Effect of Deficit Irrigation on the Growth, Water Use Characteristics and Yield of Cotton in Arid Northwest China

Pedosphere 25(6): 910–924, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press

Effect of Deficit Irrigation on the Growth, Water Use Characteristics and Yield of Cotton in Arid Northwest China YANG Chuanjie1,2 , LUO Yi1,∗ , SUN Lin1 and WU Na2,3 1 Key

Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101 (China) 2 University of Chinese Academy of Sciences, Beijing 100049 (China) 3 Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, Xinjiang 453002 (China) (Received June 4, 2014; revised May 14, 2015)

ABSTRACT Water shortage is a key constraint to sustainable agricultural production in Xinjiang, Northwest China. To enhance the use efficiency of valuable irrigation water resources, a 2-year experiment (2010–2011) was conducted to quantify the response of cotton (Gossypium hirsutum L.) growth and yield to different degrees of deficit irrigation (DI) regimes; to determine the effects of DI on the characteristics of water use for cotton, seasonal water use, available soil water in the root zone, soil water depletion, evapotranspiration (ET)-based water use efficiency and irrigation-based water use efficiency, and to determine the best DI regime for optimal water-saving and yield output. The plots were irrigated at 100% (100ET), 85% (85ET), 70% (70ET), 55% (55ET) and 45% (45ET) of the regional ET of cotton in northern Xinjiang. The effect of DI irrigation on water use characteristics was evaluated by analyzing available soil water and soil water depletion in the root zone along with water use efficiencies of cotton. The study showed that the growth, water use characteristics and yield of cotton varied with irrigation regime. Seasonal ET and seed cotton yield were linearly correlated with irrigation amount. The second-order polynomial equation best approximated water-yield relationship of cotton in the study area. Cotton yield response factor was 0.65, suggesting limited water conditions were suitable for cotton cultivation. Economic evaluation of DI treatments confirmed that the yield loss was less than 10% under 70ET and 85ET, which was acceptable for greater sustainability. The results suggested that proper DI schemes were necessary for sustainable cotton production in the region. While irrigation at 85ET was safe for high cotton yield, irrigation at 70ET was a viable alternative under limited irrigation water availability. Key Words:

available soil water, evapotranspiration, soil water dynamics, water use efficiencies, water-yield relationship

Citation: Yang C J, Luo Y, Sun L, Wu N. 2015. Effect of deficit irrigation on the growth, water use characteristics and yield of cotton in arid Northwest China. Pedosphere. 25(6): 910–924.

INTRODUCTION Northwest China is facing severe water shortage, worsened by high aridity and rapid desertification (Ling et al., 2013). In the region, shortage of water resources for agricultural production has become a key sustainability issue for irrigated agriculture (Zhou et al., 2012). To address this problem, the adoption of improved water-saving agricultural practices is required (Pereira et al., 2009). The core issue with water-saving agricultural is achieving high yields in irrigated farmlands with minimum water use (Deng et al., 2006). ∗ Corresponding

author. E-mail: [email protected].

Although irrigation affects soil quality (Fallahzade and Hajabbasi, 2012; Ghafoor et al., 2012; Johnston et al., 2013; Gao et al., 2014;) and induces soil erosion in some regions (Cerd`a et al., 2009), it is still a useful way of enhancing crop productivity. This is especially critical in arid regions where water shortage is a severe constraint to sustainable agricultural production. A widely used agricultural practice in recent years is deficit irrigation (DI), which is defined as the application of water below full crop water requirement for evapotranspiration (ET) (Pereira et al., 2002; Howell et al., 2004; Fereres and Soriano, 2007; Garc´ıa-Vila et

DEFICIT IRRIGATION EFFECT ON COTTON

al., 2009; Oweis et al., 2011; Mushtaq and Moghaddasi, 2011). Successful and beneficial use of DI requires better understanding of crop response to various degrees of water stress and developing feasible water-yield relationships for optimal crop yield. Several studies on crop response to water deficit have focused on growth, yield and water use characteristics of crops (Aujla et al., 2005; DaCosta and Huang, 2006; Shock et al., 2007; ¨ u et Lovelli et al., 2007; Geerts and Raes, 2009; Unl¨ al., 2011; Kang et al., 2012). These studies have noted that DI could increase crop water productivity (CWP) without severe reductions in crop yield. Cotton (Gossypium hirsutum L.) is a crop that is highly adaptable to limited water conditions and no significant reductions of cotton yield were found under appropriate limited water conditions (Falkenberg et al., 2007; DeTar, 2008). According to Zwart and Bastiaanssen (2004), global CWP is 0.65 kg m−3 for seed cotton and 0.23 kg m−3 for lint cotton. Global CWP range is 0.41–0.95 kg m−3 for seed cotton and 0.14–0.33 kg m−3 for lint cotton, suggesting that a tremendous opportunity exists for increasing cotton productivity with 20%–40% reduction in water use (Zwart and Bastiaanssen, 2004). Cotton is a major cash crop in Xinjiang region of arid Northwest China, cultivated in over one third of total agricultural acreage in this region. With the introduction of drip irrigation, the area of land under cotton is rapidly expanding in arid Northwest China (Wang et al., 2004; Hou et al., 2009; Kang et al., 2012). In the water-scarce region, DI is a viable way of addressing. Under DI, water is applied at a level that maximizes yield within irrigation constraints by reducing subsurface drainage and deep percolation (Cetin and Bilgel, 2002; Ibragimov et al., 2007). However, DI is seldom used in the study area of arid Northwest China due to the lack of appropriate guidance. The main objectives of this study were to: i) quantify the response of cotton growth and yield to different degrees of DI regimes; ii) determine the effects of DI on the characteristics of water use for cotton, seasonal water use, available soil water in the root zone (ASW), soil water depletion (SWD), ET-based water use efficiency (WUEET ) and irrigation-based water use efficiency (WUEI ) and iii) determine the best DI regime

