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Low-cost drip irrigation—A suitable technology for

southern Africa? An example with tomatoes using saline irrigation water

Louise Karlberga, Johan Rockströma, John G. Annandaleb and J. Martin Steynb

aStockholm Environment Institute (SEI), Box 2142, 103 14 Stockholm, Sweden bDepartment of Plant Production and Soil Science, University of Pretoria, Pretoria 0002,

South Africa

Abstract Using saline water for irrigation increases water productivity by freeing up fresh water

that can be allocated to domestic or other uses. Drip irrigation is widely regarded as the

most promising irrigation system in combination with saline water. Simple drip irrigation

kits that are affordable for smallholder farmers have successfully been implemented for

irrigation of vegetable gardens in several countries in sub-Saharan Africa. The possibility

of using low-cost drip irrigation with saline water to successfully irrigate a common

garden crop, tomatoes, was tested in this study. Two low-cost drip irrigation systems with

different emitter discharge rates (0.2 and 2.5 l h−1) were used to irrigate tomatoes

(Lycopersicon esculentum Mill. cv. “Daniella”) with water of three different salinity

levels (0, 3 and 6 dS m−1). In addition, plastic mulch to minimise soil evaporation was

also compared to a “bare soil” or uncovered treatment. Two consecutive tomato crops

(spring and autumn) were produced during two growing seasons, starting from September

2003 and ending in April 2004, at the Hatfield Experimental Farm in Pretoria, South

Africa. An average yield of 75 Mg ha−1 was recorded for all treatments and seasons,

which can be compared with the average marketable yield for South Africa of

approximately 31.4 Mg ha−1. Even at the highest irrigation water salinity (6 dS m−1), a

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yield above the average marketable yield was achieved, indicating that low-cost drip

irrigation works well in combination with saline water. Furthermore, the study showed

that the choice of drip irrigation system with regard to discharge rate is of minor

importance when irrigating with saline water. However, combining low-cost drip

irrigation with plastic mulch increased the yield by on average 10 Mg ha−1 for all

treatments. For the bare soil treatments, rainfall had an important role in the leaching of

salts from the soils. Finally, the study showed that specific leaf area was higher at high

irrigation water salinities, which is contrary to results from other studies. To be able to

generalise the promising findings from this study, there is a need to mechanistically

model the impact of different climates, soils and irrigation management practices, as well

as the long-term sustainability of these systems.

Article Outline 1. Introduction

2. Materials and methods

2.1. Experimental set-up and management

2.2. Design of drip irrigation systems

2.3. Treatments

2.4. Measurements

2.5. Analysis of data

3. Results

4. Discussion

5. Conclusion

Acknowledgements

References

1. Introduction Access to safe water in large areas of sub-Saharan Africa is one of the major challenges

in the efforts of achieving the UN Millennium Development Goals (MDGs) of halving

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the number of poor and malnourished (Falkenmark and Rockström, 2004). Three-quarters

of the world's poor live in rural areas and depend largely on agriculture as their main

source of income (IFPRI, 2004). In sub-Saharan Africa that figure is as high as 80%.

Rural livelihoods are strongly dependent on water, both for domestic and agricultural

purposes (FAO, 2000 and Rockström, 2000). The water productivity, i.e. the amount of

fresh water required to produce a unit biomass, of these agricultural systems is low;

however, studies have shown that several techniques such as the use of supplementary

irrigation (Barron, 2004), fertiliser (Stoorvogel and Smaling, 1990) and strategies to

minimise soil evaporation (Daamen et al., 1995 and Rockström, 2003), can increase

water productivity substantially. Using saline water for irrigation is another way of

increasing the water productivity by enabling more fresh water to be used for domestic

and other purposes. Saline groundwater from boreholes unsuitable for human

consumption is a potential water resource for agriculture in areas of water scarcity in sub-

Saharan Africa (Karlberg and Penning de Vries, 2004).

Saline water up to 11 dS m−1 has been used successfully for commercial irrigation of a

number of crops globally (Hoffman et al., 1990 and Rhoades et al., 1992). However, in

order to assure maximum yields from crops irrigated with saline water, it is necessary to

develop special management procedures (Pasternak and De Malach, 1994). Presently,

drip irrigation is widely regarded as the most promising irrigation system to use with

saline water (e.g. Meiri et al., 1992 and Suarez, 1992). Several factors contribute to the

good results obtained with saline water when using drip irrigation (Dasberg and Or,

1999): (i) less water use (high application efficiency) results in less salt deposited on the

field, (ii) avoidance of leaf burn, (iii) high frequency drip irrigation prevents the soil from

drying out between irrigation events, thereby avoiding peaks in salt concentration and

concomitant high osmotic potentials and (iv) salts are continuously leached out from the

wetted section and accumulate at the wetting front away from the active root zone.

Simple drip irrigation kits that are affordable and easy to assemble and maintain have

been implemented successfully for irrigation of vegetable gardens of small-scale farms in

several countries in sub-Saharan Africa, e.g. Kenya, Zimbabwe and South Africa (Du

Plessis and Van der Stoep, 2000, Kabutha et al., 2000 and Chitsiko and Mudima, 2002).

