- OVERHEAD IRRIGATION
This type of irrigation is the application of water to the land using a system of sprinklers which simulate natural rainfall. The system has a number of advantages over flood irrigation:
- It is much more economical in the use of water and the application rates of water can be controlled exactly as flood irrigation can waste enormous amounts of water.
- By controlling the size and speed of the water droplets, damage to the soil structure can be avoided.
- Fertilizer nutrients can be fed through the system in a soluble form and applied directly to the crop together with the water.
- Sprinkler irrigation can be used to cover crops with a layer of water which, on cold nights, will freeze and protect the crop from frost damage.
The designing of an irrigation system for a farm is a job for experts and no farmer should try to design his own system. Pipes and sprinklers alone can cost a great deal of money per hectare so that even a small scheme will cost large amounts of capital. The important factors which are considered by an irrigation engineer would include the following:
- The soil texture on the farm; sand, sandy loam clay, etc.
- The soil structure and amount of organic matter in the soil. Many soils have a poor soil structure and when they dry out will form a crust or cap on the surface causing poor germination of seeds and penetration of water. This capping factor, or ‘t’ factor can be affected by the size of the drops of water applied.
- The sub‐soil and underlying rock on the farm. Rock outcrops and impermeable layers in the soil can cause poor drainage, waterlogging and soil erosion problems.
- The permeability and infiltration rate of the soil, as this will govern the rate of application of water and the frequency of irrigations. Sandy soils need smaller amounts of water applied more frequently than clay soils because they have a lower water‐holding capacity.
- The amount of water to apply is based on the root depth of the crop being grown and the water holding capacity of the soil. The requirements for a deep‐rooted crop like lucerne will be different from that of a shallow‐rooted crop such as lettuce or most vegetable crops.
- The maximum water requirement which will vary according to the crop being grown and the area of the country. This will be based on the evaporation and transpiration figures for crops in the hottest months, when temperatures are high and before the rains have started
- The considerations of the farmer, such as the size of his labour force, other farming operations and the type of equipment to be used.
- Safety measures to allow for labour interruptions, breakdown of equipment particularly pumps and drought years.
- The total amount of water that will be required for a year so that the source of the water, dams, rivers, boreholes, etc, will be sufficient.
- LAYOUT OF A SCHEME
PLANNING AN OVERHEAD SPRINKLER IRRIGATION SCHEME
When planning an irrigation scheme it is necessary to ensure that the scheme meets the crop demand at the most critical growth stage (e.g. maize at flowering in January), when water stress could reduce yield substantially (see Table 1 below.) Data of the crop, climate and soil is required to plan an efficient irrigation scheme.
Table 1: Data required for planning
|Maize Peak moisture demand (Jan) Crop factor (CF) = 1.0 Allowable depletion = 50% Rooting depth = 600–800mm
|Evaporation (Eo) = 6mm/day Evapotranspiration (Et) = 6mm/day (Eo x CF)
|Texture = 25% clay Available moisture (a.m.) = 120mm/m Infiltration rate = 8.4 mm/hr Effective depth = 700mm
Example scheme for maize:
- Total available moisture (120mm x 0.7m) = 84mm
- Allowable depletion (84mm x 0.5) = 42mm
- Irrigation cycle (42mm ÷ 6mm = 7 days
- Working cycle per week = 6 days
- Stand time (42mm ÷ 8.4 ÷ 8.4mm infil.) = 5 hrs + 0.5 hrs (moving lines)
- Sets/day (16.5 hrs ÷ 5.5 hrs) = 3
- Sets/cycle (6 days x 3 sets) = 18
- Area to irrigate (648m x 192m) = 12.4ha
- Sprinkler spacing = 12m x 18m
Figure 1: Layout of an overhead sprinkler system
- No. Lateral sp. Positions (648m x 2 ÷ 18) = 72
- No. Lateral sp. Lines 72 ÷ (6 days x 3)) = 4
- No. Sprinklers/lateral (96m ÷ 13m) = 8
- Total number sprinklers (4 x 8) = 32
- Number of hydrants (72 ÷ 6) = 12
- Water demand 12.4ha x 42mm x 10 x 1 000 = 20 litres per sec
- 6 days x 15 hrs x 60 x 60 x 0.8 eff
- Pipe size: √20 litres/sec x 25 = 112mm diam./nearest size
- Operating pressure (350 kPa) = 35.0
- Main line (30% of 35m) = 10.5m
- Sprinklers (20% of 35m x 0.75) = 5.2m
- Bends and fittings (10% of 10.5 M) = 1.0m
- Static head = 5.0m
- Suction head = 3.0m
- Total head = 70.0m
- Power demand = 20 litres/sec x 70m head = 17.5kW
100 x 0.8 eff.