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for optimal water-saving and yield output. MATERIALS AND METHODS Study site The experiments were conducted in the Experimental Irrigation Station (85◦ 59′ E, 44◦ 19′ N) of Shihezi University in Xinjiang, China, during cotton growing seasons in 2010 and 2011. The station is located in the middle reach of Manas River oasis, which is in the warm-temperate arid zone and has a continental climate. Meteorological data were obtained from the Wulanwusu Station, which is a long-term monitoring station in the south of Shihezi and a typical temperate continental climate zone. Based on the statistics of 43year (1963–2006) meteorological data, average annual sunshine duration is 2 861 h with 170 d of frost-free crop growing season in the study area. Annual mean temperature is 7 ◦ C, with mean maximum of 25.4 ◦ C in July and minimum of −15.5 ◦ C in January. Relative humidity during summer months is 30%–50%, with an annual precipitation of 210 mm and pan evaporation of 1 664 mm. The depth to water table in the Experimental Irrigation Station persistently exceeds 8 m. Annual mean temperature was 8.0 and 8.4 ◦ C, relative humidity in summer months was 48% and 49% and precipitation was 340 and 205 mm in 2010 and 2011, respectively. Although annual precipitation in 2010 exceeded the long-term average, most of the precipitation (∼229 mm) occurred during off-season period. Meteorological datasets from the Wulanwusu Station (air temperature, relative humidity, wind speed, sunshine hour and precipitation) were used to compute reference evapotranspiration (ET0 ; mm d−1 ) via the FAO Penman-Monteith equation (Allen et al., 1998): 900 µ2 (es − ea ) T + 273 ∆ + γ(1 + 0.34µ2 )

0.408∆(Rn − G) + γ ET0 =

(1)

where Rn is the net radiation at the crop surface (MJ m−2 d−1 ); G is the soil heat flux (MJ m−2 d−1 ); T is the mean daily air temperature at 2 m height (◦ C); u2 is the wind speed at 2 m height (m s−1 ); es is the saturation vapour pressure (kPa); ea is the actual vapour pressure (kPa); ∆ is the slope vapour pressure curve (kPa ◦ C−1 ); and γ is the psychrometric constant (kPa

C. J. YANG et al.

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C−1 ). Daily ET0 and precipitation data for cotton growing seasons in 2010 and 2011 are shown in Fig. 1. Total ET0 for cotton growing seasons (773 mm in 2010 and 796 mm in 2011) was obtained by summing up the daily values of ET0 . Total precipitation for cotton growing season was 111 mm in 2010 and 132 mm in 2011. Field experiment A local field was selected for the field experiment. For several years before 2010, the field was cultivated with cotton. The soil contained 1 020 mg kg−1 total

nitrogen, 16.5 mg kg−1 available phosphorous and 186 mg kg−1 available potassium. Selected physical characteristics of the soil were shown in Table I. Cotton (cv. Xinluzao 7) was sown in early May in alternating row spacings of 30 cm × 50 cm and plant spacing of 10 cm and then harvested in mid-September. The ground surface of the plots was about 72% mulched with 0.08 mm thick plastic films during the entire growing season (Fig. 2). The dimension of the main plot of each treatment was 5.4 m × 20.0 m, with three 1.3 m × 20.0 m planting beds and 0.5 m separation distances. Each of

Fig. 1 Daily reference evapotranspiration (ET0 ) and precipitation in 2010 and 2011 during cotton growing seasons in the study area of arid Northwest China. TABLE I Selected physical properties of soil profile in cotton fields in the study area of arid Northwest China Depth

cm 0–10 10–20 20–40 40–60 60–80 80–100

Sand

Silt

49.8 48.2 49.6 43.2 47.7 30.8

% 39.4 38.5 33.7 37.4 31.4 42.2

Clay

10.8 13.3 16.7 19.4 21.5 27.7

Bulk density g cm−3 1.59 1.69 1.73 1.76 1.77 1.74

Field capacity

Permanent wilting point m3 m−3

0.33 0.32 0.32 0.31 0.31 0.31

0.11 0.10 0.10 0.09 0.09 0.09

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Fig. 2 Layout of the crop planting in the study area of arid Northwest China. Capital letters of A, B and C indicate the sites where soil water content was measured.