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However, Karlberg and Penning de Vries (2004) conclude that research on the use of

saline water in low-cost drip irrigation systems is generally lacking.

Low-cost drip systems utilise gravity as the force to push water through the pipes.

Normally a head of only 0.5–1 m is used for bucket kits (Sijali, 2001). Using a head of

between 0.5 and 1.5 m, and 10 m long laterals results in an average emitter discharge of

approximately 0.2–3.0 l h−1 depending on the drip system (Chigerwe et al., 2004). This

figure can be compared with flow rates under conventional drip of approximately 2.0–

8.0 l h−1 (Dasberg and Or, 1999) irrespective of the length of the laterals. Low emitter

discharge rates lead to longer irrigation application periods, and therefore a more even

salt concentration compared to high emitter discharge rates. On the other hand, a very

low discharge rate might result in higher soil evaporation since the soil surface is wetted

for a longer period of time. The latter process leads to a lower irrigation efficiency and

higher soil salinities. To conclude, discharge rate might be a factor to take into

consideration when choosing between different low-cost drip-systems, although it is

unclear which discharge rate should be superior.

Irrigation efficiency could also be improved by using plastic mulch. A mulch prevents the

soil from drying out between irrigation events, and thus peaks in salt concentration in the

root zone can be avoided. Such peaks are believed to have a negative impact on growth,

and the use of mulch can therefore be expected to have a positive effect on yield under

saline water irrigation; however, it is unclear if this effect has any significant effect on

crop yield.

The aim of this paper is to test the possibility of using low-cost drip irrigation with saline

water to successfully irrigate a common garden crop, tomatoes, in South Africa. More

specifically, the objective was to compare the impact of irrigation water salinity on crop

yield using these systems. Furthermore, additional objectives were to test the impact of

discharge rates and the use of mulching on soil evaporation, soil water content, soil

salinity and crop yield.

2. Materials and methods A field experiment was carried out at the Hatfield Experimental Farm, Pretoria

University, South Africa (25°45′S, 28°16′E) during two growing seasons, the first from

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September to December 2003, and the second from January to April 2004. These periods

coincide largely with the rainy season that occurs from October to March, during which

most of the average annual rainfall of 670 mm falls. The climate is characterised by dry,

mild winters and hot, wet summers. The hydro-climate is sub-tropical and semi-arid, with

an annual potential evaporation of 1650 mm year−1. A large part of the precipitation can

fall during intense thunderstorms, resulting in large quantities of run-off from the fields.

In addition, rainfall is erratic with frequent dry-spells.

2.1. Experimental set-up and management

Before planting in September, soil samples were analysed to design a fertiliser

programme. The soil has been classified as a sandy clay loam, consisting of

approximately 60% sand, 10% silt and 30% clay (Tesfamariam, 2004). Furthermore, the

mean field capacity is 28.4% volumetric water content, mean wilting point is around

15.8% volumetric water content and mean bulk density is 1.52 Mg m−3 in the uppermost

metre of soil. Prior to the experiment, the electrical conductivity (EC) of the water in the

dam used for irrigation was determined to be 0.22 dS m−1. Fields were ploughed and

fertiliser incorporated 2 weeks before planting using 165 kg N ha−1, 33 kg P ha−1 and

165 kg K ha−1. Raised production beds (6 m × 1.8 m) were made, each separated by a

0.6 m path (Fig. 1). Finally, the drip irrigation kits were assembled, black plastic mulch

rolled out and secured at the edges with stones, and each bed was pre-irrigated with 65 l

of water of treatment specific salinity. The same beds were also used during the second

season. Six-week-old tomato seedlings (Lycopersicon esculentum Mill. cv. “Daniella”)

were planted on the 9th of September for the spring season and on the 12th of January for

the autumn season. Each plot was planted with a total of 42 seedlings in 2 rows, with a

spacing of 1.2 m between rows and a spacing of 30 cm in the row, to match the location

of the emitters. This resulted in a density of 2.9 plants m−2. During the growing season,

additional fertiliser was applied as fertigation (spring: 37.6 kg N ha−1, 15.4 kg P ha−1,

111.4 kg K ha−1; autumn: 42.8 kg N ha−1, 15.4 kg P ha−1, 126.6 kg K ha−1) and sprayed

on the foliage (3.1 kg N ha−1 and 4.0 kg Ca ha−1 in both seasons) at regular intervals

throughout the experiment for all treatments. In addition, pesticides were applied due to

problems with cutworms and nematodes.

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Fig. 1. (a) Layout of raised production beds and irrigation systems, and location of some

of the measurement equipment. Both plots are irrigated with the high discharge rate

system (microtubes). The plot on the right is covered with a plastic mulch, whereas the

one on the left is a “bare” or uncovered soil treatment. (b) A schematic illustration of the

drip irrigation systems seen from above. Left: Microtube (high discharge rate). Right: In-

line (low discharge rate). In the in-line system, the position of the measurement

equipment is portrayed as white circles.

