Effect of drip tape placement depth and irrigation level on yield of potato

agricultural water management 88 (2007) 209–223

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Effect of drip tape placement depth and irrigation level on yield of potato Neelam Patel, T.B.S. Rajput * Water Technology Centre, Indian Agricultural Research Institute, New Delhi 110012, India

article info

abstract

Article history:

Subsurface drip irrigation (SDI) is the most advanced method of irrigation, which enables the

Accepted 17 October 2006

application of the small amounts of water to the soil through the drippers placed below the soil

Published on line 27 November 2006

surface. One of the most commonly discussed aspects of SDI system is installation depth of drip lateral. Determining the appropriate depth of installation involves consideration of soil

Keywords:

structure, texture, and crop’s root development pattern. Site-wise and crop-wise variations of

Potato

these parameters preclude the possibility of framing general recommendations for installa-

Subsurface drip irrigation

tion depths of SDI system. An experiment was conducted on potato (var. Kufri Anand) during

Depth of placement of drip tape

October–February for 3 years (2002–2003, 2003–2004 and 2004–2005) to study the effect of depth

Irrigation

of placement of drip tape and different levels of irrigation application on potato yield. Drip tapes were buried manually in the middle of different ridges. Tests for uniformity of water application through the SDI system were carried out in the month of October every year. Three different irrigation levels of 60, 80 and 100% of the crop evapotranspiration and five depths of placement of drip tape namely, 0.0, 5.0, 10.0, 15.0 and 20.0 cm were maintained in the study. The coefficient of variation (CV) of flow rates was found 0.046, 0.047 and 0.064 during 2002– 2003, 2003–2004 and 2004–2005, respectively. The low CV indicated good performance of the SDI system throughout the cropping season. The values of statistical uniformity (SU) and distribution uniformity (DU) were more than 92.0% during all the three cropping seasons. Soil water distribution at different growth stages of potato under different depths of placement of drip tape for varying irrigation levels was monitored. When drip tape was placed at surface and buried at 5.0 cm soil depth, upward movement of water takes place, 21.5% soil water content was found throughout the crop season of potato. When drip tape was buried 10.0, 15.0 and 20.0 cm below the surface, upward water movement due to capillary forces was not sufficient and soil surface remained relatively dry. The maximum yield was recorded when drip tape was buried at 10.0 cm during 2002– 2003 and 2004–2005 and at 15.0 cm during 2003–2004 that was followed by drip tape placement at 20.0, 10.0 and 5.0 cm depths in 2002–2003 to 2004–2005, respectively. Treatment 0.6T4 gave maximum IWUE of 2.07, 2.13 and 2.05 t ha1 cm1 during 2002–2003, 2003– 2004 and 2004–2005, respectively. The highest benefit cost ratio of 1.7 was obtained for treatment T3. Lowest benefit–cost ratio of 0.9 was found for treatment 0.6T5. The cost incurred for the installation of drip tape at successively higher depths, increases the annual cost of production. The placement depth of drip tape significantly affected potato yield. Maximum yield was obtained by applying the 100% of the crop evapotranspiration (23.6 cm of irrigation water) and by placing the drip tape at 10.0 cm depth. In the sandy loam soil at

* Corresponding author. Tel.: +91 11 25848703; fax: +91 11 25848703. E-mail address: [email protected] (T.B.S. Rajput). 0378-3774/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2006.10.017

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the experimental site, the gravity force predominated over the capillary force causing a greater downward movement of water. Therefore, shallow depth of placement of drip tape (10.0 cm) was recommended in potato crop to get higher yield. Appropriate depth of placement of drip tape however, will differ with crop and the change in soil type. # 2006 Elsevier B.V. All rights reserved.

1.

Introduction

India would need to produce over 400 million tonnes of food grain from gradually diminishing per capita availability of land and water in order to fulfill the requirements of population of about 1.5 billion by the year 2020. With increasing demands on limited water resources and the need to minimize adverse environmental consequences of irrigation, drip irrigation technology will undoubtedly play an important role in the future in the Indian agriculture. It provides many unique agronomic, water and energy conservation benefits that address many of the challenges facing irrigated agriculture. Consequently, the use of drip irrigation is rapidly increasing around the world. Drip irrigation system consists of drippers, which are either buried or placed on the soil surface for discharging water at a controlled rate. All micro irrigation systems have the potential to be very efficient in irrigation water conveyance, control and application. An irrigation system should apply water uniformly so that each part of the irrigated area receives same amount of water. Insufficient water leads to high soil moisture tension, plant stress and reduced crop yields. Excess water may also reduce crop yields below potential levels due to leaching of applied nutrients, increased disease incidence or failure to stimulate growth of the commercially valuable parts of the plant (Solomon, 1993). Subsurface drip irrigation (SDI) is the most advanced method of irrigation, which enables the application of the small amounts of water to the soil through the drippers placed below the soil surface with discharge rates generally in the same range as surface drip irrigation (ASAE Std., 1999). SDI offers many advantages over the surface drip irrigation such as reduction in evaporation and deep percolation losses and elimination of surface runoff (Camp, 1998). Water infiltration in the SDI takes place in the region directly around the dripper, which is small compared with the total soil volume of irrigated field. A subsurface dripper usually forms a small cavity around it into which water can freely flow (Shani and Or, 1995). Uptake of water by plant roots causes soil drying and subsequent increased soil water tension. Selected drippers discharge should not exceed the root uptake rate (Clothier and Green, 1994, 1997; Lazarovitch, 2001). Application of uniform and sufficient water to seed for good crop establishment is one of the most challenging issues of SDI. Establishment of crop in SDI relies on unsaturated water movement from the buried source to seed. The process is therefore affected by distance from water source to seed, evaporative demand and hydraulic conductivity, which is dependent on soil texture, structure and antecedent water content. One of the most commonly discussed aspects of SDI system is installation depth of drip lateral. Determining the

