Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol45.Issue3.Aug2000:
Agricultural Water Management 45 (2000) 331±342
Response of six vegetable crops
to irrigation schedules
M. Imtiyaz*, N.P. Mgadla, B. Chepete,
S.K. Manase
Department of Agricultural Research, Regional Agricultural Research Of®ce,
P.O. Box 151, Maun, Botswana
Accepted 22 October 1999
Abstract
Field studies were carried out for 2 years (1995 and 1996) on sandy soil in the north western
region of Botswana to examine the effect of irrigation schedules (18 mm of water in each irrigation
at 11, 22, 33, 44 and 55 mm of cumulative pan evaporation, CPE) on marketable yield, irrigation
production ef®ciency and economic return of cabbage, spinach, rape, carrot, tomato and onion
under sprinkler irrigation. The higher mean marketable yield of cabbage (71.65 t/ha), spinach
(33.53 t/ha), rape (73.22 t/ha), carrot (56.76 and 38.39 t/ha), tomato (46.81 t/ha) and onion (56.05
t/ha) were recorded for irrigation scheduled at CPE of 22, 11, 22, 22, 22 and 11 mm, respectively.
The irrigation at CPE of 22 mm resulted in higher irrigation production ef®ciency of cabbage
(11.32 kg/m3), spinach (3.35 kg/m3), carrot (9.83 and 6.66 kg/m3), tomato (5.90 kg/m3) and onion
(6.26 kg/m3), but rape (12.03 kg/m3) gave higher irrigation production ef®ciency at CPE of 33±
55 mm. Irrigation scheduled at 22 mm CPE resulted in a higher net return of 47 131, 104 398,
76 691 and 93 192 P/ha for cabbage, rape, carrot and tomato, respectively, but spinach and onion
gave a higher net return of 27 086 and 83 934 P/ha at 11 mm CPE(1 US$ 4.55 P). Irrigation
scheduled at 22 mm CPE gave a higher B/C ratio of 2.92, 1.94, 5.40, 4.98, 4.91 and 4.82 for
cabbage, spinach, rape, carrot, tomato and onion, respectively. Seasonal water applied and
marketable yield of vegetable crops exhibited quadratic relationships (R2 0.85±0.99). The ®tted
regression models attained the maximum yield, net return and B/C ratio at CPE of 16±18 mm. The
results revealed that rape is the most remunerative crop followed by tomato, onion, carrot, cabbage
and spinach. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sprinkler irrigation; Irrigation scheduling; Marketable yield; Net return; Vegetable crops
*
Corresponding author. Tel.: 267-660327; fax: 267-663761.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 1 0 5 - 5
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M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
1. Introduction
Botswana is a semiarid country with limited water resources. Water is the major
limiting factor for crop production in most of the agricultural regions of Botswana. Water
for irrigation, domestic and industrial needs is increasing considerably to meet the
demands for a growing population. Presently, 80% of vegetables are imported from
neighbouring countries. However, the government of Botswana through its financial
assistance programme is trying to encourage farmers to grow vegetable and fruit crops in
order to reduce imports. Therefore, it is necessary to develop efficient, reliable and
economically viable irrigation management strategies for effective use of the existing
limited water resources. Improper irrigation management practices do not only waste
scarce and expensive water resources but also decrease marketable yield and economic
return.
The irrigation scheduling which determines the amount and frequency of irrigation is
governed by many complex factors, but climate plays a major role. Therefore, it is
important to develop irrigation scheduling techniques under prevailing climatic condition.
Numerous studies were carried out in the past on the development and evaluation of
irrigation scheduling techniques under a wide range of irrigation system and management, soil, climate and crop conditions (Hagan and Laborde, 1964; Jensen et al., 1970;
Imtiyaz and Shiromani, 1990; Wanjura et al., 1990; Imtiyaz et al., 1992; Steele et al.,
1997). The meteorological-based irrigation scheduling approach, such as pan evaporation
replenishment, cumulative pan evaporation (CPE), and ratio between irrigation water and
cumulative pan evaporation etc., was used by many researchers due to its simplicity, data
availability and higher degree of adaptability at the farmers level (Prihar et al., 1974;
Singh, 1987; Pawar et al., 1991; Singh and Mohan, 1994; Imtiyaz et al., 1995, 1999;
Singh et al., 1997). In Botswana, evaporation from USWB Class A open pan is being
systematically recorded and readily available for irrigation scheduling.
Cabbage, spinach, rape, carrot, tomato and onion are the most important vegetable
crops in Botswana. Due to lack of proper irrigation scheduling techniques, the average
yield of these vegetable crops is low because of excess or deficit soil moisture regimes.
The quick-coupling sprinkler irrigation is the most common method of irrigation for
vegetable crops in Botswana, but information on economic viability of this system is
lacking. Therefore, the objectives of the present study were to examine the effect of
irrigation schedules on marketable yield, irrigation production efficiency and economic
return of vegetable crops.
2. Materials and methods
The field experiments were conducted during the winter/summer crop growing
season of 1995 and 1996 at the Etsha 6 experimental station of the Department of
Agricultural Research, Botswana (198370 S, 228170 E, 964 m above MSL). The mean
monthly maximum air temperature, minimum air temperature, relative humidity, wind
velocity and pan evaporation during the crop growing season (June to November) ranged
from 22.7 to 36.88C, 4.8±228C, 35.3±75.2%, 1.0±1.9 m/s and 3.4±11.3 mm/day,
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
333
respectively. The rainfall during the month of October and November in 1995 was 8.7 and
42.8 mm and in 1996 was 1.6 and 40.0 mm, respectively. According to rainfall and
potential evapotranspiration, the climate in this part of the country has been classified
as semiarid with mild winters and hot summers. The soil in the experimental field was
sandy with low organic matter content. The soil moisture content at field capacity
(ÿ0.03 MPa) and wilting point (ÿ1.5 MPa) was 0.1058 and 0.0345 m3/m3, respectively.
The average bulk density of soil was 1.41 g/cm3. The plant available soil moisture was
72 mm/m.
