Comparative effects of drip and furrow irrigation on ...

agricultural water management 83 (2006) 30–36

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Comparative effects of drip and furrow irrigation on the yield and water productivity of cotton (Gossypium hirsutum L.) in a saline and waterlogged vertisol Daleshwar Rajak a, M.V. Manjunatha a,*, G.R. Rajkumar a, M. Hebbara a, P.S. Minhas b a b

University of Agricultural Sciences, ARS, Gangavathi 583227, India Central Soil Salinity Research Institute, Karnal 132001, India

article info

abstract

Article history:

Field experiments were conducted on a saline vertisols during 2000–2002 for evaluating the

Accepted 18 November 2005

response of cotton (Gossypium hirsutum L.) to applied irrigation water (IW, 0.8, 1.0, 1.2 and 1.4

Published on line 28 February 2006

times the evapotranspiration, ET) with drip and furrow irrigation method in four different

Keywords:

and average WT for the blocks I, II, II and IV were 8.0 0.4, 1.25 0.08; 9.1 0.7, 1.15 0.08;

Cotton

10.4 0.5, 1.05 0.09 and 15.1 0.8 dS m1, 0.95 0.07 m, respectively. The growth and

Saline-waterlogged soils

yield performance of cotton irrigated through furrows, even though with good quality canal

Drip irrigation

water (ECw 0.25 dS m1), was poor when compared with drip irrigation with marginally

blocks varying in soil salinity (ECe, surface 0.6 m) and water table depths (WT). The initial ECe

Saline irrigation Water productivity Marginal quality waters

saline water (ECw 2.2 dS m1). The crop responded to applied water and the maximum cotton yield (1.78 Mg ha1—average for two years) was obtained from block I under drip irrigation applied at 1.2 ET while the lowest yield (0.18 Mg ha1) was from block IV when applied water equaled 0.8 ET with furrow irrigation. Due to creation of better salt and moisture regimes, water productivity also considerably improved with drip irrigation. Production

functions

developed

could

be

represented

as:

Y

(Mg ha1) = 0.2070

AW 0.0012 AW2 + 0.0807 ECe 0.0049 ECe2 0.0014 AW ECe 6.5945 (R2 = 0.974**) for drip irrigation and Y = 0.3853 AW 0.0021 AW2 + 0.0253 ECe 0.0005 ECe2 0.0016 AW ECe 14.9117 (R2 = 0.877**) for furrow irrigation where AW and ECe represent applied water and time weighted mean soil salinity, respectively. Though the gross income (US$ 223–690 ha1) was more with drip than furrow (US$ 67–545 ha1) irrigation, the net profit per unit of applied water was higher with furrow irrigation. It was concluded that the drip system provide for opportunities to enhance the use of saline waters in water scarcity areas especially those existing at the tail end of canal commands. # 2005 Elsevier B.V. All rights reserved.

1.

Introduction

Land and water are the two most important natural resources for agricultural development and economic advancement of any country. With a low per capita availability of land and water in India compared to other countries, enhancing agricultural

productivity has become essential to meet food demands forever growing population. Thus, available water for irrigation needs to be utilized judiciously. At the same time, land degradation due to soil salinity and water logging is threatening the sustainable use of these resources. Globally, more than 45 million hectares of land have been affected due to these twin

* Corresponding author. Tel.: +91 8533 271030; fax: +91 8533 271030. E-mail address: [email protected] (M.V. Manjunatha). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2005.11.005


problems with an annual loss of US$ 11.4 billion (Ghassami et al., 1995). In Tungabhadra Project (TBP) command, 80,000 hectares of land in Karnataka state has been affected by salinity and water logging (Jayashankar, 1997). In addition to this, about 60% of the groundwater in the command is having the problem of salinity and sodicity (Annonymous., 1994). Surface irrigation with these waters on heavier textured soils of the area usually leads to build up of salinity and sodicity problems and thus unsustainable crop yields. Therefore, there is need to adopt specialized and efficient methods of irrigation like micro irrigation which can help in attaining the twin objectives of higher productivity and optimum use of water. Earlier reports by Ayers et al. (1986) and Saggu and Kaushal (1991) show that saline water can be efficiently used through drip irrigation even on saline soils. Moreover, it results in considerable saving in irrigation water (Tan, 1995; Yohannes and Tadesse, 1998; Cetin and Bilgel, 2002) thus reducing the risks of secondary salinisation. However, such an option has not been studied at large with cotton crop using poor quality water in saline vertisols. Keeping this in view, a field experiment was conducted to evaluate the comparative effect of drip and furrow irrigation on salinity build up vis-a-vis cotton performance in a saline vertisols.