Irrigation took place in the evenings around sunset. Each plot was irrigated from a

separate tank located at the end of the production bed into which salt and water was

added. All treatments received the same amount of irrigation water. The irrigation

scheduling (irrigation amount and frequency) was determined from modelling of tomato

plants with the use of regional climatic data from a previous year but with zero

precipitation, using a parameterisation derived from data on saline-water, drip-irrigated

tomatoes from Israel (Karlberg et al., 2006). Since the effect of water stress was not the

focus of the study, an irrigation schedule in which water did not limit growth at any stage

during the experiment was chosen. Fixed amounts of either 15, 30 or 50 l were given to

each plot every second day in the beginning of the growing season, and daily towards the

end of each season, depending on the estimated demand. In the case of rain, irrigation

was reduced by the corresponding amount of water that had fallen as precipitation even

for the mulch treatments. For the treatments with intermediate irrigation water salinity

(3 dS m−1), 1.76 g NaCl l−1 was added to the irrigation water, whereas the corresponding

figure for the high irrigation water salinity treatments (6 dS m−1) was 3.51 g NaCl l−1.

The resulting amount of irrigation water (daily values) for spring and autumn, as well as

daily precipitation are shown in Fig. 2. During the first season, a total of 300 g salt m−2

was added to the soil for the treatments with intermediate irrigation water salinity, and

double this for the treatments with the high salinity water. The corresponding figures for

autumn were 175 and 350 g m−2, respectively. Algal growth in the tanks posed the risk of

clogging emitters, and all tanks were therefore continuously treated with algae eating

bacteria and were also cleaned by hand at regular intervals. As the plants grew bigger

they were trellised, and fruit were hand picked as they ripened. Final harvest took place

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on the 17th and 18th of December for the spring season, and from 21st to 24th of April

for the autumn season.

Fig. 2. Daily irrigation amounts for (a) spring and (b) autumn, and daily precipitation for

(c) spring and (d) autumn.

2.2. Design of drip irrigation systems

Each plot consisted of a raised production bed drip-irrigated with laterals connected to a

50 l tank (Fig. 1). A microtube system produced by International Development

Enterprises (IDE, CO, USA) was used to represent the high discharge system. This

system was developed for smallholder farmers and is therefore easy to assemble and

manage. It has an emitter discharge rate of about 2.5 l h−1at a 1-m head and a distribution

uniformity of about 85% (Chigerwe et al., 2004). Two 60 cm long microtubes extend

from the lateral and emit water at the end of each microtube. To represent the low

discharge system, we chose an in-line emitter system produced by Plastro Irrigation

Systems Ltd. (Israel). These laterals were designed for commercial farmers, but can

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easily be connected to a simple bucket or drum and used for garden irrigation. It has an

emitter discharge rate of about 0.2 l h−1 at a 1-m head and a distribution uniformity of

over 95% (Chigerwe et al., 2004). Both systems had an emitter spacing of 30 cm. The

50 l irrigation tanks were positioned approximately 90 cm above the raised surface of the

beds. After a tank was filled with water, a tap located underneath it was opened and water

gravity fed into the laterals and was discharged through the emitters. Each emitter

irrigated one plant.

2.3. Treatments

The experiment was limited to 12 treatments arranged as a randomised complete block

design with 3 factors. The two drip systems with high (H) and low (L) discharge rates

were used to irrigate the tomatoes with water of three different salinity levels (low

salinity = 01 dS m−1; intermediate salinity = 3 dS m−1; high salinity = 6 dS m−1). These

treatments were repeated with plastic mulch (M) and under bare soil conditions (B). All

treatments were replicated three times. The exact position of each treatment within a field

was randomised separately for each replicate. To avoid edge effects, the first and last

production bed of each field were also planted with tomato plants and drip irrigated with

fresh water according to the same irrigation schedule as the other plots, but were not

included in the experiment.

2.4. Measurements

Measurements were taken at different intervals before, during and after the growing

periods. Above ground dry mass was determined using destructive samples to estimate

carbon allocation to leaves, fruit and stem. One plant per plot was collected

approximately every 2 weeks, by identifying an average sized plant for each treatment

and cutting the stem at the soil surface. A total of five samples were taken from each plot.

Samples were dried in an oven at 60 °C for up to 7 days and were subsequently weighed.

At planting, 20 seedlings were used for destructive sampling, which also included root

depth and mass. Moreover, at harvest, plants for destructive sampling were dug up to

allow both above and below ground measurements. Thus, root depth and mass could also

be determined at harvest, bearing in mind that some of the root system was undoubtedly

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left behind in the soil. The destructive samples also enabled the determination of leaf area

using an LI 3100 belt driven leaf area meter (LiCor, Lincoln, NE, USA). In addition,

plant height and canopy diameter was measured throughout the experiment for all

treatments, and the onset of flowering and fruit formation was also recorded. At harvest,

total above ground biomass of the green parts (i.e. leaves and stems) was weighed in the

field for each treatment. In the same manner, total fruit biomass was weighed separately

and included both edible and inedible fruits.

Soil water content was recorded throughout the experiment at different depths using a

neutron probe model 503DR CPN Hydroprobe (Campbell Pacific Nuclear, CA, USA).

