce ET to improve WUE.
The results in this study showed that
the impact of agronomy management practices on the yield, yield gap and WUE
differed greatly. Plastic film mulching increased grain yield but also
increased ET and showed little impact on WUE and yield gap. It seems that this
is not an effective way to improve water use in this region. However, the
plastic film was laid down during the fallow period, when a large proportion of
the annual precipitation occurred (Fig. 1). Plastic film mulch reduces soil
evaporation and conserves water in the soil (Li et al. 2013). Previous studies have shown that under plastic film
mulching, soil water storage in the 0–300 cm soil layer increased by 0.37–9.75%
in wet years and by 1.83–16.07% in dry years (Li et al. 2018). As more water was in reach of crop roots, more water
was consumed during this growth period. In our study, the increased proportion
of ET and the increase in yield were similar, which led to almost no change in
the WUE. According to the study of Blum (2009),
the effective use of water, rather than the WUE, is the target for crop yield
improvement under drought stress. In terms of the yield gap under PM, though PM
led to little improvement in the yield gap, it improved the yield level. A low
yield gap with a high yield level is the optimum result, but an unchanged yield
gap with an improved yield level is also considered a success in field
management.
Among the three tillage methods, DP led
to the highest average yield during the experiments (Table 2). Moreover, the
yield gap under DP was also the lowest (Fig. 4). For SS, the average yield and yield
gap were close to those under NT. Compared with that under NT, the yield under
SS showed a wider range (1386–6382 kg ha-1), which means that SS has
the potential to obtain a higher yield at our research site. In fact, under SS,
the soil water infiltration ability improved, and hence, during wet years, more
water was available for crops (Li et al. 2014). However, in dry years, this factor is not significant,
and hence, the yield was not improved. The yield gap analysis shows that under
DP, the lowest yield gap was approximately 800 kg ha-1, which was
lower than that under NT. This result illustrates that, compared with NT, SS has greater potential to reduce the yield gap.
Such a yield gap analysis provides a
new method for field water management. The most important thing is to reduce
yield gap, increase WUE to make “more crop per drop” (Blum 2009).
However, if WUE is used as the single standard for benchmarking crop yield, it
will lead to mistakes in field management. In Fig. 3, if we draw a line that goes through
the origin and passes through as many dots as possible, all the dots on the
line will have the same WUE. However, crop WUE increased with increasing
drought stress and reduced water supply (Stewart
and Lal 2018); hence, low ET may also lead to high
WUE. Low ET with low yield will not provide the farmer with enough food,
despite the high WUE. The yield gap for a particular ET provides an additional
way to benchmark crop management (Sadras
and Angus 2006). For a given
ET, the yield gap illustrates how high of a yield is high enough. The yield gap
also indicates how much the yield can be improved. If a yield gap exists, it
means that the water was not used effectively and that there must be
constraints other than water (Grassini et al. 2009).
At last, it should be noted that,
though yield helps to benchmark field water use in dryland faming, it does not
replace WUE. The shortcoming of the yield gap is that at the same yield gap
value, the ET varies greatly. On the Loess Plateau in this study, at 200 mm of
ET with a yield of 1754 kg ha-1 and at 300 mm ET with a yield of
4114 kg ha-1, the yield gaps were both 500 kg ha-1.
Clearly, 300 mm ET with 4114 kg ha-1 yield is more acceptable for
farmers and researchers.
Conclusion
Appropriate field management can improve yields and reduce yield gaps.
Plastic film mulch in the fallow period and drilling sowing without plastic
film mulch increased wheat yield but had little impact on yield gap and WUE.
Deep ploughing increased wheat yield and WUE and reduced the yield gap.
Subsoiling had little impact on yield, WUE and yield, but it increased the
possibility of obtaining a higher yield and reduced the yield gap. Subsoiling
is a more effective technique in wet years than in dry years. Yield gap
analysis provides a supplemental method for evaluating water productivity in
dryland farming. At the same level of WUE, a higher yield gap indicates a
higher yield. With the same yield gap, the higher the WUE is, the higher the
yield will be.
Acknowledgements
We gratefully acknowledge the anonymous reviewers for their
constructive comments. This work was funded by Science & Technology
Innovation Foundation of Shanxi Agricultural University (2017YJ24), Outstanding
Doctor Funding Award of Shanxi Province (SXYBKY201749). This work was also
supported by the earmarked fund for China Agriculture Research System
(CARS-03-01-24), the National Natural Science Foundation of China (31771727).
Author Contributions
Conceived and designed the experiments:
Min Sun, Wen Lin and Zhiqiang Gao. Performed the experiments: Hao Li, Jie Zhao,
Aixia Ren. Analysed the data: Hao Li, Jie Zhao, Wen Lin. Wrote the paper: Hao
Li, Jie Zhao. All authors discussed the results and commented on the contents
of the manuscript.