2.

Materials and methods

2.1.

Experimental site and treatment details

A field experiment was conducted during 2000–2002 at Agricultural Research Station, Gangavathi, Karnataka, India which is situated in the north–eastern dry zone of the state (158150 4000 N latitude; 768310 4500 E longitude and altitude of 419 m above mean sea level). The soil of the site was clay in texture (clay, silt and sand 47.6, 29.5 and 22.9%, respectively) having an infiltration rate of 14 mm h1 and a bulk density of 1.31 g cm3 (Manjunatha et al., 2002). Due to existence of shallow water table and its variable salinity, differential salinity gradients have been naturally created along the slope of the land. Thus, the area was sub-divided into four blocks (each of 20 m 20 m) based on initial soil salinity (ECe, surface 0.6 m) and average water table depth (WT) viz., block I (ECe 8.0 0.4 dS m1, WT 1.25 0.08 m), block II (ECe 9.1 0.7 dS m1, WT 1.15 0.08 m), block III (ECe 10.4 0.5 dS m1, WT 1.05 0.09 m) and block IV (ECe 15.1 0.8 dS m1, WT 0.95 0.07 m). The experiment was laid out in each block with two methods of irrigation i.e. drip and furrow method in main plot and four quantities of applied irrigation water, IW viz. 0.8, 1.0, 1.2 and 1.4 times the evapotranspiration, ET in the sub-plots. The quantities of IW were fixed on the higher side to meet both the crop water and leaching requirement of the saline soil. Each treatment consisted of four lines (each 20 m in length) of cotton with a buffer strip of 1.5 m in between treatments to minimize the effects of lateral water and salt movement. Cotton (cultivar: Laxmi) was sown on August 1 and 2 during 2000 and 2001 keeping spacing of 0.75 m 0.30 m, respectively and harvested on March 29, 2001 and April 3, 2002. Recommended package of other agronomic practices and plant protection measures were followed. Applied fertilizers equaled 80, 40 and 40 kg ha1 of nitrogen, phosphorus and potash, respectively. Rainfall received during the cropping period of 2000–2001 and 2001–

31

2002 was 467.5 and 483.5 mm while the evapotranspiration was calculated to be 570 and 580 mm, respectively. The crop was irrigated with the available canal water (ECw 0.2 dS m1) under furrow irrigation while well water (ECw 2.2 dS m1) was used for irrigation through drips. The lateral lines were laid parallel along each row, and the spacing of the ‘on line’ emitters (4 L h1) along the lateral was 0.6 m. Water table depths were monitored at weekly at the observation wells installed in the study area. Ground water samples were also collected from these observation wells and were analyzed for salinity.

2.2.

Estimation of irrigation water requirements

Reference evapotranspiration (ET0) was calculated using modified Penman method (Doorenbos and Pruitt, 1977). Since, experimentally determined crop factor values were not available, the values were estimated to be 0.45, 0.75, 1.15, 0.85 and 0.70 for initial, developmental, mid season, late season and harvest stages, respectively (Doorenbos and Pruitt, 1977). The actual evapotranspiration was then estimated by multiplying reference evapotranspiration and crop factor for different months based on crop growth stages. The irrigation water requirements for the crop were estimated by subtracting the effective rainfall from the calculated crop evapotranspiration on daily basis using relationship; IR ¼ ET0 Kc Re

(1)

where IR, ET0, Kc and Re refer to net depth of irrigation (mm d1), reference potential evapotranspiration (mm d1), crop factor and effective rainfall (mm d1), respectively. At Gangavathi, the average monthly rainfall during August–October was 115.0, 182.7 and 172.0 mm, respectively (Table 1). Since crop season falls during this period, 60% of the total rainfall received during this period was considered as effective rainfall. Net volume of water required per plant for drip irrigation was calculated using relationship; V ¼ IR A B

(2)

where V, IR, A and B refer to net volume of water required by a plant (l d1 plant1), net depth of irrigation (mm d1), area under each plant (m2) and fraction of area (A) covered with foliage, respectively. Water productivity was calculated by dividing the cotton yield per hectare by the depth of water applied including the effective rainfall.

