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Published online 27 April 2005
Published in Agron J 97:783-790 (2005)
DOI: 10.2134/agronj2004.0194
© 2005 American Society of Agronomy
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Water Use Efficiency

Improved Water Use Efficiency Associated with Cultivars and Agronomic Management in the North China Plain

Xiying Zhang*, Suying Chen, Mengyu Liu, Dong Pei and Hongyong Sun

Shijiazhuang Inst. of Agric. Modernization, The Chinese Academy of Sciences, 286 Huaizhong Rd., Shijiazhuang 050021, China

* Corresponding author (xyzhang{at}ms.sjziam.ac.cn)

Received for publication July 16, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Both winter wheat (Triticum aestivum L.) and maize (Zea mays L.) are the two staple crops of the North China Plain (NCP) that are combined in a single-year rotation. While annual evapotranspiration increased slightly, field studies conducted at Luancheng Station indicated that crop yield improved by 50% and resulted in significant water use efficiency (WUE) increases from 1982 to 2002. Water use efficiency has improved from 10 to 15 kg mm–1 ha–1 for winter wheat and from 14 to 20 kg mm–1 ha–1 for maize in the Piedmont of Mt. Taihang in the NCP. Yield increase was associated with the increase in kernel numbers per unit area without alteration of the weight of the kernels for both winter wheat and maize. Number of kernels per spike of winter wheat was increased from about 22 for cultivars used in 1980s to about 28 for cultivars used presently. Number of kernels per ear of maize was increased from about 300 for cultivars used in 1980s to about 450 presently. From the early 1990s, combine had been used to harvest winter wheat, allowing straw mulch to be applied to maize. Measurements of WUE from 1987 to 1992 and again from 1997 to 2002 showed that WUE of maize under mulch was significantly higher than that without mulch. Mulching reduced soil evaporation loss by 40 to 50 mm per annum measured by microlysimeters, and WUE was averagely improved 8 to 10% for the 12 seasons. An improvement in irrigation scheduling had also improved WUE. Irrigation applications to winter wheat were reduced from about eight times in 1980s to about four times presently. Field tests from 1999 to 2004 still showed that reducing the present number of seasonal wheat irrigations to either three, two, or one depending on seasonal rainfall would benefit both grain production and WUE of winter wheat.

Abbreviations: LAI, leaf area index • NCP, North China Plain • TWU, total water used or evapotranspiration • WUE, water use efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
WITH THE ECONOMIC development of China, there has been an increase in the total volume of water used. The increasing water scarcity and competition from other sectors have put irrigated agriculture under great pressure; grain production is facing an unprecedented challenge (Yang and Zehnder, 2001). The NCP is one of the most important agricultural regions of China with an area of 320000 km2 and being home to more than 200 million people. The NCP now supplies more than 50% of the nation's wheat and 33% of its maize (Natl. Bureau of Stat. of China, 1999). The NCP comprises the alluvial plains of the Yellow, Huai, and Hai Rivers. The main soil type is a loam of Aeolian origin, which has been relocated by the rivers over geological time. Potential evapotranspiration greatly exceeds the annual precipitation of 450 to 650 mm. The monsoon climate dominates the region, and the distribution of precipitation is uneven, with more than 70% of the annual rainfall falling from July to September. The water deficit is especially acute during the dry, windy spring season. For centuries, farmers accommodated the deficit by producing only two to three crops every 2 yr (Yang, 1991). With the development in irrigation facilities, irrigated area was increased, and production had increased to two crops every year since the late 1970s. The double-cropping system is largely based on winter wheat and summer maize, both named after the season in which they are planted. Irrigation is essential for the practice of this multiple cropping, especially to winter wheat. In the NCP, 64% of irrigated areas rely on groundwater (Yang and Zehnder, 2001). The massive extraction of groundwater in the NCP has led to a rapid decline in the groundwater table. Groundwater levels are declining more than 1 m annually, and in some places, land subsiding has occurred (Kendy et al., 2003). In addition, there has been an increase in urban and industrial water use, leading to water shortages in most places of the NCP. To cope with the water shortage problems in the NCP, it is important to improve WUE of crops to reduce agricultural water use or to guarantee the increase in grain production to meet the demand of the increasing population without greatly increasing water utilization.