The experiment was laid out in a randomised block design with three replicates. The
experimental plots were sub-divided for six vegetable crops. The size of plots for
cabbage, spinach, rape, carrot, tomato and onion were 45, 45, 45, 25, 45 and 25 m2,
respectively. A buffer zone spacing of 2.0 m was provided between the crops. Prior to
planting, the experimental field for vegetable crops received 94.3 kg/ha phosphorus
(P2O5) and 62.9 kg/ha potash (K2O). The tomato and carrot received an additional
amount of 62.5 kg/ha K2O. Before planting, the experimental field for cabbage, spinach,
and rape received 50 kg/ha nitrogen, whereas carrot, tomato and onion received 63, 75
and 42 kg/ha nitrogen, respectively. Spinach (Var. Fordhook Giant) and carrot (Var.
Brazillia) were sown on the 30th June in both 1995 and 1996 with plant and row spacings
of 0.15 m 0.3 m and 0.05 m 0.4 m, respectively. Cabbage (Var. Grandslam), rape
(Var. Giant Essex), tomato (Var. Sixpack) and onion (Var. Texas Grano 502 PRR)
seedlings were transplanted from 11th to 14th July in 1995 and 1996 with plant and row
spacings of 0.5 m 0.5 m, 0.3 m 0.4 m, 0.3 m 0.75 m and 0.1 m 0.3 m, respectively. The experimental plots for cabbage, rape and spinach received 50 kg/ha of
nitrogen after 3, 5 and 8 weeks of transplanting. The experimental plots for tomato and
onion received 37.5 and 42 kg/ha of nitrogen after 4 and 8 weeks of transplanting, but
carrot received 21 kg/ha of nitrogen after 8 weeks. Spinach and carrot emerged during the
7th to 9th of July and thinning was done after 3 weeks to maintain desired plant
population. During the first 3 weeks, the crops were irrigated daily at 75% of pan
evaporation losses in order to establish newly planted seedlings.
The experiment consisted of five treatments formed by irrigation schedules at CPE
(cumulative pan evaporation) of 11, 22, 33, 44 and 55 mm during the entire crop growth
period with the depth of 18 mm at each irrigation. The above-mentioned CPE values were
selected in order to create excess and deficit soil moisture regimes as well as match the
frequency of irrigation, i.e. days. The cumulative pan evaporation was calculated as a sum
of daily 7-years recorded evaporation from USWB Class A open pan. The pan is located
at the climate station adjacent to the irrigation experimental field with moderate grass
cover. The irrigation in the respective treatments were applied when CPE reached
approximately 11, 22, 33, 44 and 55 mm. The irrigation during the crop growing season
was applied by quick-coupling sprinkler irrigation system.
Cabbage, spinach, rape, tomato, carrot and onion were harvested from 4th to 15th
October, 13th September to 2nd November, 31st August to 2nd November, 1st October to
16th November, 2nd to 4th October and 14th to 16th November in both 1995 and 1996.
For economic analysis, both fixed and operating cost were included. The total
production cost, gross revenue and net return under different irrigation schedules were
estimated on the following assumptions:
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M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Salvage value of the components 0
Useful life of engine, pump, pump house and irrigation system 12 years
Interest rate 14%
Repair and maintenance 5%
No. of crops/year 1
The fixed cost including engine, pump, pump house and quick-coupling sprinkler
irrigation system was calculated. The annual fixed cost was calculated by the following
approach (James and Lee, 1971):
CRF
i 1 in
1 in ÿ 1
Where CRF is the cost recovery factor; i is the interest rate (fraction) and n is the useful
life of the component (years).
Annual fixed cost/ha CRF fixed cost/ha
The operating cost, including labour, land preparation, seeds, fertilizers, chemicals,
diesel and oil, and repair and maintenance, was calculated. The total cost of production
under different irrigation schedules was estimated by adding fixed and operating cost.
The gross revenue for different irrigation schedules was estimated taking into account the
marketable yield and market price of cabbage, spinach, rape, carrot, tomato and onion.
Subsequently, the net return under different irrigation schedules was calculated
considering total cost of production and gross revenue. The benefit cost ratio (B/C)
under different irrigation schedules was calculated as follows:
B
Gross revenue P=ha
C Total cost of production P=ha
3. Results and discussion
3.1. Yield and irrigation production ef®ciency
In both years, maximum heads/m2, head weight and marketable yield of cabbage were
recorded when irrigation was applied at cumulative pan evaporation (CPE) of 22 mm
(Table 1). Further increase in irrigation level resulting from CPE of 11 mm did not
increase the marketable yield significantly. Irrigation at CPE of 33, 44 and 55 mm
reduced the marketable yield by reducing heads/m2 and head weight. The results revealed
that the head weight (28±66%) was more affected by irrigation levels compared to heads/
m2 (12±42%). Irrigation at CPE of 22 mm resulted in higher irrigation production
efficiency. A further increase or decrease in irrigation levels resulting from CPE of 11, 33,
44 and 55 mm reduced the irrigation production efficiency significantly.
In both years, irrigation at CPE of 11 mm produced a significantly higher marketable
yield of spinach (Table 2). Marketable yield decreased significantly with decrease in
irrigation levels resulting from CPE of 22, 33, 44 and 55 mm. However, irrigation at CPE
of 22 mm gave a higher irrigation production efficiency because yield reduction (20.8%)
335
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 1
Effect of irrigation schedules on yield, yield components and irrigation production ef®ciency of cabbage in 1995
and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean
marketable
yield (t/ha)
Mean
marketable
head (mÿ2)
Mean head
weight (kg)
Mean irrigation
production
efficiency (kg/m3)
95
96
95
96
95
96
95
96
95
96
1115
629
467
377
323
±
1140
636
474
384
330
±
73.34
70.91
45.51
26.02
14.71
3.04
74.26
72.39
45.23
24.69
12.97
7.05
3.75
3.71
3.27
2.65
2.21
0.21
3.79
3.74
3.28
2.55
2.09
0.22
1.96
1.91
1.39
0.98
0.67
0.06
1.97
1.94
1.38
0.97
0.63
0.14
6.58
11.26
9.75
6.90
4.55
0.39
6.51
11.38
9.54
6.43
3.93
1.22
was less than the seasonal water application (45.3%). The difference in irrigation
production efficiency between CPE of 22 and 33 mm was statistically similar. Irrigation
production efficiency decreased severely (41±64%) with irrigation at CPE of 44 and
55 mm due to severe reduction in marketable yield.