2.3.

Soil salinity

Soil samples, down to 0.6 m at 0.15 m intervals, were drawn initially at sowing, 90 days after sowing and finally at crop harvest (240 days). Samples were air-dried and ground to pass a mesh of 2 mm size and were analyzed for soil salinity (1:2.5 soil: water extract). Time weighted mean salinity (TWMS) was calculated as; TWMS ¼

fðECei þ ECe90 Þ=2g 90 þ fðECe90 þ ECef Þ=2g 150 N (3)

where ECei, ECe90 and ECef refer to soil salinity (dS m1) initially at sowing, after 90 days of sowing and finally at crop harvest (dS m1), respectively while N is total crop period (240 days).

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Total 483.5

2.4.

Total 467.5 0.95 0.42 3.60 0.13 1.05 0.08 ECe, soil salinity; WT, water table; ECwt, salinity of ground water.

2.45 0.07 1.15 0.09 2.20 0.08 1.25 0.08 Mean

Benefit-cost analysis

Benefit-cost analysis was done by considering the fixed cost of the system, cost of cultivation, water used and marketable yield of produce. The fixed cost of the system-included pump, filter, main line, sub main line, bypass assembly, emission devices and other accessories. The useful life of the drip system was considered to be 15 years (Raina et al., 1998) and thus its fixed cost was computed to be US$ 1690.2 ha1. The seasonal cost of irrigation i.e. US$ 160.1 ha1 includes depreciation, prevailing rate of interest and repair and maintenance cost of drippers during the crop period was computed as 6.67, 10 and 1% per annum of the fixed cost, respectively. The cost of cultivation, worked out by considering the cost of land preparation, seeds cost, inter-cultivation, fertilizers, plant protection measures, energy cost, etc. was US$ 233.4 and 272.3 for drip and furrow irrigation, respectively. Thus, total seasonal cost of drip irrigation was US$ 393.5 ha1. Income from the produce was estimated using prevailing average market price @ US$ 355.5 Mg1. The total cost of production, benefit-cost ratio and net profit per centimetre of water used were then estimated using the procedure outlined by Rao (1994).

5.60 0.73

85.5 134.5 263.5 – – – – – – 144.5 231.0 80.5 – 11.5 – – – – 3.95 5.00 5.50 9.07 9.17 6.37 3.85 3.65 3.65 0.95 0.70 0.55 0.85 0.95 1.00 1.10 1.15 1.35 3.45 2.10 2.07 4.05 6.17 4.75 3.47 3.37 3.07 1.10 0.80 0.65 0.85 1.10 1.15 1.20 1.25 1.40 2.20 2.45 2.43 3.40 2.67 2.35 2.20 2.25 2.10 1.15 0.90 0.70 1.05 1.15 1.25 1.30 1.35 1.50 2.10 2.08 2.06 2.07 2.52 2.77 2.12 2.05 2.00 1.30 0.95 0.85 1.15 1.30 1.35 1.40 1.45 1.55 August September October November December January February March April

2001–2002 2000–2001 ECwt (dS m1) WT (m) ECwt (dS m1) WT (m) ECwt (dS m1) WT (m) ECwt (dS m1) WT (m)

Block III (ECe 10.4 0.5 dS m1) Block II (ECe 9.1 0.7 dS m1) Block I (ECe 8.0 0.4 dS m1) Month

Table 1 – Water table depth and salinity of ground water (average data of the years 2000–2001 and 2001–2002)

Block IV (ECe 15.1 0.8 dS m1)

Rainfall (mm)


3.

Results and discussion

3.1.