To obtain the average sprayline pressure, place the pressure gauge a third of the way down the line. Pressure drop between the first and last nozzles must not exceed 20% of the operating pressure.
FERTILISERS THROUGH SPRINKLERS
Since not all fertilisers are suitable and some expertise is necessary in their use it is sensible to seek advice from trained consultants.
S X L X N X F = kg fertiliser per setting 10 000
S = sprinkler spacing L = lateral spacing
N = number of sprinklers F = fertiliser per ha
250kg N/ha = 12 x 18 x 25 x 250 /10000= 135kg fertiliser per setting
Water is pumped into the night storage dam from boreholes as the dam is filled during the night and emptied during the day. The dam is built on ground which is higher than the lands being irrigated so that only a booster pump is needed to maintain the pressure in the pipeline. In some cases, it is possible to place the dam high enough so that gravity alone will provide sufficient pressure to the sprinklers. The lines of sprinklers are attached to the hydrants and can be moved when required. The illustration below shows a line of sprinklers in operation.
Figure 2: Overhead irrigation sprinklers
On large irrigation schemes, a main pipe is laid underground, and this covers a circle around the irrigation lands with hydrants at intervals on the main pipe. The underground main pipes are normally made of cement, and the portable pipes which have to be moved about are made of aluminium or rigid PVC These pipes are light and can be carried by hand and are joined together by various patent fittings.
- SPACING OF SPRINKLERS
The rates of precipitation of water are:
- 6mm per hour for heavy soils;
- 8mm per hour for medium soils and;
- 10mm per hour for light sandy soils.
In order to achieve these rates of application, the spacing of the sprinklers will vary according to the pressure of the water in the system and the size of the nozzles in the sprinklers. The table below gives various spacings according to nozzle size and pressure.
Table 2: Nozzle Sizes and Pressures for Different Application Rates of Water
|NOZZLE SIZE mm
|3.2 x 2.5
|12 x 12
|3.5 x 2.5
|4.0 x 3.2
|12 x 18
|18 x 18
There is an enormous variety in the size and output of sprinklers. For over 40 years manufacturers in Europe, Israel and the USA have developed a range of sprinklers which vary from those that can apply minute amounts of water (for seedling production) to those which apply very large amounts of water (for bananas and sugarcane). Sprinklers are manufactured from bronze, brass or aluminium. Sprinklers have to be well made because during the annual irrigation life of one crop, each sprinkler head performs 129 600 revolutions and 31 104 000 beats. The normal life of a sprinkler should be 5 years provided the wearing parts, particularly washers are replaced twice a year.
Current trends in overhead irrigation are to reduce the spacing of the sprinklers, nozzle sizes and water pressures in order to achieve uniform precipitation with the lowest running costs. Low rates of water application using sprinkler lines that remain in the same position for 12 hours help to reduce the labour needed for irrigation.
The amount of water discharged from the nozzle and the diameter of the wetted circle will vary according to the equipment used and can be obtained from the manufacturer’s performance charts. The pressure of water in the sprayline can be measured using a pressure gauge.
Figure 3: Some examples of sprayline fitting
- THE RAINGUN
This is an irrigation sprinkler which is very large with nozzles up to 60mm in diameter and capable of discharging up to 3 500 litres per minute. Some rainguns have a tapered bore nozzle to give greater range in windy conditions. The bigger the nozzle the further the water is thrown and the bigger the droplets must be to retain enough energy to travel long distances.
