DESIGNING, CONSTRUCTION, USE AND MAIN-TENANCE OF CONTOUR WATER HARVESTING

SYSTEM

Extension Bulletin No: 177

2012

National Agricultural Extension and Research Liaison Services,

Ahmadu Bello University,Zaria

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Acknowledgement

The authors highly acknowledged the support of the man-agement of NAERLS during the writing and in sponsor-ing the publication of this bulletin. We also wish to ac-knowledge the meaningful suggestions of Mal. M. P. Buba of Agricultural Engineering and Irrigation Depart-ment, NAERLS, Mal. U. D. Idris of Samaru College of Agriculture (DAC), Ahmadu Bello University, Zaria and all the authors whose work were cited in this bulletin.

A. I. Arab and G. B. Murtala

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Table of Content

Acknowledgement2

1. Introduction4

2.0. Contour water harvesting techniques5

2.1 contour stone bunds, living barriers on Translines5

2,.2 Contour Furrows for Crops10

3.3 Contour bunds for trees15

3.0 Contour furrows for crops20

Biblography 19

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1.0 IntroductionWater in most cases is the major constraint to agricultural production in the drier part of Northern Nigeria. An in-crease in the quantity of water available to crops and live-stock improves the reliability of production as well as the level of production with up to 40-50% (Murtala, 2010). It can make crop and livestock production possible where none is viable. The water availability for crop and live-stock production can be improved through water harvest-ing techniques. Water harvesting is defined as the collection and storage of roof top runoff, overland flow, and stream flow for crop, livestock and other domestic use. Water harvesting is accomplished by diverting runoff from normal channels or water courses and directing the flow over adjacent stor-age system (tanks, flood plains etc.). The diversion and directing may or may not be controlled by a system of dikes and ditches designed to accommodate a calculated rate and volume of flow. The dikes and ditches are usu-ally constructed to enable automatic operation whenever rains fall. Water harvesting is mostly practiced for the purpose of:

i) Crop productionii) Forage productioniii) Domestic and livestock water supply

Water harvesting is not new, in fact, it may predate actual irrigation. Examples of water harvesting can be seen found in many States in Nigeria for difference purposes such as forage production, field and tree crops produc-tion. Water harvesting is usually mainly practiced in Northern states of Nigeria where the following conditions exist:

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a. Site with available run-off water supply: Both volume and frequency of flow are important. Areas having 300 – 700mm average annual rain-fall are suitable.

b. Topography: The ideal water harvesting site is a broad, smooth gently sloping surface without gul-lies. Select sites with slopes not exceeding 5% to prevent uneven distribution of run-off, soil erosion and high cost of the structural support require-ment.

c. Suitable storage facility: Select the most suited storage or utilization facility for the water to be harvested –( i.e. tanks above ground, excavated tanks/dams, flood plains or gently sloping areas deep enough to allow sufficient moisture storage capacity on which harvested water will be con-veniently spread for efficient utilization).

There are many types of water harvesting techniques and each type has definite function adaptable to specific con-dition. This bulletin gives detailed discussion on the vari-ous types of contour water harvesting system and import-ant steps in its design, construction, use and management. This will aid extension agents, extension technologist and engineers to understand the suitability, design, construc-tion, field layout, management/maintenance and socio-economic factors of various contour water harvesting sys-tems. It will also assist farmers and other clientele to im-prove their agricultural production.

2.0 Contour Water Harvesting TechniquesWater harvesting techniques in which the bunds follow the contour line are called contour systems. These in-clude:

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1. Contour stone bunds, living barriers or trash lines2. Contour furrows for crop3. Contour bunds for trees.