the three beds had four plant rows and two irrigation drip lines. The experimental plot (5.4 m × 20 m) was cultivated, irrigated and fertilized in accordance with local practices to ensure germination and seedling during early growth stage. Prior to sowing, 120 kg ha−1 of urea was uniformly applied as base fertilizer. Additional 575 kg ha−1 of urea (with 460 g N kg−1 ) and 305 kg ha−1 of triple superphosphate (with 420 g P2 O5 kg−1 ) were applied via drip irrigation during the growing seasons. Irrigation plot-test design Regional ET for the entire cotton growing season in northern Xinjiang of Northwest China is estimated at 525 mm (Chen et al., 2001; Liu et al., 2012; Zhou et al., 2012). Based on this value, five water treatments were established in the field experiment, including full irrigation (FI) at 100% ET (100ET) and 4 levels of DI at 85% ET (85ET), 70% ET (70ET), 55% ET (55ET) and 45% ET (45ET), respectively. Irrigation was applied through surface drip irrigation system (Fig. 3) and 6 drip tapes (20 m) were in each main plot with two

Fig. 3

drip tapes in each crop bed. The separation distance of every two drip tapes in a bed was 0.8 m (Fig. 2). For the irrigation treatment of 70ET, irrigation was done anytime to keep soil water at around 70% of field capacity. The irrigation dates and amounts are shown in Table II along with actual soil water content and crop growth stage. As there was far more rainfall in 2011 than in 2010 in the early stage of cotton growth in the study area, less irrigation was applied in 2011 than 2010. All the treatments were completely randomized block design with three replicates. Sampling and measurements In each replication, 10 plants were sampled at 10-d intervals during the period from July 2 to September 4, 2010 and from July 3 to September 7, 2011. The dry weights of cotton roots, stems, leaves, squares and bolls were measured after oven drying for 72 h at 80 ◦ C. Leaf area (LA) was measured using automatic area meter (AMM-7, Hayashideno Inc., Japan) and leaf area index (LAI) was calculated from LA per unit ground surface area. Root auger was used to take root samples

A sketch diagram depiction of the drip irrigation system used in the study area of arid Northwest China.

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TABLE II Irrigation regimes of different water treatments during cotton growing seasons in 2010 and 2011 in the study area of arid Northwest China Year

Treatmenta)

May 29

Jun. 20

Jul. 3

Jul. 18

Jul. 28

Aug. 8

Aug. 20

Sep. 4

Total

2010

100ET 85ET 70ET 55ET 45ET

20 18 17 13 17

66 62 73 69 63

85 77 68 47 30

92 87 62 55 48

mm 70 67 37 32 18

72 62 74 51 31

74 73 34 33 26

54 41 25 19 19

533 487 390 319 252

Year

Treatment

Jun. 6

Jun. 15

Jun. 25

Jul. 2

Jul. 14

Jul. 23

Aug. 3

Aug. 14

Total

2011

100ET 85ET 70ET 55ET 45ET

13 15 10 10 8

52 47 39 30 24

52 47 39 30 24

52 47 39 30 24

mm 63 57 47 36 29

73 67 55 42 34

78 71 59 45 37

73 67 55 42 34

456 418 343 265 214

a) 100ET

= full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

at budding, flowering and bolling stages to the depths of 40 cm, 70 cm and 100 cm and soils in the auger cylinder were subdivided at 10-cm intervals. Root length was determined using ARC/INFO image analysis software following the procedure proposed by Zheng et al. (2004). Seed cotton was picked by hand three times at 7-d intervals and the total weight was obtained by simple summation. Soil water content of each depth layer was measured using neutron probe CPN 503DR (CPN International Inc., USA). Three access tubes of the neutron probe were installed at sites A, B and C (Fig. 2) and measurements taken at each 10 cm layer down to the depth of 100 cm below the ground surface. For the top 20 cm layer, soil samples were collected and the soil water content gravimetric was measured in the laboratory and converted into volumetric soil water content via soil bulk density (Table I). The measurements were taken before and after every irrigation event and also after every precipitation event. The actual crop ET was estimated using the dual crop coefficient approach of the interactive SIMDualKc model as Rosa et al. (2012a, b): ETa = (Ks × Kcb + Ke ) × ET0

(2)

where ETa is the estimated crop evapotranspiration (mm d−1 ); Kcb is the basal crop coefficient; Ke is

the soil evaporation coefficient; Ks is the water stress reduction coefficient and ET0 is the reference evapotranspiration (mm d−1 ) computed using the FAOPM equation for daily weather data (Allen et al., 1998). The recommended values of Kcb under sub-humid climate with average relative humidity (RH) of 45% and wind speed of 2 m s−1 were well documented in the literature (Allen et al., 1998). For climatic conditions with higher RH and wind speed, the Kcb value for mid or late season period larger than 0.45 can be adjusted using the following function: [ Kcb = Kcb(Tab) + 0.04(µ2 − 2) − )]( h )0.3 0.004(RH min − 45 3