appropriate depth of installation of drip laterals requires consideration of soil structure, texture and crop’s root development pattern that hinders in providing general recommendations for installation depths of SDI systems (Burt and Styles, 1994). Solomon (1993) reported that in SDI, irrigation water and injected fertilizers are supplied directly to the roots of crop. This is specially advantageous in case of nutrients that have low mobility into soil. In SDI, top 20.0 cm of soil have lower soil water content when laterals are buried at 45.0 cm soil depth, resulting in reduced evaporation (Phene et al., 1983; Solomon, 1993). A relatively dry soil permitted farm equipment movement during the whole cropping season and eliminated weed growth (Schwankl et al., 1990). SDI is also not exposed to sun and extreme weather condition ensuring a longer life of the system. Shani et al. (1996) mentioned that the water discharge rate from the SDI drippers would have to be controlled according to the soil hydraulic conductivity. Ruskin (2000) reported that SDI system could be used to apply water in small amounts and at higher frequency. He achieved a saving of 46% of water in comparison to surface drip in medium and heavy textured soil in which the water movement occurred mainly due to capillary forces. Lamm and Trooien (2003) found that corn yield was the highest under SDI at irrigation level of 75% crop evapotranspiration. Phene et al. (1992) also studied the environmental impact of SDI system in clay loam soil when drip tubes were placed at 45.0 cm below the soil surface. It was observed that soil water remained at the root zone for utilization of plants and was not lost due to deep percolation. The uniformity of water application from a SDI system is affected both by the water pressure distribution in the pipe network and by the hydraulic properties of the drippers used. Dripper flow rate depends on its hydraulic properties, its design and water temperature. A major concern with SDI is evaluation of its performance and measurement of uniformity parameters of its discharge. The performance of the SDI system should be quantified in relation to its design, management, operation and efficient use of water. Quantification allows the users to determine and control the dripper discharge, amount and timings of application of irrigation water so that the crop water requirements are met in a planned and effective manner (Burt et al., 1997; Camp et al., 1997; Ayars et al., 1999). Estimation of uniformity coefficient for surface drip irrigation is straightforward but it is difficult in SDI system in which the laterals are buried below the soil surface. Magwenzi (2001) found the statistical uniformity (SU) and the distribution uniformity (DU) varying from 89.0 to 93.0% and 85.0 to 92.0%, respectively, for drip tapes discharging at 1.6 Lph and buried at 15.0–20.0 cm below the sugarcane set at a spacing of 0.92 m. The performance of the SDI system,

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overhead, center pivot and surface drip were evaluated by Reinders (2001) for sugar growing areas of South Africa. DU and SU of SDI was found 68.0 and 74.0%, respectively. Various studies in the past have rated most of the SDI systems as excellent on the basis of their performance (Ayars et al., 1999; Magwenzi, 2001). Measurable indices of the degree of uniformity include CV (Wu, 1997), DU (Kruse, 1978) and SU (Bralts et al., 1981). Potato (Solanum tuberosum) is an important crop of India with a production of 25.0 million tonnes (1 tonne = 1000 kg) from 1.34 million hectare area. India produces almost 13% of world’s vegetable output occupying fifth rank in potato production. The average yield of potato in India is 19.0 t ha1, 1, which is much below the potential productivity (FAO, 1998). Potato can be grown in all soil types but it prefers well-drained sandy loam soil. Potato is largely grown in cool regions where mean temperature does not normally exceed 18 8C. Optimum temperature for potato growth and development ranges between 15 and 25 8C. Potato is conventionally grown through vegetative reproduction of its tuber (Kashyap and Panda, 2003). Previous research shows that the yield and quality of potatoes improved through drip irrigation (Yuan et al., 2003; Onder et al., 2005; Kaur et al., 2005). The potato crop evapotranspiration vary from 30.0 to 70.0 cm, depending on the environment and crop growth stages (Shock and Feibert, 2000). Potato has a shallow root zone and has low tolerance for water stress (Schapendonk et al., 1989; Van Loon, 1981). Drought severity, timing and duration of water stress during the different growth stages of potato crop influences the crop yield. King et al. (2003) studied the effect of two levels of irrigation 80 and 60% of water requirement on yield of potato (var. Russet Burbank tubers). The 3 weeks water stress intervals was maintained during early, mid and late bulking stage of crop. They reported that the total potato yield reduced mainly when deficit irrigation was applied during early mid and mid late bulking, regardless of water stress intensity. However, in some circumstances, potatoes can tolerate limited deficit irrigation before tuber set without significant reductions in external and internal tuber quality (Shock et al., 1992). Expected potato yield of 79% and relative water use efficiency of 1.06 was obtained when 25% deficit of evapotranspiration was prevailed for the whole season of potato (Kirda, 1982). The present experiment was conducted for the first time in India to study the influence of subsurface drip tape on the yield of potato. Drip manufacturers in India do not produce drip tape. For the reported experiment, therefore, drip tape was imported from T-tape systems, Australia. The objectives of this study were (1) to evaluate the performance of SDI system, (2) to study the effect of different levels of

Fig. 1 – Average temperature during crop season.

application of irrigation water on yield of potato and (3) to determine the optimal depth of placement of drip tape on the basis of shape and position of the wetting zone beneath it.

