Borewell Size for Agriculture: Casing by Flow Rate

The right borewell size for agriculture follows the flow rate your crop needs, not the size of the farm. A Washington State well-construction table pairs a 6-inch casing with yields under 100 gallons per minute (gpm), an 8-inch casing with 75 to 175 gpm, and a 12-inch casing with 300 to 700 gpm. Work out your peak flow first, then read the casing size off that table.

In the United States and Canada a “borewell” is usually called a drilled well, a borehole or an irrigation well. In the UK, Australia and South Africa “borehole” is the usual word. The sizing logic is the same everywhere: the casing must be wide enough to hold a pump that can deliver the flow, with room for water to pass the motor and for tools to go down the hole later.

This guide gives you the casing table, the formula extension engineers use to find the flow, a calculator you can fill in with your own numbers, and a checklist for drilling, testing and upkeep.

How borewell size for agriculture is chosen

Well size is set by three numbers. The first is the flow you need at the peak of the season. The second is the diameter of the pump that can deliver that flow. The third is the casing diameter that holds that pump with enough clearance. Depth is a separate question: it depends on where the water-bearing layer sits, not on how much water you need.

Engineers start with the pump because the pump decides the hole. Pump makers sell submersible and turbine pumps in nominal bowl sizes of 4, 5, 6, 8, 10 inches and up. Each bowl size has a band of flow where it runs well. The casing is then chosen one or two sizes larger than the bowl.

Step 1: find your peak flow

The University of Nebraska-Lincoln NebGuide G1851 gives a simple formula for gross system capacity in gpm per acre: peak water use in inches per day, times 453, divided by pumping hours per day times application efficiency. The number 453 converts acre-inches per hour into gallons per minute.

The NebGuide’s own example uses a peak crop water use of 0.32 inches per day, 22 pumping hours a day and 85% efficiency. That gives 7.75 gpm per acre, so a 130-acre center pivot needs about 1,008 gpm. The same guide says peak use across Nebraska runs from about 0.32 inches per day in the east to 0.36 in the west.

Step 2: match the flow to a pump bowl

Once you know the flow, choose a pump whose curve gives that flow at your total head. Total head is the lift from the pumping water level to the surface, plus pipe friction, plus the pressure your sprinklers or drip system need. A pump that is too small cannot reach the flow. A pump far too large wastes energy and can pull the water level down too fast.

Step 3: read the casing size

The casing must be larger than the pump so the pump can be lowered and pulled without jamming. For submersible pumps, water also needs to flow past the motor to cool it. Pump makers publish cooling tables by motor size and casing diameter, so check the maker’s sheet for your model.

Try the numbers for your own farm below.

Borewell size chart: casing diameter by yield

Washington’s well-construction rule, WAC 173-160-211, publishes recommended well diameters for each yield band. It is a clear public table of this kind, and its bands are set by nominal pump bowl size. The table below shows the full set.

ID means inside diameter and OD means outside diameter. The metric column is our conversion (1 gpm is about 0.227 m³/h). The bands overlap on purpose. If your flow sits in an overlap, the larger casing gives more room for a bigger pump later, while the smaller one costs less to drill and case.

Two worked examples show how this plays out. The Nebraska pivot above, at about 1,008 gpm, falls in the 12-inch bowl row, so it would sit in a 16-inch casing. A small 10-acre vegetable block that needs 117 gpm sits in the 5-inch bowl row, so an 8-inch casing would do.

Chart

Recommended yield range by casing size

gpm, low and high end of each band; selected rows (the 30-inch row, 3,000-6,000 gpm, not shown)

How much water each acre needs

Flow per acre depends on the crop’s peak water use, the soil, the climate, how many hours you can pump and how much water your system loses. A table from Kansas State University’s irrigation formulas sheet (MF2404) shows the effect of efficiency. To put 0.30 inches a day into the root zone you need 5.66 gpm per acre at 100% efficiency, 7.07 at 80% and 11.31 at 50%.

Soil matters too. Sandy soils hold less water, so the system has to keep up with the crop day by day. Heavy soils store more water and can carry the crop through short peaks. The Nebraska NebGuide lists net capacities that keep corn out of stress in nine years out of ten, by soil type. In the drier of its two regions, the figure runs from 4.62 gpm per acre on loam over silt loam to 5.89 on fine sands. Replacing the full average peak use there needs 6.60.

The Texas A&M center-pivot question sheet puts the same idea in field terms. A pump giving 6 gpm per acre, run all day, supplies about 0.32 inches a day or 2.25 inches a week. At 4 gpm per acre that drops to about 0.21 inches a day.