(3)

where Kcb(Tab) is the value of Kcb when wind speed is 2 m s−1 and minimum relative humidity is 45%; µ2 and RHmin are the average wind speed (m s−1 ) and minimum relative humidity (%) values, respectively, observed during the mid or late season period and h is the average crop height during the mid or late season period (m). Soil evaporation coefficient (Ke ) was calculated from daily water balance of surface soil layer Ze (Eq. 71 of FAO-56 paper) (Allen et al., 1998) as follows: Ke = Kr × (Kcmax − Kcb ) ≤ few Kcmax

(4)

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where Kr is the dimensionless evaporation reduction coefficient that is dependent on the cumulative depth of water depleted from the top soil; Kcmax is the maximum Kc following rain or irrigation event and few is the fraction of the soil that is both exposed and wetted. The equations for the calculations of Kcmax , Kr and few are detailed in Rosa et al. (2012a). Soil water stress coefficient Ks was expressed as a linear function of root zone water depletion that is in excess of readily available water in effective root zone: Ks =

TAW − Dr TAW − Dr = TAW − RAW (1 − p)TAW for Dr > RAW

Ks = 1 for Dr ≤ RAW

(5) (6)

where ASWr is the available soil water in the root zone (mm); θ is the water content of the soil layer (m3 m−3 ); θwp is the water content at wilting point (m3 m−3 ) and Zr is the root depth (m). Root-zone SWD is a useful element in expressing crop water use characteristics. In this study, SWD was calculated using a traditional bulk layer model based on soil water balance in the root zone: SWDi = SWDi−1 + ETi − Pi − Ii

where Pi is the precipitation on day i (mm); Ii is the irrigation on day i (mm) and SWDi and SWDi−1 are the root-zone water depletions at end of day i and previous day i − 1 (mm), respectively. WUEET and WUEI were calculated using the following expressions:

where TAW and RAW are the total and readily available soil water (mm), respectively and p is the depletion fraction at water stress initiation.

WUEET =

Calculated parameters

WUEI =

The selected parameters in this study for evaluating the effects of DI on water use characteristics and yield of cotton were calculated as given in the following equations according to Stewart et al. (1977): (

1−

( ETa ) Ya ) = Ky 1 − Ym ETm

(7) −1

where Ya is the actual harvested yield (kg ha ); Ym is the maximum harvested yield (kg ha−1 ); Ky is the yield response factor; ETm is the maximum evapotranspiration (mm) corresponding to Ym ; 1 − (Ya /Ym ) is the relative yield decrease and 1 − (ETa /ETm ) is the relative evapotranspiration deficit. The response of crop to DI in terms of water use characteristics was determined from the available soil water in the root zone (ASW), SWD, and water use efficiency. In this study, daily ASW and SWD were obtained from dual crop coefficient approach using the interactive SIMDualKc model. Root-zone ASW is the difference between soil water content and permanent wilting point of the soil layers, computed as: ASWr = 1000(θ − θwp )Zr

(8)

(9)

Y ETa

(10)

Y − Yd Ia

(11)

where WUEET and WUEI are, respectively, the ETbased water use efficiency and irrigation-based water use efficiency (kg ha−1 mm−1 ); Y is the total seed cotton yield (kg ha−1 ); Yd is the dry cotton yield under no irrigation (kg ha−1 ); and Ia is the applied irrigation in each treatment (mm). Crop water productivity (CWP; kg ha−1 mm−1 ) was calculated using the following equation (Pereira, 2009): CWP =

Y TWU

TWU = P + GC + ∆SW + Is

(12) (13)

where TWU is the total water use (mm); P is seasonal precipitation (mm); GC is the seasonal groundwater contribution (mm) which was set at 0 due to the low water table; Is is the seasonal irrigation (mm) and ∆SW is the difference in soil water content between planting and harvesting (mm). In this study, the values of WUEET , WUEI and CWP from the above functions were converted from kg ha−1 mm−1 to kg m−3 by dividing the former by 10.

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RESULTS AND DICUSSIONS Irrigation and evapotranspiration The estimated daily ET for cotton is plotted in Fig. 4, where estimated ET was highly correlated with irrigation. The differences in estimated ET could be due to the differences in irrigation and hence in available crop water. The large estimated ET differences were noted for mid-season periods mainly due to the concurrence of limited available crop water and highcrop water demand under DI conditions. As summarized in Table III, the average ET was 539 mm in 2010 and 556 mm in 2011 for all treatments. For the 2010 experimental period, ET was 652 mm under the 100ET treatment and 256 mm under the 45ET treatment. Then for the 2011 experimental year, ET was 689 mm under the 100ET treatment and 217 mm under the 45ET treatment. The highest ET (652 mm) was in 2010, consistent with the 640 mm noted by Kang et al. (2012) for FI in the study area. The highest ET (652 mm) for the two experimental years in the study area was consistent with that noted in other different areas under similar conditions (Pereira