2.

Materials and methods

2.1.

Location and soil of experimental field plot

The experiment was conducted at the research farm of Water Technology Centre, Indian Agricultural Research Institute, New Delhi, India, during October–February for 3 years (2002– 2003, 2003–2004 and 2004–2005). Soil samples from surface down to 45.0 cm at 15.0 cm interval were collected. Hydrometer method was followed to determine the sand, silt and clay percentage of soil. The soil of the experimental area was deep, well-drained sandy loam comprising 69.3% sand, 14.1% silt and 16.6% clay (Table 1). The bulk density of soil was 1.53 g cm3 and saturated hydraulic conductivity 1.11 cm h1, respectively. Weather data of the experimental site for 3 years 2002–2003, 2003–2004 and 2004–2005 are shown in Figs. 1–4.

2.2.

System installation and experimental treatments

A field plot of size 27.0 m  50.0 m was selected for experimental studies. The field plot was divided into three equal plots of 9.0 m  50.0 m. Each plot of 9.0 m  50.0 m size was divided into 15 equal plots of 0.60 m  50.0 m, representing a single treatment. The experiment was laid out following the split plot design with 15 treatments (3 irrigation levels and 5

Table 1 – Particle size distribution of soil of experimental field Soil depth (cm) 0.0–15.0 15.0–30.0 30.0–45.0

Sand (%)

Silt (%)

Clay (%)

FC (cm3 of water/cm3 of soil)

PWP (cm3 of water/cm3 of soil)

71.2 68.8 68.0

14.3 15.0 12.8

14.5 16.2 19.2

0.198 0.207 0.218

0.106 0.114 0.127

EC (dS m1) 0.50 0.50 0.45

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Fig. 2 – Rainfall during the crop season.

depths of placement of drip tape) and 3 replications (R1, R2 and R3) of each treatment (Fig. 5). Installation of the SDI system commenced in September 2002 with control facility, which included hydro cyclone filter, screen filter, sand media filters, back flush mechanisms, fertilizer injection system, i.e. venturi and a control panel (Model Pro-C, Hunter). Main lines were connected to sub-mains for each 15 rows plot through a valve tree, which included a solenoid valve, ball valve connector and pressure release valve. The 15 rows in a block represented 5 treatments and 3 replications. Flush manifolds were connected at the lower end of each 15 rows of a block. All solenoid valves were direct wired to the control panel. Care was taken to place the drip tape straight in the ridges with openings on the upper side of the drip tapes. The research study consisted of three levels of irrigation including, 100% crop evapotranspiration (V), 80% of crop evapotranspiration (0.8V) and 60% of crop evapotranspiration (0.6V) as main treatments and 5 depths of drip tape placement including one at the surface and at 5.0, 10.0, 15.0 and 20.0 cm soil depths as the sub treatment. The layout indicating different treatments is shown in Fig. 5. Tubers of 30 g weight of potato (var. Kufri Anand) was sown at the depth of 15.0 cm in the raised ridges prepared during the third week of October at a tuber and ridge spacing of 30.0 cm  60.0 cm, respectively. The base width and height of ridges were kept 60.0 and 30.0 cm, respectively. Drip tape of 12.0 mil thickness (0.3 mm) (T-Tape, Australia, model TSX 515-30-250) was buried manually at depths of 0.0, 5.0, 10.0, 15.0 and 20.0 cm in the middle of ridges formed for sowing of potato under different treatments. The hydraulic characteristics of installed drip system are given in Table 2. The installed drip system

Fig. 3 – Maximum relative humidity during crop season.

had drippers spaced at 30.0 cm, each with an application rate of 0.72 Lph. Time domain reflectometry was used in this study for determination of soil water content. Three access tubes, one at the middle of ridge and two at 15.0 and 30.0 cm away from the middle of ridge were installed. Access tubes were placed at the middle of the row up to a depth of 1.0 m and water content (volumetric) was measured in all treatments. Total 135 access tubes made of PVC were installed in the experimental field.

2.3.

Estimation of uniformity of drip system

Tests for uniformity of water application the drip system were carried out, in the month of October every year. For each testing, 54 drippers were selected from head, middle and tail ends of drip tape, randomly. Uniformity of water application was determined from the dripper outflow collected in cans for a known duration. The uniformity of water application was calculated from the statistical distribution of dripper flow rates in terms of coefficient of variation (CV), distribution uniformity (DU) and statistical uniformity (SU) using Eqs. (1)– (3), as follows:   s (1) CV ¼ q DU ¼

  qlq q 

SU ¼ Table 2 – Hydraulic characteristic of the drip irrigation system Irrigation system Drip tape

Characteristics Wall thickness (mil) Tape inner diameter (mm) Operating pressure (kg/cm2) Dripper discharge (Lph) Flow per 100 m length (Lph) Spacing between two drippers (cm) Spacing between two tapes (cm) Depth of placement of drip tape (cm)

1

 100

(2)

 s  100 q

(3)

Description 12 16.0 0.5 0.72 250 30.0 60.0 0, 5.0, 10.0, 15.0, 20.0

Fig. 4 – Solar radiation during crop season.

agricultural water management 88 (2007) 209–223

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Fig. 5 – Layout of drip system.

where s is the standard deviation of drippers discharge (Lph); q the mean dripper flow rate (Lph) and qlq is the mean of lowest one-fourth of drippers discharge (Lph). Five microirrigation uniformity classifications, ranging from excellent to unacceptable, recognized by the American Society of Agricultural Engineers (ASAE, 1996a,b) were used to evaluate the SDI system.