The instrument had been calibrated before the start of the experiment (Tesfamariam,

2004). Twenty-four neutron probe access tubes were installed in pairs in one of the

replicates, one in the wet area next to a plant and one in the middle of the plant bed, for

all 12 treatments. The neutron probe measured soil water content every 20 cm in the

uppermost metre of the soil profile. In addition, the diameter of the wetted area of the soil

surface around each dipper was measured for the plants adjacent to the access tubes

during the spring experiment.

Microlysimeters (e.g. Daamen et al., 1993) were used to measure soil evaporation from

the wetted area next to the plants and from the centre of the plant beds, for all bare soil,

low and high salinity treatments. Each lysimeter consisted of an outer liner pipe

permanently located in the soil, and an inner pipe (length: 150 mm; diameter: 82 mm),

which is the actual lysimeter used for sampling (Fig. 3). Before planting, all liner pipes

were positioned in the soil so that the upper part of the cylinder was level with the soil

surface. One-half of the liners were located in the wet area next to an emitter, and the

other half in the dry area in the middle of the production beds. Soil evaporation was

measured daily, starting from sunrise the morning after an irrigation event and ending at

sunset on the onset of the next irrigation event. During this cycle, consisting of 1–4 days,

one soil sample per lysimeter was used. Samples were collected at sunrise following an

irrigation event, by pressing the lysimeter into the soil until the entire cylinder was filled

with soil, and then carefully digging it up. Each sample was taken in the edge plots at the

same location as the outer pipe in the corresponding treatment plot, i.e. a lysimeter was

filled with wet soil if it was positioned in a liner located in the wet area of the soil and so

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forth. Once excavated, the lysimeter was trimmed flush at the base, placed on a sheet

metal base plate and sealed with waterproof tape. The microlysimeters were then

weighed with a portable balance that weighed to ±1 g, and mounted in the corresponding

liner tube. Every evening at sunset they were collected, weighed and returned to their

locations in the field. At sunrise the morning after an irrigation event, a new cycle was

started and new soil samples were taken in the same manner. In case of rain, a cycle had

to be interrupted and a new cycle could only be started once the rain had ceased. As the

weighing process is extremely wind sensitive, all samples were weighed inside a box

placed in the field. Soil evaporation was determined from the difference in mass between

two measurements, and from the soil surface area of the sample. Two to three cycles of

soil evaporation were collected three times during spring (September, October and

November) and twice during autumn (January and March), resulting in a total of five

series.

Fig. 3. Left: Microlysimeters with soil samples in the field. The microlysimeter in the

lower part of the picture is located in the middle of the production bed, whereas the

second is located next to an emitter under the plant canopy. Right: The inner pipe of the

microlysimeter turned upside down with the metal base plate attached to the bottom of

the lysimeter with tape.

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During the experiment, 12 ceramic cup soil water samplers (Soil Moisture Equipment

Co., CA, USA) were used to measure the electrical conductivity of the soil water close to

the stem and at the edge of the wetted zone at the soil surface, at two different depths (20

and 40 cm). The treatments chosen for sampling (all without mulch) were: high discharge

rate, high salinity; low discharge rate, high salinity; and low discharge rate, intermediate

salinity.

Intercepted Photosynthetically Active Radiation (PAR) was measured twice during the

spring experiment with the use of a sunfleck ceptometer (Decagon Devices Inc., WA,

USA). In addition, humidity, wind speed, solar radiation, air temperature and

precipitation was recorded on an hourly basis at a weather station adjacent to the fields.

2.5. Analysis of data

All statistical analyses were performed in STATISTICA ver. 6. Multifactor analysis of

variance (ANOVA) was used to compare the different treatments. Unplanned pairwise

comparisons were preformed using Tukey's HSD test to interpret interactions between

factors. The data-set on yield and the destructive samples from the autumn experiment, as

well as the soil water salinity measurements, contained missing values. In the case of

yield data, imputation by interpolating from other measurements was used to substitute

five values. However, the destructive samples data-set from the autumn season contained

too many missing values due to problems related to the calibration of the scales,

imprecise sampling and mixing of samples, and therefore the whole data-set was

excluded from the analysis. To overcome the problem with missing data in the soil

salinity measurements, mean values for each month were used. In order to achieve

homogeneous variances for the biomass data from the destructive samples, the ANOVA

was done on the logarithm of the masses.

3. Results Total yield, including inedible fruit, for each treatment is presented in Fig. 4. During

spring, the average marketable yield was approximately 94 Mg ha−1 whereas during

autumn the corresponding figure was 55 Mg ha−1, representing a significant difference

between seasons (p < 0.0001). Even the yield from the plants irrigated with the highest

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salinity level (6 dS m−1), was above the average marketable yield of 31.4 Mg ha−1 for

South Africa recorded between 1098–2002 (FAO, 2004). Yield was significantly higher

at low irrigation water salinity than at high salinity levels (p < 0.0001). An unplanned

pairwise comparison using Tukey's HSD test for the various salinity treatments revealed

that this difference was only significant between low and high irrigation water salinity

(p = 0.0001), and between medium and high irrigation water salinity (p = 0.013). Despite

the reduction in yield under highly saline conditions (6 dS m−1), these treatments still

yielded on average 66 Mg ha−1 as compared with 80 Mg ha−1 for the fresh water

treatments. Mulch also had a significant effect on yield for all salinities and irrigation

systems (p < 0.0001), increasing the yield by 10 Mg ha−1 on average over all treatments.