Vegetative growth and yield parameters

The overall growth monitored in terms of parameters like plant height, number of branches and bolls per plant was better under drip irrigation as compared with furrow irrigation (Table 2). Plants also showed better growth when applied irrigation water, IW equaled 1.2 ET. Better growth ultimately resulted in higher cotton yields from drip-irrigated crop where the yields obtained were 38% higher than the furrow irrigated crop. Cetin and Bilgel (2002) have earlier reported that a frequent and consistent application of water in the vicinity of root through drippers provides better soil moisture in the root zone and thus resulting in higher yields. Considering the yield of cotton when applied water equaled 0.8 ET i.e. as recommended for normal conditions (RY), improvements in the yields obtained were 11, 33 and 15% when the quantities of applied water equaled 1.0, 1.2 and 1.4 times ET under drip irrigation. Similarly, the respective yields improved by 17, 65 and 30%under furrow irrigation indicating more benefits of enhanced irrigation water supplies under the latter. It may be stated here that the calculated values of leaching fraction (LF) were 0.10, 0.19, 0.27 and 0.33 at applied water quantities of 0.8, 1.0, 1.2 and 1.4 ET under drip irrigation and the counter values for furrow irrigation were 0.22, 0.28, 0.33 and 0.37. Thus, provision for higher LF’s vis-avis leaching requirements were certainly beneficial in enhancing the yields. On an average, the reductions in yields under the blocks of higher salinity i.e. 9.5 dS m1 (block II), 10.4 dS m1 (block III) and 15.1 dS m1 (block IV) were 13, 22 and 56% when compared with yields from block I (ECe 8.0 dS m1, reference for relative yield) when irrigated with drips. Similarly, the reductions in yields with furrow irrigation of higher salinity blocks II, III and IV equaled 13, 32 and 66%,

33


Table 2 – Growth and yield of cotton as influenced by different treatments Irrigation method

Applied water (ET)

Block I 2000– 2001

2001– 2002

63.2 64.1 74.1 72.3 61.2 62.7 62.4 68.0

(b) Number of bolls per plant Drip 0.8 11.5 1.0 13.4 1.2 15.8 1.4 15.0 Furrow 0.8 10.2 1.0 11.6 1.2 14.4 1.4 13.0