2.4.

as the correct depth for placement of drip tape in sandy loam soil.

2.5. Estimation of water requirement and irrigation application Weather data were collected from an automatic weather station located at 30 m away from the experimental field.

Determination of optimal depth for drip tape

The major design parameters controlling the shape and the position of the wetting zone beneath a SDI tape (lateral) are water application rate, dripper depth, dripper spacing and irrigation duration. Normally, SDI system is placed at the shallowest depth possible, consistent with avoiding damage from operations such as cultivation. However, if the lowest available dripper rate is greater than the soil intake rate, water surfacing can often result from irrigation. This can be reduced by burying the SDI system deeper in the soil (Battam et al., 2003). The shallowest effective depth of burial can be found by keeping the drip tape at varying depths. Accordingly in the present study, drip tapes were placed at five depths of 0.0, 5.0, 10.0, 15.0 and 20.0 cm below the soil surface. The depth at which potato yield was found maximum was taken

Fig. 6 – ET0 during crop season.

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Reference crop evapotranspiration (ET0) was calculated on a daily basis by using Penman–Monteith’s semi-empirical formula (Allen et al., 1998 and Fig. 6). Potato is about 130 days duration crop and may be divided in to four stages namely initial: 25 days, developmental: 30 days, middle: 45 days and tuber maturity: 30 days. The actual evapotranspiration was estimated by multiplying reference evapotranspiration with crop coefficient (ET = ET0  KC) for different months based on crop growth stages. The crop coefficient during the crop season 2002–2003, 2003–2004 and 2004–2005 was adopted as 0.50, 0.65, 1.15 and 0.75 at initial, developmental, middle and tuber maturity stages, respectively (Allen et al., 1998). Methodology formulated by Allen et al. (1998) was used for daily irrigation scheduling. During the initial and developmental growth stages, until tuber formation, it is essential, that soil is kept constantly and uniformly wet to a depth of at least 10.0–15.0 cm. The frequency of irrigation during this period was daily. During the second growth phase, i.e. tuber development and tuber maturity, irrigation frequency was reduced and water was applied once in every 3 days to allow efficient plant respiration for intensifying growth rate. In all the treatments, no disease was noticed throughout the crop season during all the 3 years of experimentation and no insecticide and fungicides were applied.

2.6.

Nutrient management

To meet the nutritional requirements of crop, 180 kg N ha1, 100 kg P2O5 ha1 and 150 kg K2O ha1 was applied as suggested by Grewal et al. (1991). In sandy loam soils, where potassium may be lost due to leaching, it was recommended to apply K in two splits (half at planting and half at earthing up) because this practice gives better results than if the entire dose were applied at planting (Phillips et al., 2004). Following the recommended practice of fertilizer application, nitrogen was applied into two split doses (one-third at planting and two-third at crop emergence stage).

2.7.

Potato yield

Matured potato was manually dug during 12–15 February. Yield of potato under each treatment and replications was recorded. Non-parametric test (Friedman’s test) and standard analysis of variance (ANOVA) were used to evaluate the effects of the treatments on the yield and to determine the significance of the main treatments and its interaction with sub treatments. Least significance differences (LSD) test was used for comparing the two main treatments and subtreatments.

Fig. 7 – Soil water distribution in treatment T1 at (a) initial, (b) developmental, (c) middle and (d) tuber maturity.

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Fig. 8 – Soil water distribution in treatment T2 at (a) initial, (b) developmental, (c) middle and (d) tuber maturity.

3.

Results and discussions

3.1.

Uniformity of drip system

The performance parameters of the installed drip system after layout and before sowing potato crop are shown in Table 3. The operating pressure of system was 1.0 kg cm2 during all the three crop growing seasons. The coefficients of variation of flow rates were 0.046, 0.047 and 0.064 during 2002–2003, 2003– 2004 and 2004–2005, respectively (Table 3). The low CV indicated good performance of the system throughout the cropping season. CV estimated by Decroix and Malaval (1985) and Bargel et al. (1996) for the in-line labyrinth type drippers was reported to be 0.066. Bargel et al. (1996) had concluded that

a CV between 0.05 and 0.066 indicated a good performance of the drip system. The values of SU and DU were greater than 92.0% during all the three cropping seasons (Table 3). According to Pitts (1997), SU and DU greater than 90.0 and 87.0%, respectively, implied an excellent functioning of the drip system.

3.2.