Over the last 20 yr, grain production increased linearly, despite climatic effects, in the NCP. The increase in grain yield was attributed to the renewing of varieties to a certain extent (Xu and Zhao, 2001). Brancourt-Hulmel et al. (2003) reported that genetic gains represented 33 to 63% of the national grain yield increase in France. The superiority of modern cultivars of winter wheat was associated with higher kernels per square meter and increased harvest index (Brancourt-Hulmel et al., 2003; Reynolds et al., 1999). The increase in kernels per square meter was mostly due to an increase in the number of kernels per spike (Slafer and Andrade, 1989) or due to increases in both kernels per spike and number of spikes per unit area (Austin and Ford, 1989; Perry and d'Antuono, 1989). Tollenaar and Lee (2002) reported that commercial maize yield in the USA has increased from about 1 Mg ha –1 in the 1930s to about 7 Mg ha–1 in the 1990s. They contended that most of this improvement was the result of genotype and management interaction. Yield improvement was generally accompanied with increase in total water use. But when yield increase was greater than the increase in evapotranspiration, WUE improvement would be achieved. Selecting improved varieties that could produce higher yield, use less water, and have higher tolerance to stress, especially to water stress, would be an important water-saving measure in the NCP.

The water requirements of double cropping of winter wheat and maize exceed 850 mm. Long-term average annual rainfall in the NCP ranges from 450 to 650 mm with 70% falling from July to September, the growing period of maize. During the growing period of winter wheat, rainfall could only meet 25 to 40% of the crop water requirements. Irrigation is essential to the high production of winter wheat. In this region, it had been reported that winter wheat was flood-irrigated over eight times each season to keep the soil wet (Ke et al., 1994). During the 1990s, an improved scientific irrigation-scheduling technique was applied that reduced irrigation applications. In the NCP, presently farmers usually irrigate winter wheat three to five times depending on seasonal rainfall situations. For reducing irrigation water use, it is necessary to examine the possibility of further reducing irrigation application by optimizing the irrigation scheduling. Previous studies have shown that wheat is not equally sensitive to water stress at different growing stages (Zhang et al., 1999). In the NCP, it is reported that the response of winter wheat to water stress varies at different growing stages with the period from stem elongation to milking being particularly sensitive to water stress (Li, 1990). The dormant period of winter wheat is from the end of November to the beginning of March. Farmers have traditionally irrigated winter wheat before this overwintering period. Results showed that this irrigation could be omitted due to its loss to soil evaporation and its effects on increasing the noneffective tillers in spring (Zhang et al., 2003). Crop responses to water stress during different growing stages have practical implications for irrigation scheduling (English and Nakamura, 1989; Ghahraman and Sepaskhah, 1997; Zhang et al., 1999). Further optimizing the irrigation scheduling of winter wheat would also be an important aspect in improving WUE in the NCP.

With the increasing water shortage in the NCP, some other agronomic practices had been adopted by farmers to save water. An example was mulching (Zhang et al., 1994; Chen et al., 2002). Use of vegetative mulch was an effective way to prevent soil evaporation (Li, 1998; Mellouli et al., 2000). At the beginning of the 1990s, widespread use of combines in the region made it possible to mulch maize using wheat straw. One of the greatest challenges for agriculture is to develop technology to improve WUE (Wallace and Batchelor, 1997). There was considerable potential for improvement in WUE through improved agronomic practice and genetic breeding for improved transpiration efficiency (Turner, 1993). Objectives of this study were to examine the changes in water use, grain yield, and WUE of winter wheat and maize from 1982 to 2002 and to analyze the effects of cultivar selection and mulching on WUE improvement in the NCP. Optimizing the irrigation scheduling of winter wheat was performed from 1999 to 2004 to test the possibility of further reducing irrigation application.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Site Description and Long-Term Investigations of Water Use Efficiency
The study was conducted from 1982 to 2002 at Luancheng Agro-Eco Experimental Station of the Chinese Academy of Sciences, which is located in the central part of the plain at the base of Mt. Taihang in the NCP (37°53' N, 114°40' E; 50 m above sea level). The average annual rainfall is 482 mm with 73% falling in June, July, August, and September, the growing season of maize. Rainfall during October to May is about 130 mm and coincides with the growing season of winter wheat. Soil is a moderately well-drained loamy soil with a deep profile that is considered highly suitable for crop production. Average water-holding capacity is 38% (v/v), and wilting point is 13% (v/v) for the 2-m profile.