Rainguns are generally used as mobile irrigators. The machine is made up of a raingun and a wheeled chassis with a water‐driven motor, which is used to winch the system along a tethered cable. The water is supplied by a trailing hose, 100mm in diameter and 200 metres long and made of rubber reinforced with nylon. The advantage of this is the large area that can be irrigated with the minimum of labour. The larger models can apply 2mm of water over 11ha in a day. The disadvantages are the high cost of the hose and the fact that the large droplet size and high water pressure can do a lot of damage to the soil structure.
- OTHER SYSTEMS OF SPRINKLER IRRIGATION
In recent times, the emphasis in irrigation on commercial irrigated farms has gone from the flood and furrow method, to the use of sprinklers on portable pipelines. Sprinkler irrigation has gained favour because of the better control of water applications, savings of water and can be used on sandy soils and sloping land. Due to the need to reduce the use of both labour and water, systems were developed in countries like Israel and Australia for use on high value, permanent crops such as tea, coffee, fruit and vegetables. Two of these systems used in the region are the solid‐set and dragline systems. Both methods use low precipitation sprinklers, and the advantages are:
- Labour savings of up to 75% over the conventional sprinkler method where lateral pipes are being moved continuously around the lands;
- The elimination of the need to move pipes over wet soil;
- Irrigation can be done at night without the need to move pipes
- Less soil compaction and crusting therefore greater efficiency of water use; and
- Better aeration of the soil because air is never excluded as happens under heavy precipitation systems. Many Israeli experts consider this fact to be of greater importance than the uniformity of sprinkler irrigation.
THE SOLID‐SET SYSTEM
This system is suited to closely planted permanent crops such as tea and coffee. It consists of small diameter 25mm plastic pipes covering the whole land. The pipes are buried at 20 metre spacings along the plant rows with both the main and the laterals lines being buried. Risers are placed at intervals along the lateral pipes and these are fitted with valves which can be adjusted to allow for the same pressure settings. This gives a uniform sprinkler discharge, even when used on sloping downhill layouts. The sprinklers are moved from one riser to the next and no pipes have to be moved, saving time, labour and damage to the trees in the plantation. The system is expensive to install but together with the advantages mentioned above maintenance costs are greatly reduced.
THE DRAGLINE SYSTEM
This is particularly suited to the more widely‐spaced permanent crops such as citrus and deciduous fruit trees. The method is an under‐tree system and the advantages are that it is protected by the trees, suffers less from wind interference, makes better use of water, reduces evaporation and provides very good water distribution.
The system consists of 1 or 2 low‐capacity sprinklers mounted on skids and connected to the water source by a length of polythene hose. Each move is made by pulling the sprinklers by the hose from one position to the next.
Without being disconnected the hose can irrigate a row of trees until the field main is reached and can then be used to irrigate the row on the other side. As it has a low‐pressure system the main lines can be of a small diameter PVC or polythene pipe buried in the ground. Using a correctly designed layout and very low capacity sprinklers, capital and operating costs can be greatly reduced. The capital costs of the system are less than for a conventional sprinkler layout and a further advantage is that irrigation can be carried out at night without having to move pipes about.
Figure 4: The Master Sprinkler
The centre pivot irrigation unit consists of a long span of pipes which is pivoted in the centre of the field. The spans are supported on wheeled structures called towers and rotate in a circle. The sprinkler emitters are suspended from the spans. The rate of application is controlled by the speed of rotation. Centre pivots are expensive but are automatic, efficient and require minimal labour.
Figure 5: Centre pivot irrigation unit (adapted from Irritech, 2003)
A 6 span pivot with a 22.56m end overhang and spans spaced at 54.86m: Radius of pivot = 54.86m x 6 + 22.56 m = 351.72 m
Area of pivot = πr2 = 3.14 x 351.72m x 351.72m / 10 000 = 38.8ha
No. of sprinklers = 351.72m = 122
2.88m (spacing) Pipe diameter = 127mm
Systems capacity = 6 mm application per 24 hrs area ha x application rate mm x 10
= 38.8ha x 6mm x 10 = 97 m3 per hour (26.9 litres/sec) 24 hrs