A short Texas A&M example for pasture in South Texas uses a peak demand of 0.25 inches a day, which is 6,789 gallons per acre per day. Pumping 24 hours at 100% efficiency, 10 acres need 47 gpm and 100 acres need 470 gpm. Pumping only 12 hours at 80% efficiency, the same 10 acres need 117 gpm and 100 acres need 1,175 gpm. Short pumping windows push the well size up fast.

Chart

Net flow per acre for corn by soil type

gpm per acre, net capacity to avoid stress 9 years in 10, NebGuide Region 2 (higher-demand region)

Borewell size by farm type

The table below turns the formula into common cases. The flows are worked out from the sources above. They are starting points for a talk with your driller, not a design.

Chart

Flow needed per acre as daily water need rises

gpm per acre for net irrigation in inches per day, at two system efficiencies

For the market-garden row we applied the K-State figures to 2 acres at 0.25 to 0.30 inches a day and 80 to 90% efficiency, pumping 12 to 24 hours. Drip blocks can be split into zones, so the well only has to serve one zone at a time.

Small farms and smallholdings

On small plots the cheapest well that meets the peak is usually a 6-inch casing with a 4-inch submersible pump. Water storage helps a lot. A tank or pond filled slowly through the day lets a small well feed a larger system for a few hours. This is also a way to use a well that tested at a low yield.

Medium and large irrigated farms

For pivots and large sprinkler systems the casing jumps to 10, 12 or 16 inches. At this scale a spare well or a second pump is worth pricing, because a pump failure at peak season can stress the whole field. Variable-speed drives let one pump serve zones of different sizes and can cut energy use when the full flow is not needed.

Livestock, washing and packing water

Stock water, dairy washdown and packing sheds add steady demand all year. Add these flows to the crop peak if they run at the same time. Water for people, animals or food washing also has to meet drinking or food-safety standards, which is a separate test from irrigation quality.

Depth, aquifer yield and drawdown

A well can only give what the aquifer gives. The size table tells you what casing a pump needs. It does not promise the ground will deliver that flow. Only a pumping test can tell you that.

Wells are deep in many farm regions. The 2023 USDA Irrigation and Water Management Survey put the average US irrigation well depth at 241 feet. Groundwater from on-farm wells supplied 54% of water applied to irrigated land in the open. US farms applied an average of 1.5 acre-feet per irrigated acre in that year.

Where pumping exceeds recharge, the water table falls. The Texas Water Development Board says that in much of the Ogallala Aquifer withdrawals exceed recharge, with declines of more than 300 feet in some areas over 50 to 60 years. A falling water level raises pumping lift, so a well that met its flow when new may fall short later.

What a pumping test tells you

• Static water level before pumping starts.
• Pumping water level at the planned flow.
• Drawdown, which is the gap between the two.
• Recovery, which is how fast the level returns after pumping stops.
• Specific capacity, which is gpm per foot of drawdown.

Set the pump intake well below the lowest expected pumping level, and size the pump for the lift at the end of a dry season, not the start.

Casing, screen and gravel pack

Steel casing is strong and used for deep or large wells. PVC casing does not rust and costs less, but has depth and pressure limits that your driller will know. In loose sand and gravel aquifers, a screen with slots matched to the sand and a gravel pack around it keeps fine grains out of the pump. A grout seal at the top keeps surface water from running down the outside of the casing.

Poor screen choice is a common cause of sand in the water. Sand wears pump parts and clogs drip emitters and filters. If your well is in fine sand, ask the driller for a sieve analysis of the formation before the screen is ordered.

Pump selection and energy use

Energy is the main running cost. The USDA survey reported that US farms spent $3.3 billion in 2023 on energy to pump well and surface water. Pump efficiency, pipe size and pressure all feed into that bill.

• Choose the pump from its curve at your real total head, not from horsepower alone.
• Keep pipe friction low. The Texas A&M pivot sheet’s friction table for center-pivot mainlines shows 1,000 gpm losing 28 psi in a 6-inch mainline but 7 psi in an 8-inch one.
• Consider a variable-speed drive where flow changes between zones.
• Solar pumping can suit small, remote wells with storage tanks.
• Have a qualified electrician install and check pump wiring and controls.

Water quality checks

Irrigation water should be tested before you design the system, because quality affects filters, emitters and crops. The FAO guide on water quality for agriculture uses electrical conductivity (ECw, in dS/m) to rate salinity, and notes that water in the moderate to high range (above 1.5 dS/m) needs careful salinity control.