In 1995, maximum marketable rape yield was obtained with irrigation at CPE of 11±
33 mm, but in 1996 irrigation at CPE of 33 mm resulted in a significantly lower yield
compared to CPE of 11 and 22 mm, probably due to variation in climatic conditions
(Table 3). In both years, marketable yield decreased significantly with decrease in
irrigation levels resulting from CPE of 44 and 55 mm. This is due to the fact that
irrigation at CPE of 44 and 55 mm reduced the plant growth as well as increased the nonmarketable yield. In both 1995 and 1996 seasons, irrigation at CPE of 33 to 55 mm gave a
significantly higher irrigation production efficiency because yield reduction was less
compared to seasonal water application. Irrigation at CPE of 11 mm resulted in minimum
irrigation production efficiency because it increased the seasonal water application
considerably (74.4%) without any significant improvement in marketable yield.
In both years, irrigation at CPE of 11 and 22 mm produced higher total and root yield
of carrot (Table 4). Marketable yield decreased significantly with decrease in irrigation
Table 2
Effect of irrigation schedules on yield and irrigation production ef®ciency of spinach in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water applied
(mm)
Mean marketable
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
33.25
27.42
19.79
9.22
4.98
2.09
33.81
25.72
16.79
9.02
4.68
2.77
2.34
3.55
3.55
2.05
1.26
0.30
2.28
3.15
2.80
1.90
1.16
0.43
336
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 3
Effect of irrigation schedules on yield and irrigation production ef®ciency of rape in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean marketable
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
72.93
72.12
68.11
57.90
50.99
5.19
73.24
74.31
65.61
55.43
46.27
6.45
5.13
9.33
12.23
12.90
12.91
1.03
4.94
9.11
10.94
11.69
11.51
1.07
levels resulting from CPE of 33, 44 and 55 mm. This is due to the fact that irrigation at
CPE of 33, 44 and 55 mm reduced the plant growth as well as increased the nonmarketable yield. Maximum irrigation production efficiency was obtained at CPE of
22 mm and it decreased significantly with a decrease in irrigation levels. Irrigation at
CPE of 11 mm reduced irrigation production efficiency considerably (44%) because it
increased the seasonal water application by 43% without significant improvement in
marketable yield.
In both 1995 and 1996 seasons, irrigation at CPE of 11 and 22 mm produced higher
fruit weight and marketable yield of tomato (Table 5). Marketable yield decreased
significantly with a decrease in irrigation levels resulting from CPE of 33, 44 and 55 mm
due to reduction in fruit weight and marketable number of fruits. In both years, higher
irrigation production efficiency was obtained with irrigation at CPE of 22 and 33 mm,
thereafter it decreased significantly, because yield reduction was higher than seasonal
water application. Irrigation at CPE of 11 mm in which seasonal water application was
maximum, reduced the irrigation production efficiency considerably (51%) because it
increased the seasonal water application without significant improvement in yield.
Table 4
Effect of irrigation schedules on yield and irrigation production ef®ciency of carrot in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean marketable yield
(t/ha)
Mean irrigation production
efficiency (kg/m3)
95
Total
Total
986
557
413
341
305
±
96
1032
600
438
366
312
±
Root
Root
95
96
95
96
95
96
95
96
57.01
56.31
36.73
15.45
5.77
3.96
56.21
57.21
37.72
15.54
6.62
3.90
38.23
38.09
24.49
10.22
3.86
2.32
37.12
38.68
24.99
10.31
4.44
2.71
5.77
10.11
8.89
4.53
1.89
0.82
5.45
9.54
8.61
4.24
2.12
0.78
3.86
6.86
5.93
2.99
1.27
0.47
3.60
6.45
5.71
2.82
1.42
0.49
337
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 5
Effect of irrigation schedules on yield, fruit weight and irrigation production ef®ciency of tomato in 1995 and
1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD (P 0.05)
Mean water
applied (mm)
Mean marketable
yield (t/ha)
Mean fruit
weight (g)
Mean (irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
44.58
47.69
36.47
18.16
12.55
2.94
42.90
45.92
38.25
18.20
10.93
4.00
82.15
82.11
64.95
54.74
47.03
3.58
80.67
83.02
67.17
51.49
45.49
5.24
3.14
6.17
6.55
4.04
3.18
0.52
2.89
5.63
6.38
3.84
2.72
0.55
In 1995 maximum bulb yield of onion was recorded with irrigation at CPE of 11 mm,
but in 1996 yield difference between CPE of 11 and 22 mm was statistically similar
(Table 6). Marketable bulb yield decreased significantly with a decrease in irrigation
levels resulting from CPE of 33 to 55 mm due to a reduction in bulb weight. In 1995,
irrigation at CPE of 33 mm resulted in higher irrigation production efficiency, but in 1996
the difference between 22 and 33 mm of CPE was non-significant. The results clearly
indicated that irrigation above 33 mm of CPE reduced the irrigation production efficiency
significantly due to higher yield reduction compared to seasonal water application.
Furthermore, irrigation at CPE of 11 mm, in which seasonal irrigation was maximum,
resulted in minimum irrigation production efficiency because increase in yield was
considerably less than the seasonal water application.
Despite of some variation, the overall results showed that a fixed amount of 18 mm
of irrigation (IW) application at cumulative pan evaporation (CPE) of 22 mm resulted
in higher marketable yield and irrigation production efficiency of vegetable crops
(Tables 1±6). Imtiyaz et al. (1999) reported similar results for broccoli, cabbage, carrot
and rape under drip irrigation. Singh and Mohan (1994) reported a reduction in sugarcane
yield when irrigation was applied beyond a IW/CPE ratio of 1.0. Singh et al. (1997)
Table 6
Effect of irrigation schedules on bulbs yield and irrigation production ef®ciency of onion in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water applied
(mm)
Mean marketable bulbs
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
57.16
49.96
40.80
25.15
20.59
3.49
54.94
49.31
37.97
24.60
18.31
6.01
4.02
6.46
7.33
5.60
5.21
0.58
3.71
6.05
6.33
5.19
4.55
0.86
338
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
observed that the reduction in plant growth and yield of palmarosa at higher irrigation
levels resulted from IW/CPE ratios of 1.1 to 1.5.