Winter wheat was generally sown during early October with row spacing of 0.16 m and a density of 300 seeds m–2. Initial tillering begun at the beginning of November and continued through November. There was a long dormant period in December, January, and February. In March, winter wheat begun to recover at the end of the dormancy. The jointing stage was at the beginning of April, followed by booting during the rest of April. The ear emergency and flowering stages occurred at the end of April or beginning of May, followed by the milk stage. Harvesting typically occurred during the first 10 d of June. Chemical fertilizers (N and P) were applied as base fertilizer. Nitrogen was applied again at the jointing stage. Maize was interplanted manually to winter wheat 5 to 7 d before harvest to prolong the growth period. The plant density of maize was 60000-70000 plants ha–1, with a 0.6-m row spacing. At the middle of July, maize was fertilized with N and then harvested at the end of September. At the beginning of October, land was plowed to prepare for winter wheat planting, and the two-crop rotation was repeated. Depending on rainfall amounts during the rainy season, land was irrigated in some dry years to ensure that soil moisture conditions were favorable for sowing wheat.

In this experimental work, surface irrigation was applied using a low-pressure tube water transportation system with a flow meter to record the irrigation applied to each plot. Plots were harvested manually and then threshed using a stationary thresher. Grain was air-dried before recording weights. Before harvest, spike (ear) numbers per unit area were counted, and 40 spikes for winter wheat and 10 ears for maize were collected from each plot to determine kernel numbers per spike (ear). Field experiments were managed similarly to the surrounding local farmland, with collected data being used for the analysis of WUE. Soil volumetric water contents were monitored weekly in 20-cm increments to a depth of 2 m using the neutron meter (IH-II, Didcot Instrument Co., Wallingford, UK) with access tubes installed in the center of the plot. Surface soil moisture (0–20 cm) was measured by taking soil cores. Total water use was identified from initial soil water content minus final soil water content, precipitation, irrigation, runoff, drainage, and capillary rise using the following equation (Zhang et al., 1999):

[1]
where TWU = total water use during duration of crop growth, mm; P = precipitation, mm; I = irrigation, mm; {Delta}W = soil water content when the crop is sown minus that at harvest for the 2-m depth, mm; R = runoff, mm; D = drainage from the root zone (mm), and CR = capillary rise to the root zone, mm. Due to the deep soil profile and large water-holding capacity, runoff was never observed in the field. Drainage and capillary rise were negligible and are not considered further (Zhang et al., 1999). Thus TWU = P + I + {Delta}W was used under our experimental conditions. Rainfall was recorded at a meteorological station about 100 m away from trial plots. Water use efficiency (kg mm–1 ha–1 or kg m–3, kg mm–1 ha–1 = 10 kg m–3) was defined as crop yield divided by total water use:

[2]
where Y is grain yield (kg ha–1).

Table 1 recorded the cultivars used and seasonal rainfall during the study. Crop cultivars changed during the 20-yr study. In addition, irrigation application was based on seasonal rainfall (Table 1). Crop water requirements were met each year with a combination of natural precipitation and irrigation supplies. From 1992, combines began to be used by local farmers to harvest their winter wheat, which allowed the winter wheat straw to remain in the field, and maize was mulched. From 1996, a straw chopper was also introduced, which made it possible to directly return maize straw to the soil after maize was harvested manually. Before these innovations, local farmers burned the crop residue after harvesting the wheat and maize. Mechanization in harvesting and plowing allowed practices to change.


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  		Table 1. Cultivars, irrigation amounts, and seasonal rainfall from 1982 to 2002 for winter wheat and maize grown at Luancheng Station.

 
Mulching Effects on Soil Evaporation and Water Use Efficiency of Maize
When a combine was first introduced in the region, experiments were conducted to compare the WUE of straw-mulched maize and maize without mulching. Water use efficiency of the two treatments was calculated by Eq. [2] and based on recording irrigation, soil water depletion, rainfall, and grain yield from 1987 to 1992. Recently from 1997 to 2002, WUE of mulched and nonmulched maize was further compared. In 1999, a microlysimeter was used to measure soil evaporation occurring in mulched and nonmulched maize. Four locally made microlysimeters (0.12-m diam. and 0.2-m length) were placed in one of the plots of each treatment. The microlysimeters contained small isolated volumes of soil and were mounted flush or slightly above the soil surface (Daamen et al., 1993; Leuning et al., 1994; Liu et al., 2002). They were weighed daily (or more frequently) to determine water loss using an electronic balance with a 0.001-kg precision. To keep the soil moisture within the microlysimeters similar to that surrounding them, the soil in the instruments was changed every 1 to 2 d.