Near coasts, heavy pumping can draw seawater into fresh aquifers. The USGS explains saltwater intrusion as what happens when too much fresh water is pumped and the salt-fresh boundary moves inland. A well close to the coast is more exposed.

If the well also supplies drinking water, the US EPA’s page on well water contaminants lists nitrate among the risks, and says high levels are most serious for infants. Test drinking wells on the schedule your health department advises.

• Salinity (EC) and sodium for crop and soil effects.
• Iron and manganese, which can clog drip lines.
• Hardness and pH, which cause scale.
• Sand and silt, which wear pumps.
• Nitrate and bacteria if people or animals drink the water.

Permits and local rules

Most places regulate new wells and how much water they take. US states set their own well-construction and water-right rules, and many also require a licensed driller. In England and Wales, larger groundwater abstractions need a licence from the Environment Agency or Natural Resources Wales. Australian states run their own bore licences. Check with your local water authority before you drill, and follow the spacing rules from septic systems, other wells and property lines. This guide is general information, so get advice from a licensed driller or water engineer for your site.

How satellite data helps with irrigation wells

A well is sized once, but the field it serves changes every season. Satellite images show where a crop is greener or weaker than the rest of the field. A dry patch that follows a sprinkler line or the end of a drip block can point to low flow or pressure, not low rainfall. The Farmonaut Satellite + AI Farm Monitoring System shows vegetation and moisture indices for each field, which helps you check whether the whole field gets water after a new well or pump goes in.

Satellites do not see the aquifer or the pump. They show crop response. Use them alongside a flow meter, a water-level reading and a look at the field.

▶ How satellite crop health maps show stress in a field before it is visible on the ground. · 4:09

Common mistakes when sizing a farm well

• Sizing for average demand. Size for the peak week, then check the pumping hours you can really run.
• Forgetting efficiency. At 50% efficiency you need about twice the flow of a 100% system, per the K-State table.
• Picking casing before the pump. Choose the pump, then the casing.
• No pumping test. The size table does not tell you what the aquifer can give.
• Ignoring future demand. If you may add acres, a casing one size up can save a new well later.
• Skipping water tests. Iron, sand or salt can ruin a drip system in one season.

▶ A walk-through of reading satellite field data in the Farmonaut web app. · 10:16

Installation and upkeep checklist

1. Work out the peak flow with the NebGuide formula or the calculator on this page.
2. Check local permits, spacing rules and water rights.
3. Hire a licensed driller and ask for the drilling log.
4. Run a pumping test and record drawdown and recovery.
5. Test water quality before buying filters or emitters.
6. Choose the pump from its curve, then confirm casing clearance and motor cooling.
7. Fit a flow meter and a way to read the water level.
8. Each season, compare flow, pressure and water level with the first readings.

A drop in flow at the same pressure can mean pump wear, a clogged screen or a falling water level. Catching it early costs far less than a failed pump at peak season.

Related pages on our site: farm management dashboard, field monitoring app, farm carbon footprint tracking, crop loan and insurance verification, satellite and weather API documentation, farm fleet management, product traceability.

FAQ

What is the best borewell size for agriculture on a small farm?

For flows under 100 gpm, the Washington table pairs a 4-inch pump bowl with a 6-inch casing. That covers many small drip and market-garden plots. Work out your peak flow first, because pumping hours and efficiency can push it higher.

How many gpm per acre does irrigation need?

It depends on crop, soil, climate, hours and efficiency. The Nebraska NebGuide example for corn under a pivot gives 7.75 gpm per acre at 22 hours a day and 85% efficiency. Its soil table gives 3.85 to 5.89 gpm per acre of net capacity, depending on soil and region.

What casing size does a center pivot need?

The NebGuide’s 130-acre pivot example needs about 1,008 gpm, which is above the 10-inch bowl band (500 to 1,000 gpm) and inside the 12-inch bowl row (800 to 1,800 gpm) of the Washington table, so it calls for a 16-inch casing. The final size depends on your pump curve.

Does a wider well give more water?

Only a little. Yield is set mainly by the aquifer. A wider casing lets you fit a bigger pump, but it will not create water the ground cannot supply. A pumping test shows the real yield.

How deep should an irrigation well be?

Deep enough to reach a reliable water-bearing layer and keep the pump below the lowest pumping level. The average US irrigation well was 241 feet deep in 2023 (USDA), but local depth varies widely, so use nearby well logs and your driller’s advice.