Soil water distribution

Desired wetting patterns of soil can be obtained by selecting the appropriate dripper discharge and spacing (Lubana and Narda, 2001). Subsurface application of water, aimed directly at the root zone moves by soil matrix suction, eliminating the effect of surface infiltration characteristics and saturated

Table 3 – Irrigation water requirement at different stages of crop, coefficient of variation (CV) and statistical uniformity coefficients of SDI system Year

2002–2003 2003–2004 2004–2005

Irrigation water requirement at different growth stages of crop (mm day1) Initial (25 days)

Developmental (30 days)

Middle (45 days)

Maturity (30 days)

1.7 1.9 2.0

1.9 1.7 1.8

2.2 2.0 2.3

1.3 1.4 1.3

Coefficient of variation (CV)

Statistical uniformity coefficients (%) DU

0.046 0.047 0.064

94.2 92.3 92.0

SU 95.4 95.3 93.6

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Fig. 9 – Soil water distribution in treatment T3 at (a) initial, (b) developmental, (c) middle and (d) tuber maturity.

condition of ponding water during irrigation. Water distribution in the soil around a buried dripper mainly depends on soil texture, dripper discharge and root water uptake. Soil samples were collected from the six points selected randomly from the experimental field at depths of 0.0–15.0, 15.0–30.0 and 30.0– 45.0 cm. On soil texture, Friedman’s non-parametric statistical test revealed that the sand proportion was significantly different at different depths (P < 0.01) but sand, silt and clay contents were not significantly different at different locations of the experimental field (P < 0.01). Soil water distributions at different growth stages of potato at different depths of placement of drip tape are shown in Figs. 7–11. During the early growth phase, when tuber formation had not started, 10.0–15.0 cm depth of soil was kept. The potato crop up to developmental stage was irrigated daily. The downward movement of water was more than its lateral movement at all growth stages of crop due to gravity force playing a predominant role in comparison to the capillary force in the sandy loam soil of the experimental plot. Vered (2002) had found that potato roots had spread up to a radius of 25.0 cm from the dripper discharge point and most were contained within 30.0–40.0 cm width and 25.0 cm depth. It was observed during the present experiment that water content of soil increased up to developmental stage of crop but then it started decreasing at all soil depths (Figs. 7–11). The SDI system, installed in the potato crop was a low discharge

system and irrigation frequency was relatively low (every 3 days). Therefore the soil around the dripper was almost at field capacity throughout the crop season. The evidence of wetting of the soil surface appeared in case of surface placement of drip tape and also in case of depth of placement of 5.0 cm (Figs. 7 and 8). When drip tapes were buried at 10.0, 15.0 and 20.0 cm depth, the soil surface remained relatively dry. Soil surface appeared moist but did not get saturated when depth of placement of drip tape was more than 5.0 cm at all growth stages of potato. Soil water content at the surface at initial, developmental, middle and maturity stage of the crop were found 16.5, 17.2, 14.9, 14.4%, respectively in treatment T3, 15.5, 15.9, 14.5, 14.0%, respectively in treatment T4 and 12.0, 13.0, 12.9, 12.3%, respectively in treatment T5 (Figs. 9–11). When drip tape was placed at the surface, the water content at 15 cm depth was greater than 18.0%. But at 30.0 cm depth the soil water content was about 12.0% (Fig. 7). When drip tape was buried at a depth of 5.0 cm, the soil water content at the surface varied from 20.5 to 22.5% at different growth stages of crop (Fig. 8). At this shallow depth of placement, water moved upwards keeping the surface soil moist with adequate amount of soil water, which was true at the 15.0 cm soil depth also (av. 21.5%) at different stages of potato. In treatment T3, when drip tape was buried at the depth of 10.0 cm, the upward capillary movement of water

agricultural water management 88 (2007) 209–223

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Fig. 10 – Soil water distribution in treatment T4 at (a) initial, (b) developmental, (c) middle and (d) tuber maturity.

was not sufficient and soil water content at the surface decreased significantly (av. 15.5%) in comparison to treatments T1 and T2 (Fig. 9). More than 16.0% soil water was available at the ridge at all points in T1, T2 and T3. Wetted soil bulb of 20.0 cm in width and 25.0 cm depth had more than 18.0% soil water content, which was very conducive for good tuber formation resulting in higher yields in treatments T1, T2 and T3. In treatment T4 and T5, the higher soil water content was observed till the soil depth of 30.0 cm in all growth stages of crop (Figs. 10 and 11). In treatment T4, adequate amount of water was available around the plant roots but in treatment T5, crop was under stress and lesser amount of water was available for the plant, as water moved beyond the ridge base, i.e. below 30.0 cm soil depth (Fig. 11). In treatments T1–T4, very little soil water content moved beyond the ridge base. In potato, the tubers formation was found confined within the ridge, i.e. only up to 30.0 cm depth from ridge top. Water that moved beyond the ridges was not available for plants at any stage of growth. Higher yield can be achieved by maintaining relatively high water content conducive to good plant growth that is achievable under shallow placement of drip tape. The high water content of the soil around the drippers facilitates better water transmission to the surrounding soil and keeps on replenishing the crop root zone (Segal et al., 2000). Therefore, keeping the drip tape within the crop root zone and

sufficiently below the soil surface replenishes the root zone effectively due to gravity flow in light soils and simultaneously cuts of evaporation losses due to restricted upward capillary flow.

3.3.