(a) Plant height (cm) Drip 0.8 1.0 1.2 1.4 Furrow 0.8 1.0 1.2 1.4

(c) Cotton yield (Mg ha1) Drip 0.8 1.0 1.2 1.4 Furrow 0.8 1.0 1.2 1.4

1.22 1.35 1.82 1.61 1.05 1.15 1.48 1.41

Block II Mean

2000– 2001

72.3 76.7 86.9 81.8 72.3 73.2 78.4 74.1

67.7 70.4 80.5 77.0 66.8 67.9 70.4 71.0

63.7 71.2 76.1 71.8 48.6 48.9 69.2 56.4

70.2 74.1 82.5 74.2 63.6 68.7 73.9 74.2

66.9 72.7 79.3 73.0 56.1 58.8 71.5 65.3

12.0 12.0 15.8 14.2 8.8 11.2 14.8 11.4

11.7 12.7 15.8 14.6 9.5 11.4 14.6 12.2

9.8 12.6 17.2 14.7 9.5 13.1 13.7 13.6

11.2 11.2 13.2 12.0 9.8 10.0 11.8 10.0

10.5 11.9 15.2 13.4 9.7 11.6 12.7 11.8

1.50 1.55 1.74 1.65 1.10 1.21 1.59 1.28

1.36 1.45 1.78 1.63 1.07 1.18 1.53 1.34

1.16 1.29 1.55 1.39 0.87 0.91 1.35 1.07

2001– 2002

Block III

1.30 1.33 1.47 1.34 1.05 1.13 1.32 1.17

Mean

1.23 1.31 1.51 1.36 0.96 1.02 1.33 1.12

2000– 2001

Block IV

2001– 2002

Mean

49.1 50.9 64.3 56.7 45.4 49.0 56.1 54.0

74.8 77.9 82.0 74.7 55.3 67.7 81.6 66.5

9.0 10.0 16.2 11.1 6.0 6.9 9.5 6.9

13.0 13.2 14.8 13.4 9.6 13.0 13.4 11.0

0.82 1.04 1.24 1.07 0.54 0.56 1.00 0.74

1.29 1.34 1.58 1.42 0.59 0.94 1.51 1.11

2000– 2001

2001– 2002

Mean

61.9 64.4 73.2 65.7 50.3 58.3 68.9 60.3

35.4 41.2 50.7 47.5 29.8 31.7 47.2 44.2

57.6 63.2 76.6 55.2 49.6 50.6 62.0 57.0

46.5 52.2 63.6 51.3 39.7 41.1 54.6 50.6

11.0 11.6 15.5 12.2 7.8 9.9 11.4 8.9

3.0 3.0 6.7 3.7 1.5 1.7 3.0 4.2

13.4 19.8 20.8 17.2 10.4 13.2 18.2 10.2

8.2 11.4 13.7 10.4 5.9 7.4 10.6 7.2

1.05 1.19 1.40 1.24 0.57 0.75 1.25 0.93

0.30 0.32 0.62 0.35 0.10 0.18 0.36 0.19

0.85 1.09 1.13 0.82 0.27 0.46 0.66 0.30

0.57 0.70 0.87 0.58 0.18 0.32 0.51 0.24

Block I = ECe 8.0 0.4 dS m1, WT 1.25 0.08 m; block II = ECe 9.1 0.7 dS m1, WT 1.15 0.08 m; block III = ECe 10.4 0.5 dS m1, WT 1.05 0.09 m and block IV = ECe 15.1 0.8 dS m1, WT 0.95 0.07 m.

respectively indicating more severe effects in the latter. With in the different salinity blocks, the maximum yield (1.78 Mg ha1) was recorded from block I when irrigated with drippers at 1.2 ET while the minimum yield (0.57 Mg ha1) was recorded from block IV irrigated at 0.8 ET. Similarly, the highest yields (1.34 Mg ha1) with furrow irrigation were

recorded with 1.4 ET from block I and the lowest (0.18 Mg ha1) with 0.8 ET from block IV. It may be stated here that the calculated values of leaching fraction were 0.10, 0.19, 0.27 and 0.33 at applied water quantities of 0.8, 1.0, 1.2 and 1.4 ET under drip irrigation and the counter values for furrow irrigation were 0.22, 0.28, 0.33 and 0.37. Lower salinities in

Table 3 – Water applied, water saving and water productivity of cotton (average data of the years 2000–2001 and 2001–2002) Particulars

Irrigation water (cm) Number of irrigations Irrigation interval (days) Total rainfall (cm) Effective rainfall (cm) Total water applied (cm) Saving of irrigation water over furrow irrigation (%) Water Productivity (kg ha1 cm1) Block I Block II Block III Block IV

Drip irrigation (ET)

Furrow irrigation (ET)

0.8

1.0

1.2

1.4

0.8

1.0

1.2

1.4

35.3 51 3 47.5 28.5 63.8 21.5

42.6 51 3 47.5 28.5 71.2 16.3

49.9 51 3 47.5 28.5 78.4 12.3

57.3 51 3 47.5 28.5 85.8 9.1

45.0 8 23 47.5 28.5 73.5 –

51.0 9 20 47.5 28.5 79.5 –

57.0 10 17 47.5 28.5 85.5 –

63.0 11 15 47.5 28.5 91.5 –

21.3 19.3 16.5 9.0

20.4 18.4 16.7 9.9

22.7 19.3 18.0 11.2

19.0 15.9 14.5 6.8

14.6 13.0 7.7 2.6

14.8 12.8 9.4 4.0

17.9 15.5 14.6 6.0

14.7 12.2 10.1 2.7

Block I = ECe 8.0 0.4 dS m1, WT 1.25 0.08 m; block II = ECe 9.1 0.7 dS m1, WT 1.15 0.08 m; block III = ECe 10.4 0.5 dS m1, WT 1.05 0.09 m and block IV = ECe 15.1 0.8 dS m1, WT 0.95 0.07 m.

34


the root zone are expected with higher LF’s but it seems that the overall salinities dominated in determining the crop responses rather than provision for higher LF’s in higher salinity blocks. Since the maintenance of higher LF’s means the application of higher quantities of water that may simultaneous induce soil saturation and thus root aeration problems under shallow water-table conditions. These counteracting effects were more obvious in higher salinity blocks with shallower water tables where even after provision for higher leaching requirements, the yields could not match with lower salinity block, e.g. the overall benefits in RY ranged only between 0–22 and 5–31% in block IV with drip and furrow irrigation, respectively.