2.1 Contour Stone Bunds, Living Barriers or Trash Lines

Arranging stones along the contour line is the simplest form of a contour water harvesting system. Contour stone bunds are used to slow down and filter run-off, thereby increasing infiltration and capturing sediments. Bund formed by stones are permeable because water can easily pass through them, they do not pond off water, but slow down its speed, filter it and spread the water over the field. Grass or other vegetative material is planted imme-diately above the stone lines and forms, over a period of time, a living barrier. Crops residues like millet and sorghum stalks, piles weeds or branches of trees can also be used to reinforce the stone lines. In this case, the bar-rier is called a trash line.The water and sediment harvested directly improves crop performance. This technique is well suited for small scale application on farmer's fields due to the natural adequate availability of stones; they can be implemented quickly at almost no labour cost. The great advantage of the stone system is that there is no need for spillways where poten-tially damaging flows are concentrated. The filtering ef-fect of the semi-permeable barrier along the stones full length gives a better spread of runoff than the earth bunds system. Furthermore, stone bunds require lesser mainte-nance schedule.

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Plate 1: Contour stone bund (Source: FAO, 1991)

2.1.1 Technical Detailsi. Suitability: Stone bunds for cereal crop (sorghum and millet) production can be used under the following condi-tions:

Rainfall: 200mm - 750mm; from arid to semi-arid areas.

Soils: suitable agricultural soils. Slopes: preferably below 2%. Topography: need not be completely even. Stone availability: must have good local supply

of stone. ii. Limitations: Stone bunds are limited to areas where stones are available locally and can be obtain cheaply.iii. Overall configuration: Stone bunds follow the con-tour, the approximate contour, across fields or grazing land. The spacing between bunds ranges is normally from 15m to 30m depending largely on the amount of stone and labour available. There is no need for diversion ditches or provision of spillways.

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Figure 1: Contour stone bund – field layout

A bund height of at least 25cm is recommended with a base width of 30cm – 40cm. The spacing between the bunds can be estimated using the following formula:

d = (h x 100) /s d = distance between two bunds over the ground (in meter)

h = height difference between stone bunds (in meters)

s = gradient of slope

iv. Bund design: Although simple stone lines can be par-tially effective, an initial minimum bund height of 25cm is recommended with a base width of 35cm - 40cm. The bund should be set into a shallow trench of 5cm - 10cm depth to prevent undermining by the runoff. As explained in the construction details, it is important to incorporate a mixture of large and small stones. A common error is to

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use only large stones which allow runoff to flow freely through the gaps in-between. The bund should be con-structed according to the "reverse filter" principle with smaller stones placed upstream of the larger ones to facili-tate rapid siltation. Bund spacing of 20 meters for slopes of less than 1% and 15 meters for slopes of 1-2% are rec-ommended.

Figure 2: Contour stone bund: Dimensions

2. 1.2 Layout and ConstructionStep One: Determine the average slope gradient The average slope of the field is determined by a simple surveying instrument such as a water level or a line level to decide the spacing of the bunds. Each contour line is then set out and pegged individually. A horizontal spacing of approximately 20 meters apart is recommended for slopes of up to 1%, and 15 meters apart for 1 - 2% slopes. Because of variations in the slope, the lines may come closer or diverge at some points.

Step Two: Mark out contour lines and adjust them to form a smooth line

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After the exact contour is laid out, the line should be smoothed by moving individual pegs up or down the slope. As a guideline for ground slopes, up to 1% of the pegs can be moved two meters up-slope or down-slope to create a smoother curve. Not only will a gentle curve be easier to follow while ploughing, but also the amount of stone used for construction will be reduced.

Step Three: Excavate a shallow trench A shallow trench is now formed along the smoothed con-tour. The trench is made by hand tools, or ploughed by oxen and then excavated by hand. The trench should be 5cm -10cm deep and equal to the base width of the bund (35cm - 40cm). The excavated soil is placed upslope.

Step Four: Construction Construction begins with large stones laid down at the base and the down-slope side of the trench, then smaller stones laid in front and on top of this "anchor" line. Small stones should be used to plug gaps between the larger ones. Where possible, a line of small or gravel stones should run along the upslope face of the bund to create a fine filter. The key to a successful stone bund is to elimi-nate any large gaps between stones. In some areas, it nec-essary to break large stones into correct sizes of material.

