Principles of Center Pivot Irrigation
Published on by Water Network Research, Official research team of The Water Network in Technology
Center pivot irrigation is a type of crop irrigation with rotating equipment with sprinkles around the pivot.
Center pivots were water-powered in the past and now most of them are operated by electric motors.
Center pivot irrigation consists of pipes with sprinkles along them and wheeled towers for moving.
The water is fed to the sprinklers from the pivot point and can be directly leading to a groundwater source.
The outer point of the equipment does not spin at the same speed as the inner point.
Angular velocity is the rate of change of angular displacement of a rotating body.
It is a pseudovector which specifies the rotational speed of the object and the axis around the object is rotating:
The formulas clearly show that the speed is greater at the outer point of the circle.
Since equal water supply is needed everywhere, the nozzle size is the biggest at the outer circle point. The size of the nozzles drops to the inner pivot point.
Center pivots are usually not made larger than 500 m in radius.
Center pivots can be set to rotate the full circle or only a part of it, for best irrigation coverage and to avoid overlapping. Various combinations can be applied:
Pivots can even irrigate the corners without wasting water by attaching swing arms to the ends of the pivots.
Center pivot advantages:
greater water efficiency
reducing labor
easy to install and last for at least 25 years
easily movable from field to field
possibility to remotely monitor and operate the equipment
Disadvantage:
great installing costs
Center pivots can save water by applying fertilizers, insecticides, and herbicides while irrigating.
The most important thing in the design is to correctly select the nozzle type based on the type of soil. Finer nozzles should be used for fine particle soil types (silty clay). Fine droplets are achieved with higher pressure and increased diffusion plate properties. Fine droplets should be used where wind is not a concern.
Good quality sprinklers generally irrigate with large droplets to reduce evaporation and so that they are not instantaneously distributed on the soil surface.
Pivots can be applied for a variety of crops and it is easy to modify them and adjust sprinkler height and application rate to best match the soil’s intake rate and ability to absorb water.
Throw distance should match the infiltration rate of the soil or efficiency is lost due to runoff. As the wetted throw distance is increased, the rate at which water is applied can be reduced.
Pressure in the center of the pivot should be regulated for:
uniform depth of water application
controlled droplet size and throw distance
flexibility in system operations
Height of the system determines the water distribution uniformity – the higher the nozzles, better the uniformity. However, the spacing between the sprinklers should be adjusted so that there is no overlapping.
Another factor which affects uniformity is a uniform travel speed . If the tires are wet, they can be slippery. To maintain speed uniformity part circle sprinklers, combined with the use of boombacks can be applied so that the water is applied behind the direction movement.
Image source: http://www.nelsonirrigation.com/media/resources/Pivot_pocketguide.pdf
The application rate depends on the type of soil and crop.
There are 2 types of application rates:
Average application rate (AAR) – rate of water application over the wetted area.
It is the average value if the wetting uniformity is assumed, for which the application rate is applied at the outer circle.Instantaneous application rate (IAR) – peak intensity of the applied water at a point.
Image: Fixed and rotating stream
Image source: http://www.nelsonirrigation.com/media/resources/Pivot_pocketguide.pdf
Find the pivot diagram for calculations here – starting with page 14 http://www.nelsonirrigation.com/media/resources/Pivot_pocketguide.pdf .
Rotating and spinning equipment at low pressure has 2 benefits:
increasing the soak time
low application rates.
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