3.2. Water supply and marketable yield
The relationship between seasonal water applied and marketable yield of cabbage,
spinach, rape, carrot, tomato and onion are shown in Fig. 1. In both years, seasonal water
applied and marketable yield of vegetable crops exhibited quadratic relationships
(R2 0.85±0.99). Cabbage, spinach, rape, carrot, tomato and onion attained maximum
marketable yields of 86.33, 35.81, 80.70, 70.55 (total) and 47.40 (root), 57.18 and 61.82
t/ha at the seasonal water application of 890, 1248, 1103, 785, 1083 and 1190 mm,
respectively, and thereafter it tended to decline (Fig. 1). Further analysis of regression
models revealed that the above-mentioned maximum yield for different crops correspond
to CPE of 16±18 mm. The quadratic water applied yield relationship results were
probably due to poor aeration and nutrient-leaching caused by excessive soil moisture.
Imtiyaz et al. (1994, 1995, 1996, 1999) reported the quadratic relationships between
seasonal water applied and marketable yield of cabbage, carrot, onion, tomato, green
pepper, broccoli, rape and okra under sprinkler and drip irrigation. Many researchers
reported quadratic crop water production functions for vegetable and field crops under a
wide variety of irrigation systems and regimes, soil and climatic conditions (Musick et al.,
1976; Singh, 1987; Stone et al., 1996; Farah et al., 1997; Howell et al., 1997; Tiwari and
Reddy, 1997).
3.3. Economic return
The total production cost, net return and benefit cost ratio (B/C) for different crops
under different irrigation schedules are presented in Table 7. The total cost of production
increased with increase in irrigation levels. The total cost of production varied amongst
the crops mainly due to difference in crop growing period and labour requirements for
harvesting and packaging etc. Labour costs in performing major farm activities
contributed 40±64% to total cost of production depending upon the crops and irrigation
levels. The fixed cost, repair and maintenance, and pumping cost contributed 8±27.5%,
3.1±7.7% and 6.2±12.9%, respectively, to total cost of production. Cabbage, rape, carrot
and tomato gave the higher net return at CPE of 22, whereas spinach and onion resulted
in higher net return at CPE of 11 mm. Irrigation at CPE of 55 mm resulted in
considerable economic loss from cabbage, carrot and spinach because total cost of
production exceeded the gross revenue. The benefit cost ratio (B/C), which indicates
gross revenue per unit investment, was also influenced by irrigation schedules. Irrigation
at CPE of 22 mm gave higher B/C ratio for cabbage, spinach, rape, carrot, tomato and
onion (Table 7). However, according to the fitted regression models (yield versus water
applied/irrigation schedules), all vegetable crops attained the maximum gross revenue,
net return and B/C ratio at CPE of 16±18 mm. The overall results clearly show that rape is
a more remunerative crop followed by tomato, onion, carrot, cabbage and spinach.
Imtiyaz et al. (1999) reported similar results for cabbage, broccoli, rape and carrot under
drip irrigation.
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
339
Fig. 1. Relationship between water applied and marketable yield of cabbage (a), spinach (b), rape (c), carrot (d),
tomato (e) and onion (f).
340
Treatment
Total production cost (P/ha)
Net return (P/ha)
(irrigation
at CPE, mm) Cabbage Spinach Rape Carrot Tomato Onion Cabbage Spinach Rape
11
22
33
44
55
31678
24519
22140
20929
20201
a
31592
24028
21514
20235
19486
31292
23728
21214
19935
19186
26268
19272
16914
15749
15044
31371
23808
21293
20015
19265
28166 42122
20602 47131
18088 23230
16810
5426
16060 ÿ6361
B/C ratio
Carrot Tomato Onion Cabbage Spinach Rape Carrot Tom Onion
27086 96607 67920 77979
22470 104398 76691 93192
10494 95791 44936 53427
ÿ4275 79229 9914 25435
ÿ11033 65917 ÿ4669 10085
83934
78668
60682
32940
22840
2.33
2.92
2.05
1.26
0.69
1.86
1.94
1.49
0.79
0.43
4.09
5.40
5.52
4.97
4.44
3.59
4.98
3.66
1.63
0.69
3.49
4.91
3.51
2.27
1.52
3.98
4.82
4.36
2.96
2.42
The prices of cabbage, spinach, rape, carrot, tomato and onion are taken at 1.0, 1.75,1.75, 2.50, 2.50 and 2.0 P/kg, respectively, 1 US$ 4.55 P (Botswana Pula).
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 7
Economic analysis of sprinkler-irrigated vegetable crops under different irrigation schedules (average data of 2 years)a
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
341
4. Conclusion
Evaporation from USWB Class A open pan is the most common and simple approach
for scheduling irrigation for field, vegetable and fruit crops. Field extension personnel can
prepare an irrigation scheduling calender taking into account the long-term daily pan
evaporation data that can be easily adopted by the farmers. The experimental results
clearly indicated that a fixed depth of 18 mm of irrigation application at cumulative pan
evaporation of 16±18 mm (June to November) is optimum for sandy soil in order to
achieve maximum marketable yield, net return and benefit cost ratio from sprinklerirrigated cabbage, spinach, rape, carrot, tomato and onion under semiarid climate of north
western region of Botswana.
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James, L.D., Lee, R.R., 1971. Economics of Water Resources Planning. McGraw-Hill, New Delhi, p. 20.
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and soybeans. Trans. ASAE, pp. 489±493.
Prihar, S.S., Gajri, P.R., Narang, R.S., 1974. Scheduling of irrigation to wheat using pan evaporation. Indian J.
Agric. Sci. 44, 567±571.