Irrigation Scheduling to Improve Water Use Efficiency of Winter Wheat
From 1999 to 2004, experiments were conducted to omit the irrigation before entering the dormancy stage for wheat and to schedule irrigations after the jointing stage. The seven treatments were (i) two irrigations at the prewintering stage and at jointing (T2); (ii) one irrigation at jointing without the prewinter irrigation (T1N); (iii) three irrigations at prewintering, jointing, and flowering (T3); (iv) two irrigations at the same time only without the prewinter irrigation (T2N); (v) four irrigations at prewintering, jointing, heading, and early milk (T4), which was the same as the surrounding farmland and followed the normal irrigation practice of local farmers; and (vi) three irrigations at the same time only without the prewintering irrigation (T3N). A control treatment was included with no irrigation during the entire growing period (T0). Each treatment had four replicates and encompassed a plot area of 40 m2. Detailed records of irrigation, soil water depletion, rainfall, and grain production were maintained to calculate WUE by using Eq. [2]. Seasonal rainfall conditions for the five growing seasons of winter wheat were shown in Table 2. Compared with the long-term average seasonal rainfall, 1999–2000 and 2000–2001 seasons were dry, especially in 1999–2000 with rainfall less than half of the normal amount for winter wheat. The 2001–2002 season was normal while the 2002–2003 and 2003–2004 seasons were wet. Because of the heavy rainfall in October and November in the 2003–2004 season, the prewintering irrigation was omitted to all treatments. The long shady days in April and May in 2001–2002 and 2002–2003 caused some diseases that reduced the grain yield of winter wheat during those two seasons.


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  		Table 2. Monthly and total rainfall for winter wheat growing seasons from 1999 to 2004 and the long-term average from 1961–2000.

 
Statistical Analysis
All the collected data were analyzed statistically. Standard deviations for each treatment were calculated, and means of treatments for the same season were compared using least significant differences (LSD) for a probability level of P = 0.01. To compare data for different seasons, average data for the same treatment at the same year were compared with other season's results, and standard deviation was calculated for each factor.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Changes of Grain Yield and Water Use Efficiency
Grain production at the Luancheng station has increased along with evapotranspiration (Fig. 1). In many cases, variations in yield were primarily caused by weather though in this irrigated region, weather was not the primary factor. Analysis of data collected at the local meteorological station showed that temperature, solar radiation, humidity, and wind speed were relatively stable from 1982 to the present day. Occasionally long periods of cloudy days reduced grain production. From 1982 to 1990, average annual grain production (winter wheat and maize combined) was approximately 10000 kg ha–1 while presently, it was 15000 kg ha–1. Annual TWU for both winter wheat and maize was approximately 800 mm from 1982 to earlier 1990s while presently, it was about 850 mm. Rate of increase of TWU was not as great as the rate of increase in grain yield, indicating that overall WUE improved (Fig. 2). Average WUE from 1982 to 1990 was about 11.8 kg mm–1 ha–1 for winter wheat and 14.4 kg mm–1 ha–1 for maize. From 1991 to 1996, average WUE of winter wheat and maize was about 13.2 and 17.6 kg mm–1 ha–1, respectively. From 1997 to 2002, WUE values for winter wheat and maize were 14.4 and 20.3 kg mm–1 ha–1, respectively. Results also showed that improvement in WUE from 1982 to 2002 was about 40% for maize and about 20% for winter wheat, indicating that improvement in WUE for maize was greater than that for winter wheat. Improvement in grain production and crop WUE was associated with many factors such as improved cultivars and production practices.



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  		Fig. 1. Grain yield and total water use (evapotranspiration, ET) for winter wheat and maize from 1982 to 2002, measured at Luancheng Station.

 


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  		Fig. 2. Water use efficiency (WUE) for winter wheat and maize from 1982 to 2002, measured at Luancheng Station.