There was no significant difference in yield between the high and the low discharge rate

systems, irrespective of salinity level. The analysis of the destructive samples taken

during spring confirmed the findings from the analysis of the final yield (Fig. 5). It was

found that, on average, above ground biomass was significantly higher at low levels of

irrigation water salinity (p < 0.0001), and for mulch treatments (p < 0.0001), whereas

there was no difference between irrigation systems.

Fig. 4. Total yield for all treatments for: (a) spring and (b) autumn. L: low discharge rate;

H: high discharge rate; 0: EC 0 dS m−1; 3: EC 3 dS m−1; 6: EC 6 dS m−1. Vertical bars

denote one standard deviation of the mean.

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Fig. 5. Total above ground biomass from destructive samples for: (a) the mulch

treatments and (b) the bare soil treatments for the spring experiment (mean values for the

two discharge rate systems).

Soil water content was analysed for the root zone (upper 40 cm). As expected, the soil

was generally wetter closer to the emitter than in the middle of the production beds

(p < 0.0001), even though after rainfall events the soil was sometimes wetter in the

middle of the field for the bare soil treatments. Comparing only the data from the soil

close to the emitters, it was found that soil water content was higher at high salinities

(p < 0.0001) (Fig. 6). In addition, including plastic mulch affected the two irrigation

systems differently (p = 0.007). For the low discharge rate system, the soil water content

was significantly higher when mulch was included (p < 0.0001, Tukey's HSD), whereas

for the high discharge rate system there was no difference between the mulch and the

bare soil treatment.

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Fig. 6. Soil water content measured close to the emitter in the root zone (upper 40 cm) for

both seasons for: (a) the low discharge and (b) the high discharge system. 0: EC

0 dS m−1; 6: EC 6 dS m−1; M: mulch; B: bare soil.

The analysis of the soil evaporation data revealed no significant difference between

systems during spring (Fig. 7), whereas during autumn, soil evaporation was found to be

higher for the low discharge rate system compared to the system with a high discharge

rate (Fig. 8) (p = 0.041). Moreover, there was no significant difference in soil evaporation

between salinity treatments for any of the seasons. Soil evaporation was higher from the

wet soil close to the emitters than from the centre of the beds (spring: p < 0.0001;

autumn: p = 0.001). There was a significant variation between series (i.e. between

September, October and November for spring, and January and March for autumn)

(spring: p < 0.0001; autumn: p < 0.0001) for both seasons, but not between days within a

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series. Finally, measurements of the wetted area of the soil surface showed that the

diameter was around 30 cm for both systems.

Fig. 7. Soil evaporation during the spring season for: (a) the wetted soil close to the

emitters and (b) the soil in the middle of the production beds. L: low discharge rate; H:

high discharge rate; 0: EC 0 dS m−1; 6: EC 6 dS m−1.

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Fig. 8. Soil evaporation during the autumn season for: (a) the wetted soil close to the

emitters and (b) the soil in the middle of the production beds. L: low discharge rate; H:

high discharge rate; 0: EC 0 dS m−1; 6: EC 6 dS m−1.

Soil water salinity was higher at high irrigation water salinity then at low (p < 0.0001)

(Fig. 9). However, no significant difference in mean soil water salinity was found

between the two systems.

Fig. 9. Soil salinity at four locations in the soil: (a) edge of the wetted zone, 20 cm depth;

(b) close to the stem, 20 cm depth; (c) edge of wetted zone, 40 cm depth; (d) close to

stem, 40 cm depth. L: low discharge rate; H: high discharge rate; 3: EC 3 dS m−1; 6: EC

6 dS m−1.

Specific leaf area was higher at high irrigation water salinity then at low salinity during

the autumn season (Table 1), whereas during spring no such relationship could be seen.

Interestingly, specific leaf area was also significantly higher for the mulch treatments

than for the plants grown without mulch during autumn.

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Table 1.

Specific leaf area measurements for both seasons

Treatment Mean (m2 gC−1)a

Standard deviation

Range (m2 gC−1) p-Valueb n

Spring

Low discharge rate 0.0117 0.0023 0.0091–0.0141 ns 108

High discharge rate 0.0114 0.0023 0.0091–0.0131 108

0 dS m−1 0.0112 0.0022 0.0090–0.0131 ns 72

3 dS m−1 0.0115 0.0023 0.0090–0.0130 72

6 dS m−1 0.0118 0.0025 0.0093–0.0148 ns 72

Mulch 0.0116 0.0021 0.0093–0.0134 ns 108

No mulch 0.0114 0.0025 0.0089–0.0139 108

Autumn

Low discharge rate 0.0112 0.0044 0.0044–0.0167 ns 126

High discharge rate 0.0113 0.0045 0.0040–0.0174 108

0 dS m−1 0.0103 0.0037 0.0037–0.0155 0.000053 72

3 dS m−1 0.0116 0.0048 0.0043–0.0174 72

6 dS m−1 0.0118 0.0046 0.0047–0.0183 ns 72

Mulch 0.0116 0.0045 0.0047–0.0176 0.000487 108

No mulch 0.0108 0.0043 0.0037–0.0165 108

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ns: not significant at the 0.05 level. a Unit is square metre leaf per gram carbon of leaf. b Calculated using Tukey's HSD test. For the different salinity treatments, the

comparisons were only made between low and medium salinity, and medium and high

salinity. A significant difference between either low or high salinity compared to medium

salinity, implies that there was also a significant difference between low and high

salinity, and thus this comparison was not made.

