Borehole Yield Testing Explained

Once the drill rig has packed up and left your property, you have a hole in the ground. What you do not yet have is a reliable water supply. That distinction matters enormously, and it is exactly what borehole yield testing is designed to resolve before you spend another rand on a pump or pipework.

What yield testing actually measures

A yield test determines two related things: how much water the borehole produces per hour, and how the water level inside the casing responds to sustained pumping. Drillers typically express yield in litres per hour (L/h) or kilolitres per day (kL/day). The key figure is not the peak rate at which water enters the borehole, but the sustainable rate — the rate at which you can pump continuously without drawing the water level down to the pump intake.

There are two common test types:

• Step tests — the borehole is pumped at progressively higher rates in short steps, each step revealing how the water level responds to increased demand. This helps identify the point at which the aquifer struggles to keep up.
• Constant-rate tests — the borehole is pumped at a fixed rate for an extended period (often six to twelve hours, sometimes longer for commercial applications) while water levels are logged at regular intervals. The data shows whether the aquifer can sustain that rate or is declining towards depletion.

A full test also includes a recovery phase, where pumping stops and the rate at which the water level returns to its resting position is recorded. A borehole that recovers quickly is drawing on a well-connected aquifer; slow recovery suggests a more isolated fracture with limited recharge.

Why yield matters before you choose a pump

Pump selection is not guesswork. Fit an oversized pump to a low-yield borehole and it will either run dry repeatedly — burning out the motor — or your installer will have to throttle it back anyway, in which case you have overpaid. Fit an undersized pump and you leave usable yield on the table.

The sustainable yield figure from your test sets the ceiling for your pump’s operating rate. The static water level (the resting depth before pumping) and the drawdown depth (how far the level drops under load) together determine the required pump head. Without these numbers, any pump recommendation is an educated guess.

You can find a broader breakdown of how these specifications affect overall system costs in the guide to borehole costs in Johannesburg. The short version: a mismatch between pump and yield is one of the most common reasons borehole owners end up with a second round of costs they did not expect.

What counts as a usable yield in Johannesburg?

This depends entirely on your intended use. As a rough guide, a household of four people using water only for non-potable purposes — garden irrigation, toilet flushing, laundry — can work with a modest yield. If you want to run the borehole as a primary or full backup supply, the demand picture changes considerably.

It is worth knowing that South Africans use on average about 237 litres of water per person per day, above the world average of roughly 173 litres. A family of four is therefore drawing something in the region of 950 litres per day for household use alone, before you add garden irrigation. Your borehole needs to be able to sustain that rate, plus a reasonable margin, over the course of a day’s pumping cycles.

Johannesburg’s geology is varied. Parts of the northern suburbs sit over fractured quartzites and shales that can yield well; other areas overlie denser formations where yields are modest. There is no substitute for testing the specific hole on your specific property.

Knowing your yield requirements before you speak to installers puts you in a far stronger position. Compare 3 free quotes — vetted installers, no obligation.

The low-yield policy — and why you need it in writing

This is the part most homeowners skip, and it is the part most likely to cost them money.

A low-yield policy is a written term in your drilling contract that specifies what happens if the borehole yields below an agreed threshold. It should clearly state:

• The minimum acceptable yield in L/h or kL/day
• Whether the driller will attempt to deepen the borehole, hydrofracture it, or drill a replacement hole at reduced or no additional cost
• What happens to your deposit or progress payments if none of those interventions achieve the agreed minimum

Without this clause, you are in a difficult position. Drilling is a service, not a guarantee of water. Once the hole is drilled and payment is made, your leverage largely evaporates. A reputable contractor will be willing to commit to a low-yield policy because it reflects confidence in their hydrogeological assessment; reluctance to include one is itself a warning signal.

Before signing anything, ask your driller directly: What is your policy if the yield is below X litres per hour? Get the answer in writing, not verbally.

How yield testing fits into the overall borehole process

For Johannesburg homeowners thinking about the full picture, the sequence runs roughly as follows: site assessment and hydrogeological survey, drilling, casing and development, yield testing, water quality testing (against SANS 241, the 2024 South African drinking-water quality standard), then pump selection, pump installation, and surface plumbing.

Yield testing sits in the middle of that chain deliberately. It informs everything that comes after it. Skipping or skimping on it — accepting a driller’s rough estimate rather than measured data — pushes risk downstream to the pump selection and installation stages, where it becomes expensive to correct.

The full process, including what to expect from a reputable driller and what the borehole drilling process involves from site visit to completion, is worth reviewing before you commit to any contractor.

What yield test results look like in practice

A properly documented yield test should provide you with a written report including:

Parameter	What it tells you
Static water level (metres below surface)	Resting depth before any pumping begins
Sustainable yield (L/h or kL/day)	Maximum rate you can pump without depletion
Drawdown at sustainable yield (metres)	How far the level drops at that pumping rate
Recovery rate	How quickly the borehole refills after pumping stops
Recommended pump depth	Safe submersion depth for pump placement

If your driller cannot or will not provide a report in this format, ask why. A verbal summary is not sufficient documentation for an installation that may cost you R60,000 or more.

A yield report in hand makes comparing installer proposals straightforward and honest. Get three installer quotes based on your actual test data, not estimates.

Questions to ask before you sign

Before you engage a driller, having a short list of direct questions will save you considerable trouble later:

• Will you conduct both a step test and a constant-rate test, and provide a written report?
• What is the minimum yield below which you consider the borehole not worth equipping?
• What does your low-yield policy cover, and can I see it in the contract?
• Who conducts the test — your team or an independent hydrologist?
• How long is the constant-rate test, and how frequently are water levels logged?

A driller who answers these questions clearly and without hesitation is worth taking seriously. One who deflects, generalises, or tells you testing is unnecessary is demonstrating something important about how the rest of the job is likely to go.