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and nitrogen levels in a subtropical region. Agric. Water Manag. 26, 253±264.
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Response of six vegetable crops
to irrigation schedules
M. Imtiyaz*, N.P. Mgadla, B. Chepete,
S.K. Manase
Department of Agricultural Research, Regional Agricultural Research Of®ce,
P.O. Box 151, Maun, Botswana
Accepted 22 October 1999
Abstract
Field studies were carried out for 2 years (1995 and 1996) on sandy soil in the north western
region of Botswana to examine the effect of irrigation schedules (18 mm of water in each irrigation
at 11, 22, 33, 44 and 55 mm of cumulative pan evaporation, CPE) on marketable yield, irrigation
production ef®ciency and economic return of cabbage, spinach, rape, carrot, tomato and onion
under sprinkler irrigation. The higher mean marketable yield of cabbage (71.65 t/ha), spinach
(33.53 t/ha), rape (73.22 t/ha), carrot (56.76 and 38.39 t/ha), tomato (46.81 t/ha) and onion (56.05
t/ha) were recorded for irrigation scheduled at CPE of 22, 11, 22, 22, 22 and 11 mm, respectively.
The irrigation at CPE of 22 mm resulted in higher irrigation production ef®ciency of cabbage
(11.32 kg/m3), spinach (3.35 kg/m3), carrot (9.83 and 6.66 kg/m3), tomato (5.90 kg/m3) and onion
(6.26 kg/m3), but rape (12.03 kg/m3) gave higher irrigation production ef®ciency at CPE of 33±
55 mm. Irrigation scheduled at 22 mm CPE resulted in a higher net return of 47 131, 104 398,
76 691 and 93 192 P/ha for cabbage, rape, carrot and tomato, respectively, but spinach and onion
gave a higher net return of 27 086 and 83 934 P/ha at 11 mm CPE(1 US$ 4.55 P). Irrigation
scheduled at 22 mm CPE gave a higher B/C ratio of 2.92, 1.94, 5.40, 4.98, 4.91 and 4.82 for
cabbage, spinach, rape, carrot, tomato and onion, respectively. Seasonal water applied and
marketable yield of vegetable crops exhibited quadratic relationships (R2 0.85±0.99). The ®tted
regression models attained the maximum yield, net return and B/C ratio at CPE of 16±18 mm. The
results revealed that rape is the most remunerative crop followed by tomato, onion, carrot, cabbage
and spinach. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sprinkler irrigation; Irrigation scheduling; Marketable yield; Net return; Vegetable crops
*
Corresponding author. Tel.: 267-660327; fax: 267-663761.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 1 0 5 - 5
332
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
1. Introduction
Botswana is a semiarid country with limited water resources. Water is the major
limiting factor for crop production in most of the agricultural regions of Botswana. Water
for irrigation, domestic and industrial needs is increasing considerably to meet the
demands for a growing population. Presently, 80% of vegetables are imported from
neighbouring countries. However, the government of Botswana through its financial
assistance programme is trying to encourage farmers to grow vegetable and fruit crops in
order to reduce imports. Therefore, it is necessary to develop efficient, reliable and
economically viable irrigation management strategies for effective use of the existing
limited water resources. Improper irrigation management practices do not only waste
scarce and expensive water resources but also decrease marketable yield and economic
return.
The irrigation scheduling which determines the amount and frequency of irrigation is
governed by many complex factors, but climate plays a major role. Therefore, it is
important to develop irrigation scheduling techniques under prevailing climatic condition.
Numerous studies were carried out in the past on the development and evaluation of
irrigation scheduling techniques under a wide range of irrigation system and management, soil, climate and crop conditions (Hagan and Laborde, 1964; Jensen et al., 1970;
Imtiyaz and Shiromani, 1990; Wanjura et al., 1990; Imtiyaz et al., 1992; Steele et al.,
1997). The meteorological-based irrigation scheduling approach, such as pan evaporation
replenishment, cumulative pan evaporation (CPE), and ratio between irrigation water and
cumulative pan evaporation etc., was used by many researchers due to its simplicity, data
availability and higher degree of adaptability at the farmers level (Prihar et al., 1974;
Singh, 1987; Pawar et al., 1991; Singh and Mohan, 1994; Imtiyaz et al., 1995, 1999;
Singh et al., 1997). In Botswana, evaporation from USWB Class A open pan is being
systematically recorded and readily available for irrigation scheduling.
Cabbage, spinach, rape, carrot, tomato and onion are the most important vegetable
crops in Botswana. Due to lack of proper irrigation scheduling techniques, the average
yield of these vegetable crops is low because of excess or deficit soil moisture regimes.
The quick-coupling sprinkler irrigation is the most common method of irrigation for
vegetable crops in Botswana, but information on economic viability of this system is
lacking. Therefore, the objectives of the present study were to examine the effect of
irrigation schedules on marketable yield, irrigation production efficiency and economic
return of vegetable crops.
2. Materials and methods
The field experiments were conducted during the winter/summer crop growing
season of 1995 and 1996 at the Etsha 6 experimental station of the Department of
Agricultural Research, Botswana (198370 S, 228170 E, 964 m above MSL). The mean
monthly maximum air temperature, minimum air temperature, relative humidity, wind
velocity and pan evaporation during the crop growing season (June to November) ranged
from 22.7 to 36.88C, 4.8±228C, 35.3±75.2%, 1.0±1.9 m/s and 3.4±11.3 mm/day,
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
333
respectively. The rainfall during the month of October and November in 1995 was 8.7 and
42.8 mm and in 1996 was 1.6 and 40.0 mm, respectively. According to rainfall and
potential evapotranspiration, the climate in this part of the country has been classified
as semiarid with mild winters and hot summers. The soil in the experimental field was
sandy with low organic matter content. The soil moisture content at field capacity
(ÿ0.03 MPa) and wilting point (ÿ1.5 MPa) was 0.1058 and 0.0345 m3/m3, respectively.
The average bulk density of soil was 1.41 g/cm3. The plant available soil moisture was
72 mm/m.