4. Discussion All treatments resulted in a total yield above or equal to the average marketable yield of

31.4 Mg ha−1 for South Africa (FAO, 2004). Thus, during this growing season, it was

possible to use water of a salinity up to 6 dS m−1 for supplementary irrigation of tomatoes

using low-cost drip systems at a garden scale. However, the results from the field

experiment also included inedible fruit, which should be borne in mind when comparing

the yield from the experiment with the average yield for South Africa. Furthermore, a

higher yield can be expected in a controlled field experiment compared to the on-farm

situation, due to factors such as reliable availability of fertiliser and irrigation water. The

yield from the second crop was significantly lower than the first. This result is to be

expected, since the experiment started with a non-saline soil (approximately 1 dS m−1 at

saturation), whereas the salinity treatments for the second season started with more saline

soils (e.g. a soil water salinity of 9 dS m−1 from the high salinity treatments). However,

this advantage might have been counteracted by the high load of salt due to the large

amounts of irrigation water applied to the fields in combination with very low rainfall

during spring, compared to the second season. Larger problems with pests during autumn

might also have contributed to the differences in yield. All in all, since climate varies

with season, it is difficult to compare yields between spring and autumn.

Increasing salinity levels in the irrigation water led to reductions in plant size and yield.

At high irrigation water salinities soil water content was generally high. This is likely to

be the result of lower plant water uptake, either caused by the smaller plant size (toxicity

effect), or causing the smaller plant size (osmotic effect). Soil evaporation was unaffected

by the irrigation water salinity level, even though canopy size varied between salinity

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treatments. Soil samples were taken from the edge plots that were irrigated with fresh

water. Therefore, measured soil evaporation for the salinity treatments might be higher

than in reality, due to the effect of lower vapour pressure because of high soil salinity.

Soil salinity was higher for high irrigation water salinities than at lower levels, and soil

salinity was substantially influenced by rainfall. From the start of the experiment until

February, soil salinity kept increasing steadily. The heavy rainfall at the end of that

month and in March caused significant leaching of the salts and subsequently a decrease

in soil water salinity. Between February and March, the average soil salinity level in the

root zone fell between 3 and 5 dS m−1, depending on treatment, whereas soil water

content remained relatively constant during this period. Salinity levels continued to

decrease until the end of the second season, but they never returned to the initial levels at

the beginning of the experiment in September. It has been shown that the impact of an

individual rain storm can cause substantial leaching of salts from the soil (Hoffman et al.,

1990), and the onset of monsoon rains caused a sharp reduction of soil salinity in saline

water irrigated rice fields from 6 to 2 dS m−1 in Bangladesh (Mondal et al., 2001).

Salinity levels closest to the emitter seemed to fluctuate less over time compared to other

locations.

Mulching minimises soil evaporation and thus prevents large fluctuations in soil water

content, whereby peaks in soil water salinity occur. A positive effect of mulch on yield

was evident for all treatments; however, there was no difference in the magnitude of this

positive effect between irrigation water salinity treatments, indicating that fluctuations in

soil water content, rather than peaks in salinity concentrations was important for plant

growth. An additional benefit of mulch was the reduction of weeds. Mulch treatments

required a lot of maintenance in the beginning of the growing season when the plants

were small, as wind tended to lift the mulch up and cover some of the plants, which

caused occasional leaf damage. On sunny days the black plastic used in the experiment

got very hot, causing high soil temperatures underneath the sheets. This might have had a

positive effect on growth in the beginning of spring, but may otherwise have caused plant

stress. It is unclear as to how much of the precipitation reached the soil through the holes

in the plastic next to the plants, as opposed the amount that was lost as runoff. This

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apparent drawback of increased runoff from the plastic is counteracted by the

concentration of precipitation next to the plants, due to the location of the holes.

A low emitter discharge rate keeps the soil water content relatively constant, resulting in

small fluctuations in soil salinity and thus lower salinity stress in comparison to high

discharge rate systems. However, when the soil surface is wetted for a long period of

time, the cumulative soil evaporation becomes high. Since irrigation took place at sunset,

both systems had almost finished irrigating by the next morning depending on the amount

of irrigation water used, and therefore this latter effect may not be of importance.