C. J. YANG et al.

¨ u et al., 2011). Table III shows that et al., 2009; Unl¨ despite less irrigation in 2010, seasonal ET was higher in 2011 than 2010. The high precipitation in 2011 induced high root-zone soil water, and in turn resulted in high crop transpiration and soil evaporation. The high reference ET in 2011 suggests that atmospheric demand for evaporation was stronger in 2011 than 2010. Stored soil water in the study area was a vital source of water for crop transpiration and soil evaporation. Finally, there was higher soil water use by cotton crop in the study area in 2011 than 2010. Fig. 5 shows a linear correlation between estimated seasonal ET and total water (irrigation + precipitation) supply. Both of the linear correlation lines for 2010 and 2011 laid above the line of y = x, suggesting crop dependence on soil water as an adaptation strategy to water stress. Soil water at the time of planting was another key factor that influenced crop water use in the study area. Other studies have also found that cotton could reach deep into the soil profile as an adaptation strategy to water stress in water-scarce/arid regions (Orgaz et al., 1992; Oweis et al., 2011). In contrary to Oweis et al. (2011), most of cotton soil water

Fig. 4 Estimated daily evapotranspiration of cotton under different irrigation conditions during experimental periods in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

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TABLE III Seasonal irrigation and precipitation, estimated cotton evapotranspiration (ETa ), reference evapotranspiration (ET0 ), change in soil water between pre-planting and harvesting (∆SW), total water use (TWU) and seasonal crop coefficient (ETa /ET0 ) under different irrigation conditions in the study area of arid Northwest China Year

Treatmenta)

Irrigation

Precipitation

2010

100ET 85ET 70ET 55ET 45ET

535 489 373 323 256

111 111 111 111 111

2011

100ET 85ET 70ET 55ET 45ET

461 422 345 268 217

132 132 132 132 132

∆SW mm 22 32 52 36 28 91 112 84 60 52

TWU

ET0

ETa

ETa /ET0

668 632 536 470 395

773 773 773 773 773

652 644 541 469 390

0.84 0.83 0.70 0.60 0.50

684 666 561 460 401

796 796 796 796 796

689 668 566 465 393

0.86 0.84 0.71 0.58 0.49

a) 100ET

= full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

Fig. 5 Correlation between seasonal estimated evapotranspiration (ETa ) of cotton and applied water in the 2010 and 2011 experimental periods in the study area of arid Northwest China.

mining in this study was in moderately dry treatments, but not in dry treatments. This could be due to water stress damage to the ability of crop roots to absorb soil water. Cotton growth response to deficit irrigation The effect of irrigation on cotton growth was clearly evident as shown in Figs. 6, 7 and 8. Crop height, LAI and dry matter accumulation (DMA) decreased with decreasing irrigation. In the cotton growing seasons in 2010 and 2011, the maximum crop height, LAI and DMA occurred either under the 100ET or 85ET treatment where irrigation was highest. Then the mini-

Fig. 6 Plant height of cotton under different irrigation conditions during the cotton growing seasons in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

mum crop height, LAI and DMA were under the 45ET treatment where the irrigation was the lowest. Similar to several other studies (Daˇgdelen et al., 2006, 2009;

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Fig. 7 Leaf area index (LAI) of cotton under different irrigation conditions during the cotton growing seasons in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

Fig. 8 Dry matter accumulation (DMA) of cotton under different irrigation conditions during the cotton growing seasons in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

¨ u et al., 2011; DeTar, 2008; Oweis et al., 2011; Unl¨ Kang et al., 2012), this study convinced adverse effects of water stress on cotton growth in terms of crop height, LAI and DMA. As depicted in Fig. 6, mean maximum crop heights under the treatments of 100ET, 85ET, 70ET, 55ET and 45ET were 62, 64, 47, 43 and 42 cm in 2010 and 51, 55, 45, 43 and 33 cm in 2011, respectively. Because of high irrigation, only small differences existed between the 100ET and 85ET treatments for crop height. However, crop height obviously shortened with decreasing irrigation in relatively drier treatments. Kang et al. (2012) also found that maximum cotton height appeared under wetter treatments, shortening with decreasing water supply. As also indicated by other studies, DI limited LAI in the whole growing season with water stress ¨ u et al., 2011; Kang et (Daˇgdelen et al., 2006, 2009; Unl¨ al., 2012). As in Fig. 7, the highest LAI for cotton was 4.67 in 2010 and 4.65 in 2011, which occurred under

the 100ET treatment (also see Yazar et al., 2002; Daˇgdelen et al., 2009). The lowest LAI was 2.29 in 2010 and 2.49 in 2011, which occurred under the 45ET treatment. Compared with the 100ET treatment, DI resulted in 51% and 47% decreases in LAI under the 45ET treatment in 2010 and 2011, respectively. Fig. 8 depicts the dynamics of DMA for different irrigation treatments in 2010 and 2011 in the study area. As expected, the effect of water stress on DMA was more obvious under the DI treatments. The highest DMA was 69 g plant−1 in 2010 and 65 g plant−1 in 2011, which occurred under the 100ET treatment. Similar to plant height and LAI, the most severe effect of water stress on DMA loss was under the 45ET treatment. Compared with the 100ET treatment, there was respectively 47% and 40% losses in DMA in 2010 and 2011 under the 45ET treatment. Kang et al. (2012) also observed that the highest DMA (53–61 g plant−1 ) was under well-irrigated treatments. However, DMA declined with deceasing soil matric potential (SMP)