The experiment was laid out in a randomised block design with three replicates. The
experimental plots were sub-divided for six vegetable crops. The size of plots for
cabbage, spinach, rape, carrot, tomato and onion were 45, 45, 45, 25, 45 and 25 m2,
respectively. A buffer zone spacing of 2.0 m was provided between the crops. Prior to
planting, the experimental field for vegetable crops received 94.3 kg/ha phosphorus
(P2O5) and 62.9 kg/ha potash (K2O). The tomato and carrot received an additional
amount of 62.5 kg/ha K2O. Before planting, the experimental field for cabbage, spinach,
and rape received 50 kg/ha nitrogen, whereas carrot, tomato and onion received 63, 75
and 42 kg/ha nitrogen, respectively. Spinach (Var. Fordhook Giant) and carrot (Var.
Brazillia) were sown on the 30th June in both 1995 and 1996 with plant and row spacings
of 0.15 m 0.3 m and 0.05 m 0.4 m, respectively. Cabbage (Var. Grandslam), rape
(Var. Giant Essex), tomato (Var. Sixpack) and onion (Var. Texas Grano 502 PRR)
seedlings were transplanted from 11th to 14th July in 1995 and 1996 with plant and row
spacings of 0.5 m 0.5 m, 0.3 m 0.4 m, 0.3 m 0.75 m and 0.1 m 0.3 m, respectively. The experimental plots for cabbage, rape and spinach received 50 kg/ha of
nitrogen after 3, 5 and 8 weeks of transplanting. The experimental plots for tomato and
onion received 37.5 and 42 kg/ha of nitrogen after 4 and 8 weeks of transplanting, but
carrot received 21 kg/ha of nitrogen after 8 weeks. Spinach and carrot emerged during the
7th to 9th of July and thinning was done after 3 weeks to maintain desired plant
population. During the first 3 weeks, the crops were irrigated daily at 75% of pan
evaporation losses in order to establish newly planted seedlings.
The experiment consisted of five treatments formed by irrigation schedules at CPE
(cumulative pan evaporation) of 11, 22, 33, 44 and 55 mm during the entire crop growth
period with the depth of 18 mm at each irrigation. The above-mentioned CPE values were
selected in order to create excess and deficit soil moisture regimes as well as match the
frequency of irrigation, i.e. days. The cumulative pan evaporation was calculated as a sum
of daily 7-years recorded evaporation from USWB Class A open pan. The pan is located
at the climate station adjacent to the irrigation experimental field with moderate grass
cover. The irrigation in the respective treatments were applied when CPE reached
approximately 11, 22, 33, 44 and 55 mm. The irrigation during the crop growing season
was applied by quick-coupling sprinkler irrigation system.
Cabbage, spinach, rape, tomato, carrot and onion were harvested from 4th to 15th
October, 13th September to 2nd November, 31st August to 2nd November, 1st October to
16th November, 2nd to 4th October and 14th to 16th November in both 1995 and 1996.
For economic analysis, both fixed and operating cost were included. The total
production cost, gross revenue and net return under different irrigation schedules were
estimated on the following assumptions:
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M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Salvage value of the components 0
Useful life of engine, pump, pump house and irrigation system 12 years
Interest rate 14%
Repair and maintenance 5%
No. of crops/year 1
The fixed cost including engine, pump, pump house and quick-coupling sprinkler
irrigation system was calculated. The annual fixed cost was calculated by the following
approach (James and Lee, 1971):
CRF
i 1 in
1 in ÿ 1
Where CRF is the cost recovery factor; i is the interest rate (fraction) and n is the useful
life of the component (years).
Annual fixed cost/ha CRF fixed cost/ha
The operating cost, including labour, land preparation, seeds, fertilizers, chemicals,
diesel and oil, and repair and maintenance, was calculated. The total cost of production
under different irrigation schedules was estimated by adding fixed and operating cost.
The gross revenue for different irrigation schedules was estimated taking into account the
marketable yield and market price of cabbage, spinach, rape, carrot, tomato and onion.
Subsequently, the net return under different irrigation schedules was calculated
considering total cost of production and gross revenue. The benefit cost ratio (B/C)
under different irrigation schedules was calculated as follows:
B
Gross revenue P=ha
C Total cost of production P=ha
3. Results and discussion
3.1. Yield and irrigation production ef®ciency
In both years, maximum heads/m2, head weight and marketable yield of cabbage were
recorded when irrigation was applied at cumulative pan evaporation (CPE) of 22 mm
(Table 1). Further increase in irrigation level resulting from CPE of 11 mm did not
increase the marketable yield significantly. Irrigation at CPE of 33, 44 and 55 mm
reduced the marketable yield by reducing heads/m2 and head weight. The results revealed
that the head weight (28±66%) was more affected by irrigation levels compared to heads/
m2 (12±42%). Irrigation at CPE of 22 mm resulted in higher irrigation production
efficiency. A further increase or decrease in irrigation levels resulting from CPE of 11, 33,
44 and 55 mm reduced the irrigation production efficiency significantly.
In both years, irrigation at CPE of 11 mm produced a significantly higher marketable
yield of spinach (Table 2). Marketable yield decreased significantly with decrease in
irrigation levels resulting from CPE of 22, 33, 44 and 55 mm. However, irrigation at CPE
of 22 mm gave a higher irrigation production efficiency because yield reduction (20.8%)
335
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 1
Effect of irrigation schedules on yield, yield components and irrigation production ef®ciency of cabbage in 1995
and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean
marketable
yield (t/ha)
Mean
marketable
head (mÿ2)
Mean head
weight (kg)
Mean irrigation
production
efficiency (kg/m3)
95
96
95
96
95
96
95
96
95
96
1115
629
467
377
323
±
1140
636
474
384
330
±
73.34
70.91
45.51
26.02
14.71
3.04
74.26
72.39
45.23
24.69
12.97
7.05
3.75
3.71
3.27
2.65
2.21
0.21
3.79
3.74
3.28
2.55
2.09
0.22
1.96
1.91
1.39
0.98
0.67
0.06
1.97
1.94
1.38
0.97
0.63
0.14
6.58
11.26
9.75
6.90
4.55
0.39
6.51
11.38
9.54
6.43
3.93
1.22
was less than the seasonal water application (45.3%). The difference in irrigation
production efficiency between CPE of 22 and 33 mm was statistically similar. Irrigation
production efficiency decreased severely (41±64%) with irrigation at CPE of 44 and
55 mm due to severe reduction in marketable yield.