Effect of irrigation levels on potato yield

The water requirement of crop depends on the actual crop evapotranspiration. The water requirement of potato varied from 1.7 to 2.2, 1.9 to 2.0 and 2.0 to 2.3 mm day1 from the early stage to the peak demand period for 2002–2003, 2003–2004 and 2004–2005, respectively (Table 3). Irrigation water requirement on different days of the growing season for the years 2002– 2003, 2003–2004 and 2004–2005 are shown in Fig. 6. The operation duration of drip system was worked out for different levels of irrigation and values were entered in the control panel, which controls the opening and closing of solenoid valves. The total amount of irrigation water needs of potato crop was estimated as 23.7 cm (2002–2003), 22.9 cm (2003– 2004) and 24.3 cm (2004–2005), respectively. Friedman’s non-parametric statistical test was applied to investigate the difference (if any) in water requirement at initial, developmental, middle and maturity stages of potato crop during 2002–2003, 2003–2004, 2004–2005. Non-parametric test (Friedman’s test) indicated no significant difference in irrigation water requirement at any particular stage of crop in

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Fig. 11 – Soil water distribution in treatment T5 at (a) initial, (b) developmental, (c) middle and (d) tuber maturity.

different years (P < 0.05). But small differences in the amounts of applied irrigation water in the three irrigation treatments cause significant differences in the potato yield (P < 0.05). Irrigation water use efficiency (IWUE) with depth of placement of drip tape for all irrigation levels is shown in Fig. 12. Treatment 0.6T4 gave maximum IWUE of 2.07, 2.13 and 2.05 t ha1 cm1 during 2002–2003, 2003–2004 and 2004–2005, respectively. Kashyap and Panda (2003), Yuan et al. (2003) and Onder et al. (2005) also observed the similar findings for potato crop. IWUE during 2003–2004 was higher than 2002–2003 and 2004–2005. Lesser amount of applied irrigation water, gives the higher IWUE (Faberio et al., 2001). Results of the differential irrigation experiments showed that potato yield decreased with decreasing amount of irrigation water. The over all average yield of potato in treatments T1, T2, T3, T4 and T5 decreased from 30.7 to 18.5, 32.0 to 27.6, 33.3 to 27.9, 32.9 to 29.5 and 29.2 to 19.9 t ha1 (yield differences between the blocks were statistically insignificant) as applied water decreased from 100 to 60% of crop evapotranspiration (24.5–14.7 cm) (Table 4). The 100, 80 and 60% application of irrigation water significantly affected the potato yield (P < 0.05). In treatment T1, 13.7% increase in yield was recorded in comparison to treatment 0.8T1. It was observed that 20% reduction in amount of irrigation water decreased the yield by

about 13.7%. But on applying 60% amount of water (0.6T1), potato yield decreased by 65.9%. Steyn et al. (1998) had reported significant tuber yield and size reductions with the reduction of applied water, but they also pointed out

Table 4 – Potato yield observed at different irrigation levels and depth of placement of drip tape Treatments

T1 T2 T3 T4 T5 0.8T1 0.8T2 0.8T3 0.8T4 0.8T5 0.6T1 0.6T2 0.6T3 0.6T4 0.6T5

Potato yield (t ha1) during different years 2002–2003

2003–2004

30.2 31 33.5 32.4 29.8 26.5 29.2 31.8 30.4 24.4 18.5 27.5 28.5 29.5 20.2

30.8 32.2 32.6 33.7 28.7 27.5 28.5 30.2 30.5 24 19 27.2 27.8 29.2 19.7

2004–2005 31.2 32.8 33.8 32.7 29.2 27.1 29.5 30.6 31 23.7 18.1 28.1 27.5 29.9 19.7

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Table 5 – The effects of irrigation levels and depth of placement of drip tape Potato yield (t ha1) 2002–2003 Irrigation levels V 0.8V 0.6V LSD (5%)

31.4 27.3 19.6 0.99

Depth of placement of drip tape 0.0 cm (surface) 25.1 5.0 cm 29.2 10.0 cm 31.3 15.0 cm 30.8 20.0 cm 24.8 ns LSD (5%) 0.99

2003–2004 31.6 27.3 19.6 0.81

31.9 27.2 19.2 1.6

25.8 25.5 ns 28.2 27.2 24.1 0.97

25.5 25.4 ns 28.6 27.1 24.2 1.1

Irrigation levels and drip tape depth interaction V 0.0 cm (surface) 30.2 30.8 5.0 cm 31.0 ns 32.2 10.0 cm 33.5 32.6 15.0 cm 32.4 33.7 20.0 cm 29.8 ns 28.7

Fig. 12 – IWUE at different depth of placement of drip tape.

significant differences among genotypes in response to water stress. Onder et al. (2005) also had suggested that farmers could not be advised to grow potato under water deficiency of more than 33% of the irrigation water requirement. Potato yield decreased by 4.2, 8.5, 7.2 and 5.2% in treatments T2, T3, T4 and T5 in comparison treatment T1. In treatment T2, only 9.9% increase in potato yield was recorded over treatment 0.8T2 (Table 4). Difference in yield of potato was not significant in treatments T2 and 0.8T2, implying that 20% irrigation water can be saved without significantly affecting the potato yield. Application of 40% less amount of water decreased yield by 65.9, 15.9, 19.4, 11.5 and 46.7% in treatments T1, T2, T3, T4 and T5, respectively. Previous studies have shown that potato yield responds linearly to applied water (Hegney and Hoffman, 1997; Martin et al., 1992). In treatment T3, there was increase in yield by about 7.8 and 19.4%, respectively, in comparison to treatments 0.8T3 and 0.6T3. Difference in yield in treatment T3 and 0.8T3 was significant during 2002–2003 (Tables 5 and 6). During 2002– 2003, the effects of irrigation levels  depth of drip tape