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threshold during the study (Kang et al., 2012). Cotton yield response to deficit irrigation The second order correlation between seed cotton yield and irrigation could be expressed as: Y2010 = −0.0146I 2 + 17.811I + 702 (R2 = 0.98) for 2010 and Y2011 = −0.0313I 2 + 28.409I − 418 (R2 = 0.99) for 2011; where Y is seed cotton yield (kg ha−1 ) and I is seasonal water application (mm). Similarly, several other studies have noted polynomial relations between irrigation and yield for drip irrigated cotton (Wanjura et al., 2002; Cetin and Bilgel, 2002; Rajak et al., ¨ u et 2006; DeTar, 2008; Garc´ıa-Vila et al., 2009; Unl¨ al., 2011; Kang et al., 2012). Based on the polynomial regression curves, water use for cotton was below estimated threshold for maximum seed cotton yield. Cotton yield obviously declined with decreasing irrigation. The highest mean yield (6 058–6 045 kg ha−1 in 2010 and 6 071 kg ha−1 in 2011) was under the 100ET treatment. Minimum mean seed cotton yield was 4 297 kg ha−1 (4 280 kg ha−1 in 2010 and 4 314 kg ha−1 in 2011), which occurred under the 45ET treatment. Similar to DI, the polynomial regression curves also suggested that excessive irrigation could also limit cotton yield (also see Wanjura et al., 2002; DeTar, 2008). This is critical for optimization of irrigated cotton production and saving valuable water resources for sustainable irrigated agriculture in the Arid Northwest China study area. The average yield response factor (Ky ) for drip irrigated cotton in the study area in 2010 and 2011 (obtained as described by Stewart et al., 1977) was 0.65 (Fig. 9). This value was within the range of seed cotton yield (0.38–0.84) observed by Ertek and Kanber (2003) and that (0.50–0.75) by Yazar et al. (2002) in Seyhan plain and Harran plain of Turkey, respectively. However, Daˇgdelen et al. (2006, 2009) noted that the range of cotton yield response factor in Turkey is 0.78–0.95. Singh et al. (2010) found that cotton yield response factor in India is 0.95. This suggested that Ky of less than 1.00 indicates high tolerance to water stress. In other words, yield loss due to water stress was less than irrigation water loss due to ET. Water use characteristics Soil water dynamics.

The effect of DI on water

Fig. 9 Correlation between relative yield loss (1 − Ya /Ym ) and relative evapotranspiration deficit (1 − ETa /ETm ) under drip irrigation in cotton fields in the study area of arid Northwest China. Ya = actual harvested yield (kg ha−1 ); Ym = maximum harvested yield (kg ha−1 ); ETa = estimated evapotranspiration (mm); ETm = maximum evapotranspiration (mm) corresponding to Ym .

use characteristics of cotton was evaluated from ASW and SWD dynamics. To better compare the effect of DI on ASW and SWD dynamics, the ratio of ASW to TAW and that of SWD to TAW were worked out and plotted in Figs. 10 and 11, respectively. The maximum root depth and TAW for different irrigation regimes are also detailed in Table IV. TABLE IV Root depths and total available water (TAW) under different irrigation conditions in the cotton fields in the study area of arid Northwest China Treatmenta)

Root depth

TAW

100ET 85ET 70ET 55ET 45ET

m 1.0 1.0 0.9 0.7 0.6

mm 220 220 198 154 132

a) 100ET

= full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

The range of the ASW/TAW ratio for all the treatments in the cotton growing season was 1.0–0.5 (Fig. 10). The variation dynamics of the ASW/TAW ratio was driven by both irrigation regime and crop growth period. As depicted in Fig. 10, the ASW/TAW ratio decreased with decreasing irrigation from the

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Fig. 10 Seasonal variations in the ratio of available soil water to total available water (ASW/TAW) under different irrigation conditions of cotton during experimental periods in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