Figure 3: Construction of stone bundStep Five: Crop Planting Procedure

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Stone bunds are often used to rehabilitate infertile and de-graded land. In order to achieve this objective, the bunds are supported by a further technique - planting pits or "zai". These pits, which are usually about 0.9m apart, are up to 0.15m deep and 0.30 m in diameter. Manure is placed in the pits to improve plant growth and better use of the harvested water. The pits also concentrate local runoff which is especially useful at the germination and establishment phase. Regular weeding is essential to pre-vent the harvested water from being used by the ‘wrong’ plant.

Plate 2: ‘Zai’ (Source: Critchley, 1987)

2.1.3 Maintenance: During heavy runoff, stone bunds may be over-

topped and some stones dislodged, these should be replaced. A more common requirement is to plug any small gaps with small stones or gravel where runoff forms a tunnel through.

Eventually, stone bunds silt-up, and this means ex-pected water harvesting efficiency may not be

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acheved. The stone bunds silt-up takes up to 3 sea-sons or more to happen; in some cases, it could occur more rapidly where bunds are wide apart or on steeper slopes. To reduce siltation, bunds should be built up in these circumstances with less tightly packed stones while maintaining the effect of slowing runoff. Alternatively, grasses can be planted along the bund.

2.1.4 Socio-Economic FactorsOn-farm stone bund for crop production is quickly appre-ciated and adopted by farmers. The techniques involved include simple surveying can be easily learned. The amount of labour required is reasonable, and where groups are organized to work in turn on individual mem-ber's farms, fields can be transformed in a single day. The benefits of stone bunds are often clearly seen already in the first season - and this has popularized the system.

Some challenges of stone bund system include:a. Relatively, rich farmers can make use of wage labour

to treat their fields while poorer farmers may lag be-hind.

b. Differing availabilities of stones can lead to inequali-ties between neighboring areas: not everyone can ben-efit in the same way. This leads to another problem - to what extent will the cost of stone transportation be justified where they are scare?

2.0 Contour Furrows for Crops

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Contour furrows, sometimes called contour ridges or mi-cro-watersheds are small earthen banks with a furrow on the higher side which collects runoff from an uncultivated land between the ridges. Ridges follow the contour at a spacing of usually one to two meters. Runoff is collected from the uncultivated strip between ridges and stored in a furrow just above the ridges. Cereals crops (sorghum, mil-let) and legumes (cowpeas, tepary beans) are usually planted on both sides of the furrow. The system is simple to construct by hand or by machine and can be less labour intensive than the conventional tilling of a plot. The yield of runoff from the very short catchment lengths is extremely efficient and when designed and constructed correctly, there should be no loss of runoff out of the sys-tem. Another advantage is even crop growth due to the fact that each plant has approximately the same contribut-ing catchment area.

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Plate 3: Contour furrow for crop (Source: FAO, 1991)

2.2.1 Technical Detailsi. Suitability: Contour ridges for crop production can be used under the following conditions: Rainfall: 350mm - 750mm. Soils: all soils which are suitable for agriculture.

Heavy and compacted soils may be a constraint to construction of ridges by hand.

Slopes: from flat up to 5%. Topography: even area with rills or undulations

should be avoided. ii. Limitations: Contour ridges are limited to areas with relatively high rainfall, as the amount of harvested runoff is comparatively small due to the small catchment area. iii. Overall configuration: The overall layout consists of parallel or almost parallel earth ridges on the contour at a spacing of between one and two meters. Soil is excavated

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and placed down-slope to form a ridge, and the excavated furrow above the ridge ycollects runoff from the catch-ment strip between ridges. Small earth ties in the furrow are provided every few meters to ensure an even storage of runoff. A diversion ditch may be necessary to protect the system against runoff from outside.