In 1995, maximum marketable rape yield was obtained with irrigation at CPE of 11±
33 mm, but in 1996 irrigation at CPE of 33 mm resulted in a significantly lower yield
compared to CPE of 11 and 22 mm, probably due to variation in climatic conditions
(Table 3). In both years, marketable yield decreased significantly with decrease in
irrigation levels resulting from CPE of 44 and 55 mm. This is due to the fact that
irrigation at CPE of 44 and 55 mm reduced the plant growth as well as increased the nonmarketable yield. In both 1995 and 1996 seasons, irrigation at CPE of 33 to 55 mm gave a
significantly higher irrigation production efficiency because yield reduction was less
compared to seasonal water application. Irrigation at CPE of 11 mm resulted in minimum
irrigation production efficiency because it increased the seasonal water application
considerably (74.4%) without any significant improvement in marketable yield.
In both years, irrigation at CPE of 11 and 22 mm produced higher total and root yield
of carrot (Table 4). Marketable yield decreased significantly with decrease in irrigation
Table 2
Effect of irrigation schedules on yield and irrigation production ef®ciency of spinach in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water applied
(mm)
Mean marketable
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
33.25
27.42
19.79
9.22
4.98
2.09
33.81
25.72
16.79
9.02
4.68
2.77
2.34
3.55
3.55
2.05
1.26
0.30
2.28
3.15
2.80
1.90
1.16
0.43
336
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 3
Effect of irrigation schedules on yield and irrigation production ef®ciency of rape in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean marketable
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
72.93
72.12
68.11
57.90
50.99
5.19
73.24
74.31
65.61
55.43
46.27
6.45
5.13
9.33
12.23
12.90
12.91
1.03
4.94
9.11
10.94
11.69
11.51
1.07
levels resulting from CPE of 33, 44 and 55 mm. This is due to the fact that irrigation at
CPE of 33, 44 and 55 mm reduced the plant growth as well as increased the nonmarketable yield. Maximum irrigation production efficiency was obtained at CPE of
22 mm and it decreased significantly with a decrease in irrigation levels. Irrigation at
CPE of 11 mm reduced irrigation production efficiency considerably (44%) because it
increased the seasonal water application by 43% without significant improvement in
marketable yield.
In both 1995 and 1996 seasons, irrigation at CPE of 11 and 22 mm produced higher
fruit weight and marketable yield of tomato (Table 5). Marketable yield decreased
significantly with a decrease in irrigation levels resulting from CPE of 33, 44 and 55 mm
due to reduction in fruit weight and marketable number of fruits. In both years, higher
irrigation production efficiency was obtained with irrigation at CPE of 22 and 33 mm,
thereafter it decreased significantly, because yield reduction was higher than seasonal
water application. Irrigation at CPE of 11 mm in which seasonal water application was
maximum, reduced the irrigation production efficiency considerably (51%) because it
increased the seasonal water application without significant improvement in yield.
Table 4
Effect of irrigation schedules on yield and irrigation production ef®ciency of carrot in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water
applied (mm)
Mean marketable yield
(t/ha)
Mean irrigation production
efficiency (kg/m3)
95
Total
Total
986
557
413
341
305
±
96
1032
600
438
366
312
±
Root
Root
95
96
95
96
95
96
95
96
57.01
56.31
36.73
15.45
5.77
3.96
56.21
57.21
37.72
15.54
6.62
3.90
38.23
38.09
24.49
10.22
3.86
2.32
37.12
38.68
24.99
10.31
4.44
2.71
5.77
10.11
8.89
4.53
1.89
0.82
5.45
9.54
8.61
4.24
2.12
0.78
3.86
6.86
5.93
2.99
1.27
0.47
3.60
6.45
5.71
2.82
1.42
0.49
337
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 5
Effect of irrigation schedules on yield, fruit weight and irrigation production ef®ciency of tomato in 1995 and
1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD (P 0.05)
Mean water
applied (mm)
Mean marketable
yield (t/ha)
Mean fruit
weight (g)
Mean (irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
44.58
47.69
36.47
18.16
12.55
2.94
42.90
45.92
38.25
18.20
10.93
4.00
82.15
82.11
64.95
54.74
47.03
3.58
80.67
83.02
67.17
51.49
45.49
5.24
3.14
6.17
6.55
4.04
3.18
0.52
2.89
5.63
6.38
3.84
2.72
0.55
In 1995 maximum bulb yield of onion was recorded with irrigation at CPE of 11 mm,
but in 1996 yield difference between CPE of 11 and 22 mm was statistically similar
(Table 6). Marketable bulb yield decreased significantly with a decrease in irrigation
levels resulting from CPE of 33 to 55 mm due to a reduction in bulb weight. In 1995,
irrigation at CPE of 33 mm resulted in higher irrigation production efficiency, but in 1996
the difference between 22 and 33 mm of CPE was non-significant. The results clearly
indicated that irrigation above 33 mm of CPE reduced the irrigation production efficiency
significantly due to higher yield reduction compared to seasonal water application.
Furthermore, irrigation at CPE of 11 mm, in which seasonal irrigation was maximum,
resulted in minimum irrigation production efficiency because increase in yield was
considerably less than the seasonal water application.