 
Yield and Water Use Efficiency Improvement Associated with Introducing New Cultivars
From 1982 to 2002, five winter wheat cultivars were used (Table 1). Before 1993, yield of the three cultivars used was almost the same. From 1993, introduction of a new cultivar of wheat significantly increased grain yield by 22%. In 1996, the winter wheat cultivar 4185 was introduced and has been used in the study since that time. Its yield has remained at a relatively stable and higher level than ‘Jimai38’ (Table 3). Yield reduction only occurred in 1997–1998 due to long, cloudy days during the grain-filling stage that reduced seed weight. With the increase in grain production, evapotranspiration also increased as shown in Fig. 1. Average seasonal evapotranspiration increased from 400 mm in the 1980s to 450 mm at present. Overall yield improvement from that time until 2002 was 38%, with a corresponding increase in evapotranspiration of about 12%. Analysis of variance for grain yield and WUE showed that there was significant difference among the five cultivars used (Table 4). Thus, WUE and grain yield were partly improved by introducing high-yielding cultivars of winter wheat.


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  		Table 3. Average yield and yield components for different cultivars of winter wheat grown from 1982 to 2002 at Luancheng Station (values after ± are standard deviations).

 

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  		Table 4. Analysis of variance for grain yield and water use efficiency (WUE) by cultivars used for winter wheat and maize from 1982 to 2002.

 
There was a slight difference in spike numbers per unit area. It ranged from 600 to 650 spikes m–2 from 1982 to 2002. Table 3 gives kernel numbers per spike and thousand-kernel weight of the five cultivars. Thousand-kernel weight increased slightly, and it was affected by the weather conditions in May when grain-filling stage occurs. Kernel numbers per spike was significantly improved with the change from the old to the new cultivars in 1993. Average kernel numbers per spike was 21 to 22 in 1980s and improved to 27 to 28 for the two new varieties grown since 1993. Other researchers have shown that winter wheat yield improvement was associated with the increase in kernel numbers per spike (Slafer and Andrade, 1989; Duggan et al., 2000). Improvement in yield components, especially in the kernel numbers per spike, played an important role in yield improvement of winter wheat in this region.

Yield and yield components of maize for the different cultivars used from 1983 to 2002 are shown in Table 5. In relation to the previous cultivar, maize grain production was improved by 27% with introduction of the cultivar Yedan 4 in 1996. Much of this yield improvement was also attributed to increase in kernel numbers per ear. Figure 1 shows that total water consumption for maize during 1983–2002 did not change much while WUE increased by 40% (Fig. 2). Analysis of variance for grain yield and WUE showed that there was significant difference among the five cultivars used (Table 4). Thus, WUE and grain yield were also partly improved by introducing high-yielding cultivars of maize. It can also be seen in Fig. 2 that WUE of maize was better than that for winter wheat, especially during the last 10 yr. Introduction of new maize cultivars since 1996 had a larger impact on maize yield than new winter wheat cultivars have had on wheat yield.


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  		Table 5. Average yield and yield components for different cultivars of maize grown from 1982 to 2002 at Luancheng Station (values after ± are standard deviations).

 
Straw Mulching Effects on Water Use Efficiency of Maize
Changes in management practices also influenced WUE improvement. An important water-saving measure adopted was straw mulch, which conserved soil water. Figure 3 shows daily soil evaporation rate of mulched and nonmulched treatments measured by microlysimeters in 1999. The average soil evaporation rate for mulched treatment was smaller than that of nonmulched treatment, especially in the earlier growth stages of maize when leaf area was low. Average evaporation rate was about 2 and 0.8 mm d–1 for the nonmulched and mulched treatments, respectively, during June and the first half of July. During this period, leaf area index (LAI) for maize was less than 2. As LAI increased, the soil evaporation rate was reduced, especially for the nonmulched treatment. The average soil evaporation rate was about 0.7 and 0.4 mm d–1 for the nonmulched and mulched treatments, respectively, during the second half of July to the middle of September. Soil evaporation occurring under mulch was much less affected by LAI than that occurring without mulching. Average soil evaporation during the season was 0.52 and 1.17 mm d–1 for the mulched and nonmulched treatments, respectively. More than 50 mm of water was prevented from evaporating by the mulch treatment. Without mulching, soil evaporation accounted for 30% of the total evapotranspiration. With mulching, this value was reduced to approximately 15 to 20%.



