Flood or surface irrigation schemes rely on gravity to move water through the canals or pipes and onto the land but overhead or sprinkler schemes need a pumping system to force the water through the sprinklers with enough pressure to achieve a good spread of water. There are 2 types of pumps available. The turbine pump and centrifugal pump, being the one most commonly used in this region.
The centrifugal pump is a very simple device with only one moving part. It consists of a single rotating shaft with an impeller at one end. When the shaft is rotated at high speed water is thrown against the outer casing at high speed causing a vacuum to be formed at the center of the impeller. This, in turn, draws more water into the pump, and as long as the water enters the pump at the center of the impeller it will be forced against the casing. The casing is larger than the outlet pipe and because of this the velocity of the water inside the casing is turned into pressure when it leaves the pump through the smaller outlet. The two diagrammes below show how a centrifugal pump works.
Figure 1: Pump discharge
The maximum suction head of a centrifugal pump, e.g. (the height the water is raised) should be: Below 1 000 metres above sea level: 3.5 ‐ 4 metres.
Above 1 000 metres above sea level: 3.0 ‐ 3,5 metres.
The mouth of the inlet or suction pipe should be 500 mm below the surface of the water that is to be pumped and a non‐return valve fitted to the pump outlet.
All pumps have what is called an efficiency rating set by the manufacturer. This is the actual performance when it is working rather than the theoretical performance based on calculations. The efficiency rating is expressed as a percentage and most pumps have a rating of 60% ‐ 70%. This means that they will work at 60% ‐ 70% of their theoretical capacity. The difference between the actual and theoretical performance of pumps is due to factors such as friction, air in the inlet pipe, etc.
In order to calculate the capacity of a pump for an irrigation scheme, the following formula is used by irrigation engineers.
Pump Capacity in litres per second = mm (depth) X ha (area) X 2.8
f X hours per day X e
- mm (depth) is the depth of soil to irrigate.
- ha (area) is the number of hectares to be irrigated.
- F: is the flow of water.
- Hours: is the time spent each day irrigating. Normally during daylight hours.
- e: is the efficiency of the pump which should be not less than 60%
If the factor 10 is used instead of 2.8 this will give the pump capacity in cubic metres per hour (m3/h) instead of litres per second (l/s).
Where the efficiency of the pump is 70% (or 0.70), the average requirement would be 1 litre per second per hectare to be irrigated.
All pumps need power to drive them which is either supplied by an electric motor or a diesel or petrol engine. Again, engineers use a formula to work out the power requirement needed to drive a pump which is as follows:
Power in kW (kilowatts) = Flow in litres per second X head in metres
100 X e
In addition, because of the loss of power between the engine and pump, the following adjustments have to be made to allow for these transmission losses.
- 10% for direct‐coupled electric motors.
- 20% for belt‐driven motors.
- 20% for direct‐coupled engines.
- 25% for belt‐driven engines.
For example, having worked out the power requirements to drive a pump, if the power is supplied by a diesel engine driving the pump by means of a belt, the power requirement would have to be increased by 25%.
An important aspect of any irrigation scheme is the drainage of the soil. Water is being applied throughout the year particularly in summer during the rains it is possible that the soil will often become saturated, ie; filled above field capacity. Rainwater does not cause any problem but water used for irrigation may have a high content of minerals and salts and if this water is allowed to remain in the soil for too long, the result can be the formation of a brackish or alkaline soil. Over irrigating during winter can lead to soil saturation and this excess water must be removed quickly from the soil.
The two methods of draining soils are through open ditches and tile drains. The tables below give recommendations for both.
Table 1: Recommended Dimensions of Open Ditch Drains
|Soil||Maximum Velocity of Flow||Side Slopes||Depth|
|Clay||1.5 metres / second||0,5 / 1||Water surface in the drains must be below the crop root depth|
|Loam||1.0 metres / second||1 / 1|
|Sand||0.5 metres / second||2 /1|
RECOMMENDED DIMENSIONS OF TILE DRAINS
Gradient: a minimum of 1 in 500 and a maximum of 1 in 100.
- lines of drains should be 30 metres apart in clay soils
- 60 – 120 metres apart in sandy soils.
- Between 1 and 2 metres deep, depending on soil type.
Table 2: Area Drained depending on the Size of Tile Drain Pipe
Figure 2: Open ditch drains on clay, loam and sandy soils
The costs of irrigation can be divided into the costs of installing the system, the capital costs, and the day to day costs of operating the system namely, the running costs.
The capital costs of irrigation schemes are high. Usually this amount is borrowed, interest paid on the loan, and repaid over a number of years. The cost of the interest on the loan has to be added to the cost of installing the irrigation making it even more expensive.
The returns from irrigation are calculated easily. If a farmer installs irrigation and his maize yields increase from a dry land yield of 5 tons a hectare to an irrigated yield of 9 tons a hectare, the increase due to his irrigation is 4 tons a hectare. At a value of US$250 per ton gives him an increase in income of 250 x 4 = $1000 per ha. With full irrigation the farmer can grow a crop such as winter wheat during the dry season and will increase his net income for the year.
One problem with irrigation costs is calculating the running costs of the scheme. Agritex Zimbabwe devised a system which gives a reasonably accurate figure for the cost per 1 000 cubic metres of water applied to the land.
Table 3: Calculating the cost of running an irrigation scheme
One method which a farmer can use to decide whether to install irrigation is Partial Budgeting which is covered in the Farm Management Course in the section on Farm Budgeting.