100ET treatment to the 45ET treatment. Compared with the 100ET treatment, a gentle decreasing trend was noted as irrigation steadily decreased from the 85ET and the 70ET treatments. However, a sharp decreasing trend was observed as irrigation continued decreasing from the 55ET treatment to the 45ET treatment. The decline in the ASW/TAW ratio suggested increasing DI and inadequate ASW for full crop growth, which resulted in water stress. Fig. 10 also showed that the ASW/TAW ratio declined with increasing water deficit. For all the treatments, the ASW/TAW ratio was higher in 2010 than 2011 growing season. The main reason for this discrepancy was the low irrigation, high reference ET and high soil water use in 2010. Significant variations were also found in the characteristics of the ASW/ TAW ratio during the period of vegetative growth. As shown in Fig. 10, the ASW/TAW ratio was high for initial growth stages, suggesting excess ASW for crop use. The fraction of stored soil water from winter snow was the main source

of water for the early crop growth stages. However, stored water in the soil profile was not adequate for crop growth during middle growth stages. This necessitated irrigation to support crop growth, hence the periodic variations in the ASW/TAW ratio with irrigation. At the end of growing season, irrigation ceased and crops used more of soil water, leading to decreasing crop water use. The study showed that DI resulted in lower values of the ASW/TAW ratio at harvesting stage, further conserving stored water in the soil profile (Pereira et al., 2009). Appropriate use of DI could significantly enhance the use of soil water and potentially save irrigation water. Based on Eq. 9, the differences in the dynamics of SWD among the treatments were mainly driven by ET. Also SWD increases with the use of root-zone soil water by ET. Because of high soil water content and low ET, the SWD/TAW ratio was low during initial growth stages. For the initial period of soil water dry-up after first irrigation, the SWD/TAW ratio increased with decreasing ASW. For the middle growth stages, the

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Fig. 11 Seasonal variations in the ratio of soil water depletion to total available water (SWD/TAW) under different irrigation conditions of cotton during experimental periods in 2010 and 2011 in the study area of arid Northwest China. 100ET = full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET.

SWD/TAW ratio changed with irrigation. The SWD/ TAW ratio dynamics suggested that the characteristics of crop water use were mainly driven by irrigation. In the late growth stages during which time irrigation ceased, the SWD/TAW ratio gradually increased with decreasing crop water use. As also noted in this study, DaCosta and Huang (2006) observed higher SWD under DI than FI. Fig. 11 showed that the SWD/TAW ratio increased with increasing DI in the middle and late stages of crop growth in the study area. According to Allen et al. (1998), root-zone water depletion was high enough to induce water stress when SWD exceeded readily ASW. The higher the SWD, the more severe water stress was (Allen et al., 1998). Water use efficiencies. ET-based water use efficiency (WUEET ) and irrigation-based water use efficiency (WUEI ) in the study area are listed in Table V. Although both WUEET and WUEI increased with decreasing irrigation, the increase became significant (p < 0.05) when irrigation dropped down to 70ET.

In the study area, WUEET ranged 0.86 to 1.09 kg m , and was highest under the 45ET treatment and lowest under the 85ET treatment in both 2010 and 2011 (Table V). Other studies showed a WUEET range of 0.77–1.46 kg m−3 for cotton fields in Western Turkey (Daˇgdelen et al., 2009) and 0.81–1.13 kg m−3 for cotton fields in Northwest China (Kang et al., 2012) under different deficit drip irrigation regimes. The variations in WUEI under different DI regimes were similar to those in WUEET . In this study, WUEI range was 0.93–1.27 kg m−3 and it was the highest under the 45ET treatment and the lowest under the 100ET treatment in both 2010 and 2011. From year-by-year comparison of WUEET and WUEI for all the treatments, it was obvious that WUEET was lower than WUEI in the study area. This discrepancy was largely attributed to the stored water ¨ u et al., in the soil profile (Daˇgdelen et al., 2009; Unl¨ 2011). Also the discrepancy could be attributed to differences between stored soil water and rainfall in the study area (see Table III). −3

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TABLE V Evapotranspiration-based water use efficiency (WUEET ), irrigation-based water use efficiency (WUEI ) and crop water productivity (CWP) of cotton under different irrigation conditions in 2010 and 2011 in the study area of arid Northwest China Treatmenta)

2010 WUEET

2011 WUEI

CWP kg

100ET 85ET 70ET 55ET 45ET

0.93cb) 0.92c 1.01b 1.03ab 1.09a

0.93c 0.98c 1.12b 1.17b 1.26a

0.95c 0.93c 1.02b 1.03ab 1.08a

WUEET

WUEI

CWP

0.88c 0.86c 1.00b 1.06a 1.09a

0.98c 1.04c 1.19b 1.26a 1.27a

0.87c 0.89c 1.01b 1.07a 1.07a

m−3

a) 100ET

= full irrigation at 100% evapotranspiration (ET); 85ET = deficit irrigation (DI) at 85% ET; 70ET = DI at 70% ET; 55ET = DI at 55% ET; 45ET = DI at 45% ET. b) Means in each column followed by the same letter(s) are not significantly different at P < 0.05 according to Duncan’s test (n = 3).