References
Affholder F, C Poeydebat, M
Corbeels, E Scopel, P Tittonell (2013). The yield gap of major food crops in
family agriculture in the tropics: Assessment and analysis through field
surveys and modelling. Field Crops Res
143:106‒118
Blum A (2009). Effective use of
water (EUW) and not water-use efficiency (WUE) is the target of crop yield
improvement under drought stress. Field
Crops Res 112:119‒123
Ding D, Y Zhao, H Feng, RL Hill, X
Chu, T Zhang, J He (2018). Soil water utilization with plastic mulching for a
winter wheat-summer maize rotation system on the Loess Plateau of China. Agric Water Manage 201:246‒257
Edreira JIR, N Guilpart, V Sadras,
KG Cassman, MKVvan Ittersum, RLM Schils, P Grassini (2018). Water productivity of rainfed maize and wheat: A local to global perspective. Agric For Meteorol 259:364‒373
Farooq M, KHM Siddique (2017). Innovations in Dryland Agriculture.
Springer International Publishing AG, Basel, Switzerland
French R, J Schultz (1984). Water
use efficiency of wheat in a Mediterranean-type environment. I. The relation
between yield, water use and climate. Aust
J Agric Res 35:743‒764
Grassini P, J Thorburn, C Burr, KG
Cassman (2011). High-yield irrigated maize in the Western US Corn Belt: I.
On-farm yield, yield potential, and impact of agronomic practices. Field Crops Res 120:142‒150
Grassini P, AJ Hall, JL Mercau
(2009). Benchmarking sunflower water productivity in semiarid environments. Field Crops Res 110:251‒262
Hajjarpoor A, A Soltani, E Zeinali,
H Kashiri, A Aynehband, V Vadez (2018). Using boundary line analysis to assess
the on-farm crop yield gap of wheat. Field
Crops Res 225:64‒73
Hatfield
JL, BL Beres (2019). Yield Gaps in Wheat: Path to Enhancing Productivity. Front Plant Sci 10; Article 1603
Li H, JF Xue, ZQ Gao, NW Xue, ZP
Yang (2018). Response of yield increase for dryland winter wheat to tillage
practice during summer fallow and sowing method in the Loess Plateau of China. J Integr Agric 17:817‒825
Li S, Z Wang, S Li, Y Gao, X Tian
(2013). Effect of plastic sheet mulch, wheat straw mulch, and maize growth on
water loss by evaporation in dryland areas of China. Agric Water Manage 116:39‒49
Li X, M Tang, D Zhang,
W Wang, T Cui (2014). Effects of sub-soiling on soil physical quality and corn
yield. Trans Chin Soc Agric Eng 30:65‒69
Lin W, W Liu (2016). Establishment
and application of spring maize yield to evapotranspiration boundary function
in the Loess Plateau of China. Agric Water Manage 178:345‒349
Lobell DB (2013). The use of
satellite data for crop yield gap analysis. Field
Crops Res 143:56‒64
Lollato RP, DAR Diaz, E DeWolf, M
Knapp, DE Peterson, AK Fritz (2019). Agronomic practices for reducing wheat
yield gaps: a quantitative appraisal of progressive producers. Crop Sci 59:333‒350
Meng Q, P Hou, L Wu, X Chen, Z Cui,
F Zhang (2013). Understanding production potentials and yield gaps in intensive
maize production in China. Field Crops
Res 143:91‒97
Sadras VO, JF Angus (2006).
Benchmarking water-use efficiency of rainfed wheat in dry environments. Aust J Agric Res 57:847‒856
Sadras VO, G McDonald (2012). Water Use Efficiency of Grain Crops in
Australia: Principles, Benchmarks and Management. Grains Research and
Development Corporation, South Australian Research and development Institute
and University of Adelaide, Australia
Stewart B, R Lal (2018). Increasing
world average yields of cereal crops: It's all about water. Adv Agron 151:1‒44
van den Berg M, A Singels (2013).
Modelling and monitoring for strategic yield gap diagnosis in the South African
sugar belt. Field Crops Res 143:143‒150
van Ittersum MK, KG Cassman (2013).
Yield gap analysis—Rationale, methods and applications—Introduction to the special
issue. Field Crops Res 143:1‒3
Van Ittersum MK, KG Cassman, P
Grassini, J Wolf, P Tittonell, Z Hochman (2013). Yield gap analysis with local
to global relevance—a review. Field Crops
Res 143:4‒17
Wang M, L Wang, Z Cui, X Chen, J
Xie, Y Hou (2017). Closing the yield gap and achieving high N use efficiency
and low apparent N losses. Field Crops
Res 209:39‒46
Zhang S, V Sadras, X Chen, F Zhang (2013).
Water use efficiency of dryland wheat in the Loess Plateau in response to soil
and crop management. Field Crops Res
151:9‒18
Zhou S, W Liu, W Lin (2016). The
ratio of transpiration to evapotranspiration in a rainfed maize field on the
Loess Plateau of China. Water Sci Technol
Water Supply 17:221‒228
Zwart
SJ, WGM Bastiaanssen, CD Fraiture,
DJ Molden (2010). A global
benchmark map of water productivity for rainfed and irrigated wheat. Agric Water Manage 97:1617‒1627