Choosing a pump for a borehole, well, river or tank.
The water source decides the kind of pump: a submersible pump down a borehole, a surface pump beside a well or a river, or a booster beside a tank. This guide shows which suits your source, how far the water can fall before a surface pump stops working, and how to read a pump's chart so the one you buy gives the flow you need at your total head, all through the dry season.
Written for farmers, greenhouse and nursery growers, homeowners and estate and facility managers.
Flow, pressure and head
Flow is how much water passes in a set time: litres per hour (L/h) for a dripper, or cubic metres per hour (m³/h) for a pump or a pipe. One cubic metre is 1,000 litres.
Pressure is how hard the water pushes inside the pipe, measured in bar. Head is pressure given as a height of water, in metres: one bar is about 10 metres of head.
Total head is the height the pump must lift the water, plus the pressure the system needs at the far end, plus what is lost to friction in the pipes, all in metres. A pump is chosen for one flow at one total head, so both must be known before you compare pumps.
Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design; FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant.
- Borehole: a submersible pump sits down in the water and pushes it up
- Hand-dug well: a surface pump beside it sucks the water up a pipe
- River in the rains: the water stands high, so the pump sits above the floods
- River in the dry season: the level falls, and the suction pipe must still reach it
- Tank: filled from a borehole, the town supply or a tanker
- Booster pump: takes water from the tank and raises its pressure
Three kinds of pump
Each kind of pump suits a different source. A surface pump has to suck its water up, which limits how far below it the water can be; a submersible pushes the water up from below, so depth matters far less to it.
| Pump | Suits | Watch for |
|---|---|---|
| Submersible, down in the water | Deep boreholes, and wells too deep to suck from | It is cooled by the water it pumps, so it needs a cut-off for when the water falls too low |
| Surface pump, on firm ground beside the source | Water a short way below the pump, or water that flows to it | It can suck water up only a few metres, and a long suction pipe cuts that further |
| Booster, on the line after a tank or a pump | More pressure for drip lines or sprinklers, from a tank or a weak supply | It adds pressure, not water: it cannot give more than its supply |
Sources: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant; FAO (1986), Irrigation and Drainage Paper 43: Water Lifting Devices, chapter 2.
Water levels change with the seasons
A water level is never fixed. In a borehole or well it drops as soon as pumping starts, and the faster you pump, the further it drops; this drop is called drawdown. Levels in rivers, dams and reservoirs rise in the rains and fall through the dry season, as water is drawn off and evaporates.
So find the lowest level of the year, at the end of the dry season, and the drawdown at the flow you need. A borehole's pumping test record gives the drawdown; the driller or the last owner may hold it.
A pump much bigger than the source can feed draws the water down to its intake and starts to suck air. FAO warns this can soon damage the pump, and in a borehole it can pull silt through the screen and harm the bore itself. Set the intake safely below the lowest likely level, but above the borehole's screen.
On a river, keep the pump above the highest flood level, or use one that can be moved, and do not draw from a bend where sand and silt settle.
How to check what your source givesPlanning for the dry season
Sources: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant; FAO (1986), Irrigation and Drainage Paper 43: Water Lifting Devices, chapter 2.
How far a surface pump can lift
A surface pump lowers the pressure in its suction pipe, and the air pressing on the water pushes it up. Even a perfect vacuum could not raise water more than about 10.4 m at sea level, and in practice FAO puts the limit for a centrifugal pump, the common kind, at about 4.5 m at sea level with a short suction pipe.
The limit falls by about 1 m for each 1,000 m above sea level, by about 7 percent when the water is at 30 °C rather than 20 °C, and typically to about half with a suction pipe 80 m long, though a wider pipe loses less. Below the limit the pump loses its prime, the water in its suction pipe, and stops delivering.
If the water will fall too far, set the pump nearer the water, or use a submersible pump. The pump's chart also gives the suction it needs, called its NPSH required; the site must give more than that.
Sources: FAO (1986), Irrigation and Drainage Paper 43: Water Lifting Devices, chapter 2; FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant.
Reading a pump's chart
A pump's chart, or curve, shows the flow it gives at each total head. The same pump gives more water against a low head and less against a high one, down to none at all at its shut-off head. Makers usually also show the pump's efficiency, the power it takes and the suction it needs, each against flow.
- Work out the flow you need and the total head at that flow, using the lowest water level of the year.
- Find the flow along the bottom of the chart and the total head up the side. Where they meet is your duty point: the flow and head the pump must give.
- Look for a pump whose curve passes through or just above the duty point. A curve far above it means a bigger pump than you need, which may draw the source down too far.
- Check the efficiency at the duty point. FAO's manual says to choose the pump that gives your flow and head at the highest efficiency, and to reject one whose duty point falls outside its curve or at either end of it.
- Read the shut-off head, where the curve meets the head scale at no flow. A closed valve can hold the pump there, so the pipes and fittings must be rated above it.
- For a surface pump, read the suction it needs at your flow, and check the site gives more.
Pipe pressure classes and the shut-off head
Source: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant.
Power: grid, generator or solar
The power in the water is the flow times the total head: in kilowatts, the flow in litres a second times the head in metres, divided by 102. A pump and its motor or engine lose part of the power they take in. FAO puts the overall efficiency in the field at about 0.35 for an engine-driven pump and 0.50 for an electric one, and says higher figures are not realistic.
Grid power suits a site with a supply at the hours you must pump. An engine pump loses power with height and heat: FAO's manual takes off about 1 percent for each 100 m above sea level and for each 5.6 °C of air temperature above the maker's rating, then keeps 5 to 10 percent in reserve. Electric motors may need derating above 1,100 m or 37 °C; the maker says how much. Ask a generator's maker for its own derating.
A solar pump gives its water in daylight, so it needs a tank for nights and dull days.
Sizing a solar pump, worked through
Sources: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design.
What to measure
- The kind of source: borehole, hand-dug well, river, dam, tank or town supply
- The depth to the water at rest, at the end of the dry season and in the rains
- The drawdown at the flow you want, from a pumping test
- For a surface pump: the height from the lowest water level up to where the pump would sit, and the length of the suction pipe
- The height up to the highest point to be watered, or to the tank inlet
- The flow you need: that of the largest zone or block watered at one time
- The power on site: the hours of grid supply, the size of any generator, or open space for panels
- Sand, silt or iron in the water
What to compare on the spec sheet
- The curve: flow against total head, with your duty point well inside it
- The efficiency at your duty point, not only the highest on the chart
- The shut-off head, against the pressure rating of your pipes
- For a surface pump, the suction it needs at your flow
- For a submersible, the smallest borehole it fits, how deep it may be set, and its protection against running dry
- The power: the motor or engine size, the voltage and phase it needs from the grid or a generator, or the panel power for solar
- How much sand it can handle, if your water carries any
- The warranty: ask what it covers, for how long, and where spare parts and service come from
Source: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant.
When to ask for site-specific sizing
- The water level, the drawdown or what the source can give is not known
- A surface pump would be near its suction limit at any time of year
- The source is shared, or its level swings far between the seasons
- One pump must feed several zones at different flows, or lift to more than one height
- You want to combine power, such as solar with a generator
- The water carries sand or iron
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