3.2. Water requirement, water saving and water productivity Irrigation requirement of cotton crop during its eight months period (August to March) were the maximum during March due to higher temperature and evaporative demands. The net amount of irrigation water applied was ranged from 35.3 cm with drip irrigation at 0.8 ET to 63.0 cm with furrow irrigation at 1.4 ET (Table 3). Number of irrigations (each of 60 mm water) equaled 8, 9, 10 and 11 furrow irrigations were applied for 0.8, 1.0, 1.2 and 1.4 ET, respectively. The total amount of water applied as irrigation plus effective rainfall was 73.5, 79.5, 85.5 and 91.5 cm for the irrigation levels of 0.8,

1.0, 1.2 and 1.4 ET, respectively. In case of drip irrigation where the irrigation interval was 3 days, a total of 51 irrigations were applied. Here, the total amount of water applied was 63.8, 71.2, 78.4 and 85.8 cm for 0.8, 1.0, 1.2 and 1.4 ET, respectively. The net saving in irrigation water with drip irrigation was 21.5, 16.3, 12.3 and 9.1% at irrigation levels of 0.8, 1.0, 1.2 and 1.4 ET, respectively when compared with the same levels of furrow irrigation. Computations of water productivity (cotton yield/total applied water) showed that it was highest (22.7 kg ha1 cm1) in the low salinity block with drip irrigation applied at 1.2 ET and was lowest (14.6 kg ha1 cm1) in case of furrow irrigation applied at 0.80 ET (Table 3). Similar results were also observed in block II and III. In block IV, the highest and lowest water productivity of 11.2 and 2.6 kg ha1 cm1 were recorded in drip irrigation at 1.2 ET and furrow irrigation at 0.8 ET, respectively. Higher water productivity in case of different drip irrigation treatments was obviously due to higher yields accompanied by saving of irrigation water as compared to furrow method of irrigation. These results corroborated the earlier findings of Tiwari et al., (1998, 2003), Manjunatha et al., (2001), Cetin and Bilgel, (2002).

3.3.

Soil salinity

Initial soil salinity (surface 0.6 m soil) as determined at the time of sowing was highest in block IV (ECe 15.1 dS m1). After

Fig. 1 – Initial and final salinity under various quantities of applied water with drip and furrow irrigation: (a) soil depth 0– 0.30 m; (b) soil depth 0.30–0.60 m.

35


the crop harvest, higher soil salinity was observed in case of furrow irrigation than drip irrigation. In spite of a relatively deeper water table (1.25 m) in block I, soil salinity (0–0.30 m) after the crop harvest increased marginally with both the methods of irrigation where as in the other blocks, a marginal reduction in soil salinity was observed under drip irrigation (Fig. 1). The reduction was much higher for 1.2 ET level of irrigation as compared with the other levels. Even though good quality canal water (ECw 0.2 dS m1) was used for furrow irrigation, considerable build-up in soil salinity was observed at all the irrigation levels and in all the salinity blocks. Similar trend was also observed in the lower soil depth (0.30–0.60 m), although the comparative effects were lesser (Fig. 1). The calculated values of time weighted mean salinity were higher under furrow irrigation. It may be pointed out that soil samples were drawn from the crop rows to have the maximum representation of rooting zone. The capillary fluxes from the shallow and saline water table seems to have lead to evaporation and re-salinisation of ridge soils during the periods between the respective irrigations and since irrigation was applied through furrows, little leaching of the ridge soil is expected. However, the reduced irrigation interval (3 days) must have resulted in the maintenance of a constant downward flux and thus lowering the salinities under drip irrigation. Latter results agree with earlier reports of Saggu and Kaushal (1991), who reported that the salinity in the root zone of potato was lower for drip irrigation than for furrow irrigation.

3.4.

Production function

Production functions were developed separately for drip and furrow irrigation using total amount of water applied (IW, cm) and time weighted mean soil salinity (ECe, dS m1) and these could be represented as:

Fig. 2 – Predicted yields of cotton at different quantities of applied water and soil salinity under drip (—) and furrow (– – –) irrigation.

(I) Drip irrigation YðMg ha1 Þ ¼ 0:2070 AW 0:0012 AW2 þ 0:0807 ECe 0:0049 ECe2 0:0014 AW ECe 6:5945ðR2 ¼ 0:974 Þ

(4)

(II) Furrow irrigation Y ðMg ha1 Þ ¼ 0:3853 AW 0:0021 AW2 þ 0:0253 ECe 0:0005 ECe2 0:0016 AW ECe 14:9117ðR2 ¼ 0:877 Þ

(5)

Predictions using above equations (Fig. 2) show that higher yields of cotton can be maintained at given salinity and applied water especially when limited quantities of the latter are available. Also for the maximum yields with drip irrigation, the quantities of applied water should equal 80.9, 89.7, 78.5, 77.4 and 76.2 cm at ECe of 8, 10, 12, 14 and 16 dS m1,