Furthermore, the soil wetting pattern and the ability to leach salts from the soil might

have varied between the two systems, although the area of the wetted soil surface was

found to be the same for both. Since no significant difference between the two drip

irrigation systems was found, neither in the final yield data, nor in the measurements on

above ground biomass, the combined effect of these processes might have been the same

for the two systems. Nonetheless, the analysis of the water balance data shows that soil

water content was not affected by mulching in the high discharge rate system, whereas it

was higher with mulch than without in the low discharge rate system, especially at low

salinity. Assuming that transpiration is proportionate to total above ground dry matter

within each salinity treatment, and that transpiration equals plant water uptake, both

systems transpire approximately the same amount of water per salinity level. Thus, the

withdrawal of water from the soil by plant roots is not likely to have caused these

differences in soil water content in the root zone with and without mulch between the two

irrigation systems. Measurements of soil evaporation showed that the system with a low

discharge rate had a higher average evaporation rate (5.13 mm day−1) than the other

irrigation system (4.24 mm day−1) during autumn. This is probably due to the longer

duration of irrigation in the former case, causing the upper soil in the slow discharge rate

system to be wetter in the morning following an irrigation event, thus leading to higher

soil evaporation. Soil water content is also affected by drainage, which might have been

higher for the high discharge rate system due to the higher application rate. However,

since no measurements were made on drainage, this variable remains unknown.

At low salinity without mulch, water is lost from the soil through drainage, water-uptake

and soil evaporation. By covering the soil with a plastic sheet, soil evaporation is

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minimised and thus some water is saved in the soil. In the high discharge rate system this

water seems to be taken up by the plant, since soil water content remains the same as

without mulch, whereas plant growth and thus transpiration is higher. The high discharge

rate system lost less water from the soil as evaporation whereas drainage is assumed to be

higher, resulting in similar soil water contents for the two irrigation systems. Since soil

evaporation is high for the system with the low discharge rate, the inclusion of mulch in

this system has a large impact on the soil water content. In this system, the water that is

saved by mulching is partly used by the plant, expressed as a higher growth compared to

the bare soil treatment, but some water remains in the soil, causing the observed

difference in soil water contents between the mulch and bare soil treatments. It is likely

that water uptake had reached a point where water was no longer limiting to plant growth,

and this is why the plants in this treatment did not utilise all the extra water in the profile.

The higher soil evaporation and the assumed lower drainage in the low discharge rate

system as compared to the high discharge rate system, implies a higher accumulation of

salts in the former system. However, measurements of soil water salinity showed no

difference between systems. Even so, soil salinity levels seem to fluctuate more in the

high discharge rate system, which might be due to larger variations in soil water content

caused by the high infiltration rate, leading to a drying out of the surface soil between

irrigation events. Thus, a measurement taken directly before irrigation might be

substantially higher than the mean soil water salinity during the irrigation cycle. From

this perspective, a comparison of the measured values of soil water salinity from the two

different irrigation systems might be misleading. Further, the impact on growth of these

peaks in salinity concentration is difficult to estimate. Since no difference in yield was

seen between the two systems at high salinity, it is assumed that the combined effect of

all factors affecting soil salinity was of the same magnitude for both systems.

Some temporal patterns were seen in the measurements, especially for soil evaporation.

During spring, potential evaporation increases with time. As the plant grows it shades

more and more of the wetted area surrounding the emitter. The combined effect of these

two factors results in small differences over time in soil evaporation from the wetted area

during spring, whereas for the dry area the difference in evaporation between series is

more dependent on rainfall. On the other hand, during autumn soil evaporation from the

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wetted area shows a marked decrease from the beginning of the season to the middle of

the season, probably caused by a larger shading effect from the plant canopy in

combination with a lower potential evaporation. As for spring, soil evaporation from the

dry area seems to be mainly dependent on rainfall.

No strong relationship between salinity and specific leaf area was found, which

contradicts the findings of Taleisnik (1987), Brugnoli and Björkman (1992) and Marcelis

and van Hooijdonk (1999). Generally, the variation in measurements was higher at high

salinity, which was also the case in the study on radish (Raphanus sativus L.) by Marcelis

and van Hooijdonk (1999). As has been shown by others (Gary et al., 1993 and Marcelis

et al., 1998), specific leaf area is a very variable characteristic, and the absence of a

strong relationship between specific leaf area and salinity might be due to a high degree

of variation in the data.

The results of this study indicate that both irrigation systems are suitable to use in

combination with saline water, and can give good yields. However, at the end of the

second season, soil water salinity levels were higher than they were at the beginning of

the experiment, indicating an accumulation of salts in the root zone. Since the likelihood

of rainfall between April and September is low in this region, the soil water salinity levels

at the end of the experiment represents this year's annual build-up of salt. It is unclear if

this accumulation would continue if the same irrigation practices continued yet another

year, or if the soil water salinity would fluctuate around approximately the same levels.

One of the most determining factors seems to be heavy rainfall, causing leaching of salts

from the soil profile. Since rainfall is erratic in the semi-arid tropics, it is difficult to

predict the long-term sustainability of these systems without using a tool, such as a

simulation model, in which the important processes in the system are accounted for under

varying climatic conditions.

The potential yield increase from adopting the technologies used in this study must be

weighed up against the costs involved in buying, installing and maintaining the systems.