2004–2005

31.2 32.8 33.8 32.7 29.2

0.8V 0.0 cm (surface) 5.0 cm 10.0 cm 15.0 cm 20.0 cm

26.5 29.2 ns 31.8 30.4 ns 24.4

27.5 28.5 30.2 ns 30.5 ns 24.0

27.1 29.5 30.6 ns 31.0 ns 23.7

0.6V 0.0 cm (surface) 5.0 cm 10.0 cm 15.0 cm 20.0 cm LSD (5%)

18.5 27.5 28.5 29.5 ns 20.2 1.4

19.0 27.2 27.8 29.2 19.7 1.4

18.1 28.1 27.5 29.9 ns 19.7 1.5

ns: non-significant.

placement interaction on potato yield was significant in all treatments except in treatments T2, 0.8T2, 0.8T4 and 0.6T4 (P < 0.05). The effect of irrigation levels  depth of drip tape placement on yield was not significant only in treatments 0.8T3 and 0.8T4 during the years 2003–2004 (P < 0.05). However, the effects of irrigation levels  depth of drip tape placement during the years 2004–2005 were non-significant in treatments in 0.8T3, 0.8T4 and 0.6T4 (Table 6). In treatment T4, yield increased by 11.5 and 7.5% in comparison to treatments 0.6T4 and 0.8T4, respectively.

3.4. yield

Effect of depth of placement of drip tape on potato

Maximum yield of potato was recorded during the years 2004– 2005, in all the treatments, as compared to 2002–2003 and 2003–2004. Highest yield was recorded in treatment T3 (33.3 t ha1) and lowest in case of 0.6T1 (18.5 t ha1). Similar trend continued in 2002–2003 and 2004–2005 but in 2003–2004 potato yield was maximum in treatment T4 (33.7 t ha1). Potato yield was higher under SDI than surface drip irrigation

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Table 6 – Mean squares from the variance analyses of the yield Source of variation Years 2002–2003 Replication Levels of irrigation (A) Error (a) Depth of drip tape (B) AB Error (b) Total Years 2003–2004 Replication Levels of irrigation (A) Error (a) Depth of drip tape (B) AB Error (b) Total Years 2004–2005 Replication Levels of irrigation (A) Error (a) Depth of drip tape (B) AB Error (b) Total *

d.f.

Sum of squares

Mean sum of squares

2 2 4 4 8 24

24.24 230.85 1.574 344.85 101.55 13.636

12.12 160.425 0.3935 86.213 12.70 0.568

44

806.7

2 2 4 4 8 24

0.228 368.93 1.054 326.17 81.36 13.098

0.114 184.47 0.264 81.54 10.17 0.5458

44

790.84

17.97

2 2 4 4 8 24

0.569 391.084 4.263 385.496 113.385 15.855

0.2845 195.542 1.066 96.374 14.173 0.661

44

910.652

20.69

Calculated F

Tabulated F (5%)

407.69

6.94*

151.78 22.359

3.84* 2.36*

698.75

6.94*

149.39 18.6

3.84* 2.36*

183.435

6.94*

145.80 21.44

3.84* 2.36*

18.334

Significant.

system during all years of experimentation (Table 4). The effect of depth of placement of drip tape had significant effect on the yield of potato during all the 3 years excepting at 20.0 cm during 2002–2003 and 5.0 cm during 2003–2004 and 2004–2005 (P < 0.05) (Tables 5 and 6). The maximum yield was recorded in treatment T3 during 2002–2003 and 2004–2005, which was followed by the treatment T4 and T2 in 2002–2003 and 2004–2005, respectively. Potato yield was significantly affected by the placement of drip tape and maximum yield was obtained by placing the drip tape at 10.0 cm soil depth (P < 0.05). Least variation in yield was obtained in 3 years in case of treatments 0.6T2 and 0.6T3 (standard deviation 0.5) whereas highest in case of T1 (standard deviation 1.2). Difference in yield in treatment T2 and T3 was not significant (P < 0.05). It was observed that the drip tape buried either at 10.0 or 15.0 cm depth had no significantly different effect on potato yield (CD at 5% were 0.6, 0.55 and 0.66 during 2002–2003, 2003–2004 and 2004–2005). It was recommended that to achieve higher yields, drip tape should be buried at 10.0 cm depth. Levels of irrigation and depth of placement of drip tape significantly affected the mean yield of tuber in all 3 years (P < 0.05). If sufficient amount of irrigation water is available to potato growers, higher yield (33.3 t ha1) can be achieved by placing the drip tape at 10.0 cm soil depth. But in the water deficit condition, potato yield will reduce 7.8 and 12.9% by corresponding saving of 20.0 and 40.0% of irrigation water by burying the drip tape at 10.0 and 15.0 cm soil depth. One of the reasons to achieve higher yield with deficit water supply was 7.5, 3.12 and 3.0 cm of rainfalls were received during tuber development

and maturity stage of crop in 2002–2003, 2003–2004 and 2004– 2005, respectively. Similar types of results were reported by Shock and Feibert (2000). They observed that reduction in total yield of potato due to the progressive deficit irrigation treatments averaged 6.7, 10 and 14% with corresponding water savings of 25.0, 36.0 and 40.0%. In India, farmers generally do not use machines to perform the cultural operations. It was observed that when drip tape was placed at surface and at 5.0 cm depth, then the damage of drip tape was more at the time of weeding. The weed infestation was low when drip tape was buried at 10.0,15.0 and 20.0 cm soil depth because the soil surface was relatively dry throughout the crop season.