Figure 4: Contour ridges: field layout

iv. Catchment: cultivated area ratio: The cultivated area is not easy to define. It is a common practice to as-sume a 50cm strip with the furrow at its center. Crops are planted within this zone, and use the runoff concentrated in the furrow. Thus, for a typical distance of 1.5m be-tween ridges, the C:CA ratio is 2:1; that is a catchment strip of one meter and a cultivated strip of half a meter. A distance of two meters between ridges would give a 3:1 ratio. The C:CA ratio can be adjusted by increasing or de-creasing the distance between the ridges. In practice a spacing of 1.5 - 2.0 meters between ridges (C:CA ratios of 2:1 and 3:1 respectively) is generally recommended for annual crops in semi-arid areas.

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v. Ridge design: Ridges need only be as high as neces-sary to prevent overtopping by runoff. As the runoff is harvested only from a small strip between the ridges, a height of 15 -20cm is sufficient. If bunds are spaced at more than two meters, the ridge height must be increased.

Figure 5: Contour ridge dimensions

Table 1: Quantities of earthwork for contour ridges Ridge spacing Ridge & Tie height Earthworks per ha(m) (cm) (m3)1.5 15 2701.5 20 4802.0 20 360Source: FAO, 1991

2.2.2 Layout and ConstructionStep One: Determine the average slope gradient of the contour Contours are surveyed by a simple surveying instrument such as a water tube level or line level. The real contour should be smoothened to obtain a better alignment for agricultural operations.

Step Two: Mark out contour lines and stake out.

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Contour key lines should be staked every 10 or 15 meters. The alignment for the ridges is then marked in between the key lines according to selected spacing. On uneven terrain, the contours may come closer together at one point or widen at other points. It is necessary to stop lines where the contours converge or to add short extra lines in between where the contours diverge.

Step Three: Excavate the furrows The furrows are excavated using a hoe or ploughed par-allel to the marked alignments for the ridges. The exca-vated soil is placed down-slope, next to the furrow, and the ridge is formed.

Step Four: Construct small cross-ties Small cross-ties are built at intervals of about five meters dividing each furrow into a number of segments. The ties are 15-20cm high and 50 - 75cm long.

Step Five: Construct a diversion ditch A diversion ditch should be provided above the block of contour ridges if there is a risk of damage caused by runoff from outside the system. The diversion ditch should be 50cm deep and 1-1.5m wide with a gradient of 0.25%. The excavated soil is placed down-slope. The ditch should be constructed before the contour ridges are built to prevent damage from early rains.

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Figure 6: Contour furrow: layout technique

Step Six: Crop Planting ProcedureThe main crop (usually cereal) is seeded into the upslope side of the ridge between the top of the ridge and the fur-row. At this point, the plants have a greater depth of top soil. An intercrop, usually a legume, can be planted in front of the furrow. It is recommended that the plant pop-ulation of the cereal crop be reduced to approximately 65% of the standard for conventional rain-fed cultivation. The reduced number of plants thus has more moisture available in years of low rainfall.

Figure 7: Planting configuration

2.2.3 Maintenance: Repair breaches and overtopping immediately

they occur. Rebuilt ridges at the end of each season to their

original height.

2. 2.4 Socio-Economic FactorsSince the contour ridge technique implies a new tillage and planting method compared to conventional cultiva-tion, farmers may be initially reluctant to accept this tech-

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nique. Demonstration and motivation are therefore very important. On the other hand, it is one of the simplest and cheapest methods of water harvesting. It can be imple-mented by the farmer using a hoe, at no or little extra cost. External support is limited to a minimum. Alternatively, it can be mechanized and a variety of implements can be used. When used by a farmer on his own land, the system does not create any conflicts of interest between the im-plementer and the beneficiary.