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  		Fig. 3. Daily soil evaporation under mulch and without mulch, as measured by lysimeters, and leaf area index (LAI) of maize during 1999 season.

 
Comparison of WUE under mulched and nonmulched conditions measured for the period 1987–1992 and again from 1997–2002 showed that mulch significantly improved WUE by 8 to 10% (Fig. 4). Analysis of variance showed the difference of WUE between mulched and nonmulched treatments was significant at p = 0.01 for all the seasons. Yield was improved in some seasons (1987, 1988, 1990, 1997, and 2001) by mulching while during other seasons, there was not significant difference between mulch and nonmulch treatments. The mulch reduced soil evaporation by 40 to 50 mm, and the saved water was used by plants for transpiration in dry periods, explaining why yield of maize was also improved by mulching in some seasons. In early June, when wheat was harvested and maize was planted, very dry soil conditions generally required irrigation for germination and emergence of maize seedlings. During these early stages of vegetative maize growth, there was a rapid loss of water by evaporation due to lack of soil coverage by a plant canopy. Use of mulch significantly reduced soil evaporation and increased WUE.



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  		Fig. 4. Water use efficiency (WUE) of maize with mulch and without mulch during seasons of 1987 to 1992 and 1997 to 2002.

 
Effects of Irrigation Scheduling for Improvement of Water Use Efficiency
Figure 5 gives results from five seasons when yield and WUE for winter wheat were measured for irrigation treatments. This indicates significant yield and WUE responses to the irrigation treatments, and these were not consistent since rainfall conditions differed each season. Comparing nonirrigation with the most irrigated treatments, grain yield was improved by 35% in the dry seasons of 1999–2000 and 2000–2001 and by 15% in the normal season of 2001–2002 and decreased by 1.9 and 1.6% in the wet seasons of 2002–2003 and 2003–2004. Whether it was a dry or wet season, compared with the crop's overall water requirements, rainfall was deficient, and water stored in the soil profile did play an important role in guaranteeing successful winter wheat production in this region. The winter wheat root system generally reached a depth of 2 m (Zhang, 1999, p. 34–39), and the 2-m soil profile had field capacity values corresponding to more than 700 mm of water (Zhang and Yuan, 1995). Before sowing winter wheat, soil moisture was usually at 70 to 80% of field capacity. These values correspond to about 500 to 550 mm of available water capacity for the 2-m soil profile. So, rainfed winter wheat could still produce economic returns, even with minimal seasonal rainfall. Water depletion for the 2-m soil profile in the 1999–2000 and 2000–2001 season was 228.8 and 235.1 mm, respectively, for the rainfed treatment. In the other three seasons, when the seasonal rainfall was normal or higher, the rainfed treatment still consumed 150 mm or even more of soil water. For the irrigated treatments, soil water uptake still represented a considerable proportion of the total water consumed, ranging from 50 to 180 mm over the five seasons. Thus, soil water stored before sowing winter wheat was very important for a high production level of this crop.



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  		Fig. 5. Yield and water use efficiency (WUE) for different winter wheat treatments in 1999–2000, 2000–2001, 2001–2002, 2002–2003, and 2003–2004 at Luancheng Station (A = T0: without irrigation; B = T1N: one irrigation at jointing; C = T2: as T1N plus prewintering; D = T2N: two irrigations at jointing and flowering; E = T3: as T2N plus the prewintering irrigation; F = T3N: three irrigations at jointing, heading, and early milk; G = T4: as T3N plus irrigation at prewintering). In 2003–2004, irrigation at prewintering for T2, T3 and T4 was omitted because of heavy rainfall.

 
Figure 5 also shows the effects that different irrigation treatments had on yield and WUE. For all four seasons except 2003–2004, yield for treatment T2 was not significantly different from that of treatment T1N, but the WUE for T1N was significantly higher (10–18%) than for T2. For treatment T3, grain production was slightly less than that of T2N while the WUE of T2N was significantly higher than for T3, being about 8 to 15% for the four seasons. For treatments T3N and T4, T3N produced a slightly higher grain yield than T4, and the WUE of T3N was always significantly higher than T4 in all four seasons. Results showed that without the prewintering irrigation, yield and especially WUE improved when winter wheat was irrigated once, twice, and three times. Timing of irrigation was from jointing onward. Generally, the prewintering irrigation increased soil evaporation during the long dormant period, and good soil moisture conditions at the recovery stage increased tillers in spring. Those tillers were not effective and would die later. But those tillers would consume photosynthesis reserves and increase water use by transpiration. So it was preferable to arrange a limited number of irrigations from jointing to early milk and omit the prewintering irrigation. Yuan et al. (1992) and Zhang et al. (1998) also recommended reducing the irrigations at prewintering and recovery stages in this region.