Despite of some variation, the overall results showed that a fixed amount of 18 mm
of irrigation (IW) application at cumulative pan evaporation (CPE) of 22 mm resulted
in higher marketable yield and irrigation production efficiency of vegetable crops
(Tables 1±6). Imtiyaz et al. (1999) reported similar results for broccoli, cabbage, carrot
and rape under drip irrigation. Singh and Mohan (1994) reported a reduction in sugarcane
yield when irrigation was applied beyond a IW/CPE ratio of 1.0. Singh et al. (1997)
Table 6
Effect of irrigation schedules on bulbs yield and irrigation production ef®ciency of onion in 1995 and 1996
Treatment (irrigation
at CPE, mm)
11
22
33
44
55
LSD(P 0.05)
Mean water applied
(mm)
Mean marketable bulbs
yield (t/ha)
Mean irrigation production
efficiency (kg/m3)
95
96
95
96
95
96
1421
773
557
449
395
±
1482
816
600
474
402
±
57.16
49.96
40.80
25.15
20.59
3.49
54.94
49.31
37.97
24.60
18.31
6.01
4.02
6.46
7.33
5.60
5.21
0.58
3.71
6.05
6.33
5.19
4.55
0.86
338
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
observed that the reduction in plant growth and yield of palmarosa at higher irrigation
levels resulted from IW/CPE ratios of 1.1 to 1.5.
3.2. Water supply and marketable yield
The relationship between seasonal water applied and marketable yield of cabbage,
spinach, rape, carrot, tomato and onion are shown in Fig. 1. In both years, seasonal water
applied and marketable yield of vegetable crops exhibited quadratic relationships
(R2 0.85±0.99). Cabbage, spinach, rape, carrot, tomato and onion attained maximum
marketable yields of 86.33, 35.81, 80.70, 70.55 (total) and 47.40 (root), 57.18 and 61.82
t/ha at the seasonal water application of 890, 1248, 1103, 785, 1083 and 1190 mm,
respectively, and thereafter it tended to decline (Fig. 1). Further analysis of regression
models revealed that the above-mentioned maximum yield for different crops correspond
to CPE of 16±18 mm. The quadratic water applied yield relationship results were
probably due to poor aeration and nutrient-leaching caused by excessive soil moisture.
Imtiyaz et al. (1994, 1995, 1996, 1999) reported the quadratic relationships between
seasonal water applied and marketable yield of cabbage, carrot, onion, tomato, green
pepper, broccoli, rape and okra under sprinkler and drip irrigation. Many researchers
reported quadratic crop water production functions for vegetable and field crops under a
wide variety of irrigation systems and regimes, soil and climatic conditions (Musick et al.,
1976; Singh, 1987; Stone et al., 1996; Farah et al., 1997; Howell et al., 1997; Tiwari and
Reddy, 1997).
3.3. Economic return
The total production cost, net return and benefit cost ratio (B/C) for different crops
under different irrigation schedules are presented in Table 7. The total cost of production
increased with increase in irrigation levels. The total cost of production varied amongst
the crops mainly due to difference in crop growing period and labour requirements for
harvesting and packaging etc. Labour costs in performing major farm activities
contributed 40±64% to total cost of production depending upon the crops and irrigation
levels. The fixed cost, repair and maintenance, and pumping cost contributed 8±27.5%,
3.1±7.7% and 6.2±12.9%, respectively, to total cost of production. Cabbage, rape, carrot
and tomato gave the higher net return at CPE of 22, whereas spinach and onion resulted
in higher net return at CPE of 11 mm. Irrigation at CPE of 55 mm resulted in
considerable economic loss from cabbage, carrot and spinach because total cost of
production exceeded the gross revenue. The benefit cost ratio (B/C), which indicates
gross revenue per unit investment, was also influenced by irrigation schedules. Irrigation
at CPE of 22 mm gave higher B/C ratio for cabbage, spinach, rape, carrot, tomato and
onion (Table 7). However, according to the fitted regression models (yield versus water
applied/irrigation schedules), all vegetable crops attained the maximum gross revenue,
net return and B/C ratio at CPE of 16±18 mm. The overall results clearly show that rape is
a more remunerative crop followed by tomato, onion, carrot, cabbage and spinach.
Imtiyaz et al. (1999) reported similar results for cabbage, broccoli, rape and carrot under
drip irrigation.
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
339
Fig. 1. Relationship between water applied and marketable yield of cabbage (a), spinach (b), rape (c), carrot (d),
tomato (e) and onion (f).
340
Treatment
Total production cost (P/ha)
Net return (P/ha)
(irrigation
at CPE, mm) Cabbage Spinach Rape Carrot Tomato Onion Cabbage Spinach Rape
11
22
33
44
55
31678
24519
22140
20929
20201
a
31592
24028
21514
20235
19486
31292
23728
21214
19935
19186
26268
19272
16914
15749
15044
31371
23808
21293
20015
19265
28166 42122
20602 47131
18088 23230
16810
5426
16060 ÿ6361
B/C ratio
Carrot Tomato Onion Cabbage Spinach Rape Carrot Tom Onion
27086 96607 67920 77979
22470 104398 76691 93192
10494 95791 44936 53427
ÿ4275 79229 9914 25435
ÿ11033 65917 ÿ4669 10085
83934
78668
60682
32940
22840
2.33
2.92
2.05
1.26
0.69
1.86
1.94
1.49
0.79
0.43
4.09
5.40
5.52
4.97
4.44
3.59
4.98
3.66
1.63
0.69
3.49
4.91
3.51
2.27
1.52
3.98
4.82
4.36
2.96
2.42
The prices of cabbage, spinach, rape, carrot, tomato and onion are taken at 1.0, 1.75,1.75, 2.50, 2.50 and 2.0 P/kg, respectively, 1 US$ 4.55 P (Botswana Pula).
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
Table 7
Economic analysis of sprinkler-irrigated vegetable crops under different irrigation schedules (average data of 2 years)a
M. Imtiyaz et al. / Agricultural Water Management 45 (2000) 331±342
341
4. Conclusion
Evaporation from USWB Class A open pan is the most common and simple approach
for scheduling irrigation for field, vegetable and fruit crops. Field extension personnel can
prepare an irrigation scheduling calender taking into account the long-term daily pan
evaporation data that can be easily adopted by the farmers. The experimental results
clearly indicated that a fixed depth of 18 mm of irrigation application at cumulative pan
evaporation of 16±18 mm (June to November) is optimum for sandy soil in order to
achieve maximum marketable yield, net return and benefit cost ratio from sprinklerirrigated cabbage, spinach, rape, carrot, tomato and onion under semiarid climate of north
western region of Botswana.
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