2.3.0 Contour Bunds for TreesThe contour bunds for trees are very similar to the contour furrows for crops. The difference is that in the system for trees, the harvested water is collected in an infiltration pit, instead of a furrow. One of the advantages of contour bunds is their suitability to the cultivation of crops or fod-der between the bunds. As with other forms of contour water harvesting techniques, the yield of runoff is high, and when designed correctly, there is no loss of runoff out of the system.

Figure 8: Contour bunds for trees

2.3.1 Technical Detailsi. Suitability: Contour bunds for tree planting can be used under the following conditions:

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Rainfall: 200mm - 750mm; from semi-arid to arid areas.

Soils: Must be at least 1.5m, preferably 2m deep to ensure adequate root development and water storage.

Slopes: from flat up to 5%. Topography: must be even, without gullies or

rills.

ii. Limitations: Contour bunds are not suitable for un-even or eroded land as overtopping of excess water with subsequent breakage may occur at low spots.

iii. Overall Configuration: The overall layout consists of a series of parallel or almost parallel earth bunds on the contour at a spacing of between five and 10 meters. The bunds are formed with soil excavated from an adja-cent parallel furrow on their upslope side. Small earth ties perpendicular to the bund on the upslope side subdivide the system into micro-catchments. Infiltration pits are ex-cavated in the junction between ties and bunds. A diver-sion ditch protects the system where necessary.

Figure 9: Contour bunds for trees: field layout

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iv. Unit Micro-catchment Size: Common sizes of micro-catchments are around 10cm -50m2 for each tree.

v. Bund and Infiltration Pit Design: Bund heights vary, but are in the order of 20cm - 40cm depending on the pre-vailing slope. As bunds are often made by machine the actual shape of the bund depends on the type of machine; whether for example a disc plough or a motor grader is used. It is recommended that the bund should not be less than 25cm in height. Base width must be at least 75cm. The configuration of the furrow upslope of the bund de-pends on the method of construction.

Figure 10: Micro-catchment unit

Bunds should be spaced at either 5m or 10m apart. Cross-ties should be at least two meters long at spacing of two to 10 meters. The exact size of each micro-catchment is thus defined. It is recommended to provide 10m spacing be-tween the bunds on slopes of up to 0.5% and 5m on steeper slopes. A common size of micro-catchment for multipurpose trees is 25m2. This corresponds to 10 meters bund spacing with ties at 2.5m spacing or five meters bund-spacing with ties at 5m spacing.

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Excavated soil from the infiltration pit is used to form the ties. The pit is excavated in the junction of the bund and the cross-tie. A pit size of 80cm x 80cm and 40cm deep is usually sufficient.

Figure 11: Bund dimensions

Figure 12: Infiltration pit and planting sites

2.3.2 Layout and ConstructionStep One: Determine the average slope gradient of the contour and staked outThe contour is determined using a simple surveying in-strument, such as a line level or a water tube level. As contour bunds are implemented on even land, contours need only be staked out approximately every 50 meters. The real contour should be smoothened to a gentle curve. Bunds may become slightly wider apart at one end to ac-commodate any change in the contour.

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Step Two: marked out the alignment of each bund on the ground The alignment of each bund should be marked on the ground before construction starts. As recommended under "bund design" the bunds should be set at a spacing of 10m for slopes up to 0.5% and 5m for steeper slopes. When disc ploughs are used, a single disc (with the remaining one or two removed) forms an adequate bund. Where available, a reversible plough is preferred because furrows can be ploughed consecutively in both directions. Com-paction of the bunds is recommended. When no machines are available, this should be done by foot or with a barrel filled with sand.

Step Three: Dug infiltration pit and construct cross-ties The catchment size required for each seedling determines the spacing between cross-ties. For example where a 25m2

catchment area is required and bunds are 10 meters apart the cross-ties will be 2.5 meters apart. Cross-ties are made by hand. An infiltration pit of 80cm x 80cm and 40cm deep is dug in the furrow above the bund. Water collected in the fur-row will then drain into the pit and supply the adjacent seedling. The excavated material is sufficient to form a cross-tie of 2m length, 75cm base width and 25cm height. The cross-tie extends upslope from the main bund at an angle of 90° to the main bund; at least 30cm should be left between the cross-tie and the pit to allow sufficient space for planting the seedling.