Table 4 – Economic analysis of cotton under drip and furrow irrigation (average data of the years 2000–2001 and 2001– 2002) Block

Drip irrigation (ET) 0.8

Furrow irrigation (ET)

1.0

1.2

1.4

0.8

1.0

1.2

1.4

Gross income (US$) I 556.5 II 504.1 III 431.3 IV 235.7

577.4 522.0 473.0 281.1

689.8 565.0 546.4 339.4

620.1 518.9 473.6 222.9

381.9 341.3 201.6 66.8

418.9 362.7 257.0 114.1

545.0 474.3 446.5 180.9

478.2 398.2 329.6 87.1

Gross benefit-cost ratio I 1.23 II 1.11 III 0.95 IV 0.52 90.1

1.31 1.18 1.07 0.64 122.1

1.61 1.32 1.27 0.79 239.4

1.47 1.23 1.13 0.53 186.1

1.40 1.25 0.74 0.25 108.2

1.54 1.33 0.94 0.42 147.3

2.00 1.74 1.64 0.66 271.7

1.76 1.46 1.27 0.32 204.0

Net profit per cm of water used (US$) I 1.41 1.71 II 0.69 1.02 III () 0.29 0.40 IV () 2.95 () 2.00

3.05 1.59 1.37 () 1.04

2.17 1.06 0.57 () 2.15

1.47 0.94 () 0.96 () 2.77

1.85 1.13 () 0.19 () 1.97

3.17 2.36 2.04 () 1.06

2.23 1.37 0.62 () 2.02

() indicates loss; block I = ECe 8.0 0.4 dS m1, WT 1.25 0.08 m; block II = ECe 9.1 0.7 dS m1, WT 1.15 0.08 m; block III = ECe 10.4 0.5 dS m1, WT 1.05 0.09 m and block IV = ECe 15.1 0.8 dS m1, WT 0.95 0.07 m.

36


respectively while for furrow irrigation the predicted values were 87.0, 86.3, 85.5, 84.8 and 84.0 cm, respectively. These in turn mean that the quantities of applied water need to be adjusted in such a manner to avoid the initiation of aeration problems those negate the effects of more leaching with higher quantities especially under shallower water table-visa-vis higher soil salinities.

3.5.

Economic analysis

Economic analysis of drip and furrow irrigation for cotton (Table 4) revealed that in all the four different salinity blocks, gross income (including the income generated from the additional area cultivated due to saving in irrigation water) was higher for drip irrigation than for furrow irrigation. However, gross benefit-cost ratio was lower for drip irrigation than for furrow irrigation due to higher initial cost incurred in drip irrigation. In first salinity block, the benefit-cost ratios vary between 2.00 (highest) for furrow irrigation at 1.2 ET and 1.23 (lowest) for drip irrigation at 0.8 ET. The net profit per centimetre of water used was higher for furrow irrigation at 1.2 ET (US$ 3.17) followed by drip irrigation at 1.2 ET (US$ 3.05) and minimum for drip irrigation at 0.8 ET (US$ 1.41). Similar results were also obtained from blocks II and III. Nevertheless, gross benefit-cost ratio was higher under drip irrigation in block IV obviously due to enhanced yields as compared with furrow irrigation.

4.

Conclusions

Based on two years results, it was concluded that irrigation methods and levels had a significant effects on growth and yield of cotton. The higher cotton yields and water productivities were obtained with drip irrigation as compared with furrow irrigation. Yields improved with quantities of water applied but tended to decline after some maxima due to inducement of aeration problems. Higher time-weighted mean soil salinity was maintained in the root zone with furrow than drip irrigation. As the time-weighted mean salinity increased, there was a considerable reduction in crop yield irrespective of methods and levels of irrigation but the decline was more pronounced under furrow irrigation than drip irrigation. The quadratic production functions with applied water and soil salinity showed considerably higher coefficient of determination (R2 = 0.974** and 0.877** under drip and furrow irrigation). If plenty of good water is available, irrigation quantities can be 1.2 times the evapotranspiration demands with furrow irrigation. Nevertheless, it is a common observation that there is general scarcity of fresh water especially in the tail end of canal commands where the use of saline ground water seems a feasible solution through drip irrigation even though the gross benefit-cost ratios may be marginally lesser. However, long-term effects of use of saline waters for irrigation on

soil physico-chemical properties and the problem of emitter clogging need to be further evaluated for sustainability of the system.

references

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