For the dry and normal seasons of 1999–2000, 2000–2001, and 2001–2002, maximum production and WUE were not achieved with the most frequently irrigated treatment (T4). In the dry seasons of 1999–2000 and 2000–2001, the T3N treatment produced the same or a slightly higher level of grain production as compared with T4, but the WUE of the former was significantly greater than the latter. In the normal season of 2001–2002, T2N and T3N produced the maximum level of production (Fig. 5). Results showed that the commonly used irrigation scheduling of winter wheat in this region (T4) could now be still further improved by reducing one or two irrigation applications in dry or normal seasons.

For the wet seasons of 2002–2003 and 2003–2004, yield was not significantly different among all the treatments. But difference in WUE was significant (P = 0.01 for 2002–2003 and P = 0.05 for 2003–2004).With more irrigation application, WUE tended to decrease. Maximum WUE was achieved with T0 and T1N treatment in 2002–2003. Irrigation once (T1N and T2), irrigation twice (T2N and T3), and irrigation three times (T3N and T4) after overwintering in 2003–2004 didn't affect grain yield, but WUE was decreased by 10% from one irrigation application to three irrigation applications. Both yield and WUE of T1N and T2 were slightly higher than T0 because rainfall in 2003–2004 season fell in prewintering and late growing period and the irrigation application at jointing benefited yield and WUE. Then in wet seasons, application of one irrigation to winter wheat would be sufficient for optimum yield and WUE. The timing of this irrigation would be after overwintering and depend on the distribution of the rainfall.

The results showed that irrigation applied twice or three times, from jointing to milk stage, could produce maximum yield in dry or normal seasons of winter wheat in this region. The prewintering irrigation can be omitted. After the dormant period, irrigation at the revival stage of winter wheat should be suspended until the jointing stage. From jointing to milk stage, winter wheat has a high sensitivity to water stress, so irrigation during this period would favor grain production. In wet seasons, when rainfall was greater than the long-term average, irrigation applied once would guarantee the maximum yield and WUE of winter wheat. The timing of this application would depend on rainfall distribution. The common irrigation practice in this region could be further improved to reduce irrigation water use of winter wheat.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study has shown that grain yield and WUE were improved from 1982 to 2002. Field management practices, such as selecting better-yielding cultivars, reducing soil evaporation, and better irrigation scheduling, could all improve WUE to some degree. Appropriate techniques should be adopted and proper tools used to further increase the productivity attained by using limited water supplies in the NCP. More innovations would be needed to increase the WUE of water that is available. Future improvement in genetic breeding of new cultivars could further increase grain production without considerable increase in water use. Besides this, irrigation technologies and irrigation scheduling may be adapted for more effective and rational uses of limited water supplies. It is necessary to develop new irrigation-scheduling approaches, not necessarily based on full crop water requirements, but ones designed to ensure the optimal use of allowable water supplies (English and Raja, 1996; Kirda, 2002). The results from this study showed that winter wheat could attain its maximum yield with less than full application of current irrigation practice. Other research has also shown that crops such as maize and wheat are well suited to deficit irrigation (Craciun and Cracium, 1999; Waheed et al., 1999; Kirda, 2002). Yield response can vary depending on crop sensitivity for that particular growth stage. Timing the water deficit appropriately is a tool for scheduling irrigation for minimal yield reductions where a limited supply of water is available (Moutonnet, 2002). China's regional water scarcity and the depletion of water resources in the NCP are serious problems. An increase in water demand and an aggravation of the deficit were projected in the next two decades in the NCP (Yang and Zehnder, 2001). Reducing water use by improving WUE will continue to be an important matter.


   ACKNOWLEDGMENTS
 
This research was funded by the Innovation Knowledge Project of Chinese Academy of Sciences (Grant no. KZCX-SW-317-02). The authors acknowledge the anonymous referees for their valuable comments. Special appreciation is extended to Mr. Shaoren Wang at the same institute for his help in data analysis.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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