As summarized in Table V, CWP increased significantly (P < 0.05) as irrigation declined from 100ET or 85ET down to 70ET. According to Zwart and Bastiaanssen (2004), global average CWP for seed cotton was 0.65 kg m−3 . The large range of global CWP (0.41– 0.95 kg m−3 ) presented a tremendous opportunity for increasing cotton productivity with less (20%–40%) irrigation. CWP observed in this study was 0.87 kg m−3 under 100ET and 1.08 kg m−3 under 45ET irrigation treatment. This range was in agreement with the 0.81– 1.12 kg m−3 range observed by Kang et al. (2012) and the 0.91–1.16 kg m−3 range noted by Wang et al. (2014) under various DI conditions in cotton fields in Xinjiang, Northwest China. Recent studies have shown that DI could slightly improve CWP (Geerts and Raes, 2009; Pereira et al., 2009; Singh et al., 2010; Oweis et al., 2011; Kang et al., 2012; Wang et al., 2014) and the 2-year field experiment in this study also supported this trend. With severe DI, however, CWP only slightly increased due to overall reduction in total water use (TWU) and was accompanied by a large reduction in cotton yield. Under such conditions, water saving failed to adequately translate in CWP and therefore optimization of irrigation was needed for profitable CWP and crop yield. Economic analyses of water-saving strategies To determine optimal DI regime with the high water saving and yield output, extensive economic analyses of DI treatments were conducted. All variables associated with cotton yield increase and irrigation water-

saving were factored in the analyses as follows: PLoss =

[ 1 × (Ifull − Ideficit ) × Pw × 100 Yfull × Pc ] − (Yfull − Ydeficit ) × Pc (14)

wherePLoss is the cotton yield loss (%); Yfull and Ydeficit are the seed cotton yield under full and deficit irrigations, respectively (kg ha−1 ); Ifull and Ideficit are the seasonal water use under full and deficit irrigations, respectively (mm); Pw is the irrigation water price (yuan m−3 ); Pc is the cotton price (yuan kg−1 ) and 100 is irrigation conversion factor from mm to m3 . From analyses, Pw and Pc were 0.0492 yuan m−3 and 7.5 yuan kg−1 , respectively. Based on Eq. 14, the losses were 1.9%, 8%, 17.5% and 26% in 2010 and 2%, 5.4%, 16.8% and 28% in 2011 for the DI treatments of 85ET, 70ET, 55ET and 45ET, respectively. The analyses showed that the DI treatments of 85ET and 70ET were economically feasible with less cotton yield loss than 10%. Given both water saving and yield output, cotton growth was promising under DI water-saving agriculture in Xinjiang, arid Northwest China. CONCLUSIONS Plant height, LAI, DMA and seed cotton yield decreased with decreasing irrigation. When water amount decreased to 45ET, LAI and DMA decreased by nearly 50% compared with the 100ET treatment. Decreasing irrigation limited crop ET and a linear correlation existed between seasonal ET and irrigation. In the study area, cotton ET was 390–689 mm and seed cotton yield

DEFICIT IRRIGATION EFFECT ON COTTON

was 4 280–6 071 kg ha−1 . The second-order polynomial function best approximated water-yield relation for cotton in the study area. Cotton yield response factor (Ky ) was 0.65, which suggested that cotton was well adaptable to limited water conditions. Compared with FI, DI induced low ASW and enhanced SWD, WUEET , WUEI and CWP, which indicated that DI led to more efficient use of stored water in the soil profile than FI, thus saving more irrigation water. However, severe DI (at 45ET or less) led to relatively high yield loss. Conversely, moderate DI (at 85ET) enhanced irrigation water saving with low yield loss. However, moderately high DI (at 70ET) led to higher irrigation water saving with moderately high yield loss. Overall, appropriate DI was a necessary requirement for sustainable cotton production in the region. While DI at 85ET was safe for cotton production in the study area, and DI at 70ET was a viable alternative under limited water resources. ACKNOWLEDGEMENT This research was supported by the National Natural Science Foundation of China (No. 41371115) and the 100 Talents Program of Chinese Academy of Sciences (No. KZXC2-YW-BR-12). REFERENCES Allen R G, Pereira L S, Raes D Smith M. 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage, Paper 56, FAO, Rome. Aujla M S, Thind H S, Butter G S. 2005. Cotton yield and water use efficiency at various levels of water and N through drip irrigation under two methods of planting. Agr Water Manage. 71: 167–179. Cerd` a A, Gim´ enez-Morera A, Bod´ı M B. 2009. Soil and water losses from new citrus orchards growing on sloped soils in the western Mediterranean basin. Earth Surf Proc Land. 34: 1822–1830. Chen D, Xu H, Xu L, Cao B. 2001. A study on the evapotranspiration of cotton field under mulch drip irrigation in north Xinjiang. Desert Oasis Meteorol (in Chinese). 24(2): 16–17. Cetin O, Bilgel L. 2002. Effects of different irrigation methods on shedding and yield of cotton. Agr Water Manage. 54: 1–15. DaCosta M, Huang B. 2006. Deficit irrigation effects on water use characteristics of Bentgrass species. Crop Sci. 46: 1779– 1786. Daˇ gdelen N, Yılmaz E, Sezgin F, G¨ urb¨ uz T. 2006. Water-yield

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