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Step Four: Construct lateral bund to prevent loss of runoff At each side of the block, a lateral bund of 25cm - 30cm height is built to prevent loss of runoff out of the system. The earth should be excavated from inside the system and the contour bunds must be joined up with the lateral bund.

Step Five: Construct diversion ditch A diversion ditch should be provided above the scheme if there is a risk of damage by runoff from outside the block. The diversion ditch is aligned on a 0.25% slope and a common dimension is 50cm deep and 1 - 1.5m wide, with the soil piled down-slope. The diversion ditch should be constructed before the contour bunds are built to prevent damage if rainstorms occur during construction.

Step Six: Tree planting procedure with spacingTree seedlings, of at least 30cm height, should be planted immediately after the first runoff has been harvested. The seedlings are planted in the space between the infiltration pit and the cross-tie. It is advisable to plant an extra seedling in the bottom of the pit for the eventuality of a very dry year. Manure or compost can be added to the planting pit to improve fertility and water holding capac-ity. One important advantage of contour bunds for tree estab-lishment is that oxen or mechanized cultivation can take place between the bunds, allowing crops or fodder to be produced before the trees becomes productive. However, this can reduce the amount of runoff reaching the trees.

2. 3.3 Maintenancea. Repair or rebuilt damage bunds at the season.

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b. Repair any breaches immediately they occur and the repaired section compacted.

c. Prevent animal from invading the plots so as not to cause damage.

d. Grass should be allowed to develop on the bunds, thus assisting consolidation with their roots.

3.4 Socio-Economic Factors1. Contour bunds for trees are mainly made by machine;

costs of bund construction can be relatively low and implementation fast, especially where plots are large and even and the kind of mechanization well adapted.

2. However, as with all mechanization in areas with lim-ited resources, it may not be sustainable. Experience has shown that very often the machines come abruptly to a halt when the project itself ends.

3. Another aspect that must be addressed is the manage-ment after the scheme has been established (which is usually done under the auspices of a development project). This is an issue which has to be seriously considered during the planning phase. Management of a large afforestation block by the local community is in most cases a new challenge and failure or success will depend on acceptance of the technique by the ru-ral population.

Bibliography1. Critchley, W.R.S., (1987). Some lessons from water

harvesting in Sub-Saharan Africa. Report From a Workshop held in Baringo, Kenya, 13-17 October 1986. World Bank, Eastern and Southern Africa Projects Dept., Washington DC.

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2. Critchley, W.R.S.,and Seznec, A., (1992). Water har-vesting for plant production: Case studies and conclusions for Sub – Saharan Africa. Vol. II, 1992. World Bank Technical Paper no. 157. The World Bank, Washington.

3. FAO, (1991). Water harvesting techniques: In FAO Corporate Document Repository. Rome: Natural Re-sources Management and Environment Department.

4. Murtala, G. B., (2010). An overview of rainwater har-vesting technologies and utilization. In the Proceed-ings of the pre-season training for extension staff in the North-West Zone, Nigeria. Organize by the Insti-tute of Agricultural Research (IAR), Samaru and National Agricultural Extension and Research Liaison Services (NAERLS), Samaru, Ahmadu Bello University, Zaria. (7th – 11th June, 2010).

5. Murtala, G. B., and Buba, M. P. (2010). Rainwater harvesting technologies for crop production. In the Proceedings of the pre-season training for extension staff in the North-West Zone, Nigeria. Organize by the Institute of Agricultural Research (IAR),

Samaru and National Agricultural Extension and Research Liaison Services (NAERLS), Samaru, Ah-madu Bello University, Zaria. (7th – 11th June, 2010).

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