In the case of saline water irrigation, yield loss caused by salinisation must be weighed

against the benefits of being able to allocate more fresh water from irrigation to domestic

use, and the benefits relating to reduced risk of crop failure due to drought when using

supplementary irrigation. In India, a bucket drip irrigation kit irrigating over 100

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individual plants over a 25 m2 area cost US $5 per kit, including the bucket (Postel et al.,

2001), making drip irrigation an affordable technology even for resource poor farmers. A

cheap alternative to plastic mulch is to use crop residues from a previous crop to cover

the soil. Crop residues have profound water conserving effects; they increase infiltration

of rainwater, as well as reduce run-off and soil evaporation (Erenstein, 2002).

The second crop was more affected by pests than the first, which most likely caused a

significant reduction in yield. However, the target group of this study, namely the rural

farmers, would probably not grow a large monoculture twice in a row. On the other hand,

the larger problem with pests during autumn could also have been due to the fact that this

season was a lot wetter than the previous season. Nonetheless, this difference between

seasons caused increased variation in data, which complicated the analysis of the results.

Another source of error stems from the fact that soil water content and soil water salinity

were measured at the same location on each plot throughout the whole experiment. This

means that if there was any disturbance of the soil right next to the measurement points,

the whole measurement series from that treatment might be skew. For example, if the

emitter next to the neutron probe access tube was suddenly blocked, the measurement of

soil water content for that treatment will be lower than the mean value for that plot.

Evaporation from microlysimeters has been shown to be consistent with the evaporation

calculated by gravimetric methods for the first 4 days after core extraction (Daamen et

al., 1993). However, the same study showed that the largest errors in measurements are

caused by the boundary to water flow imposed at the base of the lysimeter at the time of

core extraction, and it is therefore recommended that soil samples should be renewed at

least daily. This recommendation was not followed in this study, due to time and labour

constraints, and might therefore have caused some errors in estimated soil evaporation.

Finally, the imputation used to substitute missing values in the statistical analysis

decreased the variance of the samples; however, the number of missing values were few

in relation to the whole sample which means that the effect of the imputation is probably

rather small.

It is possible that the outcome of this study would have been very different for a different

soil, climate and crop. A more salt sensitive crop could for example have preformed

differently to tomato under the prevailing climatic conditions and treatments. The first

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growing season was very dry and most of the rain came late. Higher rainfall earlier in the

season would most likely have led to higher crop yields for the salinity treatments. To

evaluate the impact of the environmental conditions, the data obtained in this study could

be used in combination with a simulation model. Several transient models for estimating

water uptake under matric and osmotic stress have been developed (e.g. Nimah and

Hanks, 1973, Letey et al., 1985, Cardon and Letey, 1992 and Pang and Letey, 1998).

Recent advances of crop growth models have led to the incorporation of more detailed

growth functions into the transient models (e.g. van Dam et al., 1997 and Stöckle et al.,

2003). Furthermore, biochemical photosynthesis models based on the work by Farquhar

et al. (1980) have been developed by several research teams (e.g. Sellers et al., 1996 and

Dai et al., 2004). Karlberg et al. (2006) incorporated a biochemical photosynthesis model

into an ecosystem modelling package, including a transient model for water uptake under

matric and osmotic stress, and a model for increased plant respiration due to ion toxicity.

Thereby, the modelling package could account for processes such as radiation saturation

at high light intensities, and reduced transpiration and photosynthesis as a function of

stomatal closure at high vapour pressure deficits; two important characteristics of dry

tropical environments. This modelling package, which was tested on data on drip-

irrigated tomato grown at various irrigation water salinities in an arid environment, could

also be combined with the data obtained in this project to evaluate the long-term

sustainability of saline water, drip irrigated systems in different environments.

5. Conclusion We conclude that low-cost drip irrigation works well in combination with saline water for

the irrigation of garden crops in semi-arid Africa. Even at quite high irrigation water

salinity (6 dS m−1), a yield above the average marketable yield was reached. Furthermore,

the study showed that the choice of drip irrigation system with regard to discharge rate is

of minor importance when irrigating with saline water. However, combining low-cost

drip irrigation with plastic mulch resulted in a higher yield for all treatments. The choice

of drip system and the inclusion of mulch must be decided upon both from a cost

perspective and from an evaluation of the systems performance in the field, e.g. risk for

blockages. Rainfall had an important role in the leaching of salts from the soils, causing a

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decrease in average soil water salinity of 3–5 dS m−1 over a 4-week period, depending on

treatment. Specific leaf area was found to be higher at high irrigation water salinities,

which contradicts earlier findings (Taleisnik, 1987, Brugnoli and Björkman, 1992 and

Marcelis and van Hooijdonk, 1999). Further, the study showed a higher specific leaf area

for the mulch treatments. Finally, it should be noted that the results of this study are only

valid for the prevailing environmental conditions at the study site. Hence, there is a need

to evaluate the impact of different climates, soils and management practices, as well as

the long-term sustainability of these systems, with a mechanistic modelling approach.

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1 The irrigation water from the dam had a salinity level of 0.22 dS m−1. Therefore, the

treatment denoted 0 dS m−1 was in actual fact equal to 0.22 dS m−1, although the former

has been used