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Getting it right: pump selection criteria for irrigation

The location of the irrigation water makes a difference to the pump that should be selected

 

Pump selection is crucial to ensuring that the farmer’s irrigation design layout meets the precise requirements for the crop and optimises irrigation efficiency. These requirements are satisfied if the pump matches the irrigation system, flow pressure is kept low, and controls are 

Different types of irrigation techniques place varied demands on how water is pumped from the source and distributed within the field. The goal is broadly speaking to supply the entire field uniformly with water using the least amount of energy, ensuring that each plant has the amount of water it needs, neither too much nor too little. Modern irrigation methods are efficient enough to achieve this goal, and having the right pump is crucial.

Irrigating an entire field uniformly is best accomplished by dividing the area to be irrigated into zones. A zone can be defined according to irrigation need, types of crops, and soil type and is often served by a single irrigation valve and one or a group of drippers or sprinklers that are connected by pipes or tubes. Irrigation systems are divided into zones because there is usually not enough pressure and available flow to run sprinklers for an entire field at any one time. An irrigation controller – either mechanical or, increasingly, electrical and online – signals a zone to turn on at a specific time and keeps it on for a specified amount of time.

 

Select irrigation equipment first

 

Different irrigation equipment requires different amounts of water and pressure, and the equipment must therefore be selected before selecting the pump. The controller must not be overlooked. With pump performance controlled, irrigation can be turned on and off during predefined periods. Water can be conserved by not irrigating in direct sunlight, or when winds are heavy.

A controller can be programmed to optimise operation with due respect to both the crop and water conservation, for example by turning the pump off for a time will allow the soil to absorb the irrigated water. Engaging it later on will improve infiltration rate and reduce run off.

Smart pump control for irrigation is becoming more widespread, as farmers overcome their scepticism of computer-controlled systems and distributors become better at including monitoring and control in their service offerings to farmers. Weather data, soil moisture sensors, and rain sensors can all play together with pressure control for fully automated irrigation systems.

 

Consider the source of water

 

Successful agriculture is dependent upon farmers having sufficient access to water. Looking back to the middle of the last century, the common perception was that water was an infinite resource. Today, we are aware that water is a resource that needs to be managed. This is not only a question of more mouths to feed, people today consume more calories and eat more meat, and this requires more water to produce food. To meet future demands, world food production must double by 2045.

Sources of irrigation water can be groundwater extracted from springs or by using wells, surface water withdrawn from rivers, lakes or reservoirs or non-conventional sources like treated wastewater, desalinated water, drainage water, or reclaimed water generally.

The location of the irrigation water makes a difference to the pump that should be selected. Deep well submersible pumps and turbine pumps are specially designed to lift water from several hundred metres underground, and a variety of pumps can be used when drawing surface water.

If submersible pumps are used when drawing water from a reservoir or lake, advantages are improved theft protection, because the pumps are submerged, and reduced noise, because noise is limited to that from the pipes and the valves.

Two basic elements are crucial to ensure flow of water to the irrigation system: the availability of water, and the crop’s need for water.

If the source is groundwater, the recommendation is to use more than one well in order to minimise drawdown. Furthermore, employing several small pumps rather than one large pump offers many benefits, including easy cut in/cut out of pumps according to flow demand, reduced aquifer drawdown, thereby reducing energy consumption as lifting height is limited, and avoiding negative influences on the aquifer.

 

A basis for pump selection

 

Typically, pumps for irrigation are over-sized. Choosing a correctly sized pump is crucial to the success of the irrigation system, and things to consider include keeping power consumption low, maintaining system pressure, and adding variable speed control and motor protection.

Today, pumps have to be much more integrated with the rest of the irrigation system. This means the pump must be designed to match the rest of the irrigation equipment, or the irrigation equipment must be designed to match the pump.

Taking power consumption first, pumps and motors have different efficiencies, and the overall efficiency should always be calculated before the final selection is made. The electricity bill will depend on how many kW the motor absorbs. Simply compare the flow and head produced by the pump with the kW consumption of the motor. Most pump manufacturers are able to provide all relevant data, so a true calculation of the efficiency can be made.

Keeping system pressure as low as possible is an effective way of reducing leakages, conserving water and reducing energy consumption. However, a specific minimum pressure for proper functioning is usually necessary, and without this, the correct performance of the irrigation equipment cannot be guaranteed.

 

How to calculate efficiency ?

 

This is how the efficiency of the pump and motor can be calculated:

 

Efficiency % =   Q x H / 3.67 X P1

 

Q = flow in m3/h

H = head (pressure from pump in metres)

P1 = the kW required by the motor*.

 

*This must not be confused with the kW output stamped on the motor nameplate.

 

The pump must do more than simply deliver water to the pipes in order to be effective. For example, adding variable speed drives improves the efficiency of groundwater withdrawal when pumping directly into an irrigation system. Surface water intake and distribution can be improved by using multi-pump pressure boosting systems, and across the board, monitoring and control systems further safeguard the reliable flow of water by protecting the pump from dry-running, motor breakdown or power supply irregularities. Motor protection ultimately saves on maintenance and service checks.

 

An integrated design means cost savings

 

All these elements must be fully integrated into the design to provide the benefits that a modern irrigation pumping system can offer the farmer. Maintaining correct pressure and flow in the pipes and at the nozzle means more water per kWh and savings on energy, which is one of the highest cost drivers in farming.

If the pump matches the irrigation system, if pressure is not higher than necessary, and if controls are used, then operating costs generally are lowered. Water delivered with greater precision to the crop results in a better harvest, increased profitability and better water management, ensuring sustainable agriculture in the future.

 

 

 

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