3.5.

Benefit–cost analysis

Benefit–cost analysis was carried out to determine the economic feasibility of using SDI system for potato. The seasonal cost of drip irrigation system was estimated considering depreciation (10%), bank interest rate (10%), repair and maintenance (2%) of the system. The life of the drip system was considered 5 years. The cost of SDI system was US$ 1750 ha1. The cost of cultivation of potato crop included cost of installation of drip tapes at different depths, field preparation, cost of seeds, sowing, fertilizer application, weeding, crop protection measures, irrigation and harvesting. The income from produce was estimated by using prevailing average market price as US$ 75 t1. The annual total cost of production, income from produce and the benefit–cost ratio under different treatments is given in Table 7.

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Table 7 – Economic analysis of various treatments of potato (pooled data of the years 2002–2005) Treatments T1 T2 T3 T4 T5 0.8T1 0.8T2 0.8T3 0.8T4 0.8T5 0.6T1 0.6T2 0.6T3 0.6T4 0.6T5

Annual total cost of production (US$)

Potato yield (t ha1)

Income from produce (US$ 75 t1)

1435 1475 1510 1585 1685 1435 1475 1510 1585 1685 1435 1475 1510 1585 1685

30.7 32 33.3 32.9 29.2 27 29.1 30.9 30.6 24 18.5 27.6 27.9 29.5 19.9

2302.5 2400 2497.5 2467.5 2190 2025 2182.5 2317.5 2295 1800 1387.5 2070 2092.5 2212.5 1492.5

The income from produce was found to be highest (US$ 2497.5) in treatment T3 followed by treatment T4 (US$ 2467.5), T2 (US$ 2400) and 0.8T3 (US$ 2317.5) (Table 7). The highest benefit cost ratio of 1.7 was obtained for treatment T3 (Table 7). Lowest benefit–cost ratio of 0.9 was found for treatment 0.6T5. The cost incurred for the installation of drip tape at successively higher depths, increases the annual cost of production. Reduced level of irrigation resulted in small reductions in cost of production because only pumping cost was reduced. In India, there is no cost of ground water to the farmer.

4.

Summary and conclusions

On the basis of low coefficient of variation of dripper flow rates to the tune of 0.046, 0.047 and 0.064 during 2002–2003, 2003– 2004 and 2004–2005, respectively, it may be concluded that the performance of the drip system was good throughout the cropping season. The values of SU and DU were found more than 92.0% during all the three cropping seasons. Soil water content distribution at different growth stages of potato at different depths of placement of drip tape was monitored. When drip tape was buried at 5.0 cm soil depth, upward movement of water took place and surface soil became moist. At 10 cm depth of placement of drip tape, upward water movement due to capillary forces was not noticeable in the sandy loam soil at the experimental site and the soil water content at the surface decreased. When drip tapes were buried at the depths of 15.0 and 20.0 cm, the soil surface remains relatively dry and soil water content increased at the 30.0 cm soil depth at all growth stages of crop and water moves beyond the ridge base. In potato, the tubers formation was confined within the ridges; therefore, the water that moved beyond the ridge base was not available for plants at any stage of growth. Treatment 0.6T4 gave maximum IWUE of 2.07, 2.13 and 2.05 t ha1 cm1 during 2002–2003, 2003–2004 and 2004–2005, respectively. IWUE during 2003–2004 was higher than 2002–2003 and 2004–2005. The highest benefit cost ratio of 1.7 was obtained for treatment T3. Lowest benefit–cost ratio of 0.9 was found for treatment 0.6T5. The cost incurred for the installation of drip

Benefit–cost ratio 1.6 1.6 1.7 1.6 1.3 1.4 1.5 1.5 1.4 1.1 1.0 1.4 1.4 1.4 0.9

tape at successively higher depths, increases the annual cost of production. Reduced level of irrigation resulted in small reductions in cost of production because only pumping cost was reduced. In India, there is no cost of ground water to the farmer. Potato yield was higher under SDI system than under surface drip irrigation system during all years of experimentation. Highest yield was recorded in treatment T3 (33.3 t ha1) and lowest in case of 0.6T1 (18.5 t ha1). The maximum yield was recorded in treatment T3 during 2002–2003 and 2004–2005, which was followed by the treatment T4 and T2 in 2002–2003 and 2004–2005, respectively. Potato yield was significantly affected by the placement of drip tape and maximum yield was obtained by applying the 23.6 cm of irrigation water and by placing the drip tape at 10.0 cm soil depth. The greater vertical movement of water in sandy-loam soil took place because of the predominant role of gravity than the capillary forces, therefore shallow depth of placement of drip tape is recommended in potato crop to get higher yield.

Acknowledgments Authors are thankful to the National Committee on the Plasticulture Applications in Horticulture (NCPAH), Department of Agriculture and Cooperation, Ministry of Agriculture, Government of India for providing the necessary funds to conduct this research. The use of trade names is for information purposes only and does not imply in any way endorsement of particular products.

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