Why yield testing matters after drilling
A borewell is drilled to produce water, not just to reach a depth. The driller’s airlift reading at the end of drilling shows that the well yields, but it is not a measure of what the well will deliver day after day under a pump. Selecting a pump from the airlift figure or from total depth alone is one of the most common reasons for wells that run dry in summer, pumps that burn out, and sand or silt problems.
A yield test links three things that are easy to confuse: what was measured, what was calculated from the measurements, and what rate was selected for operation. Keeping these separate is the heart of good practice.
How does the compressor fit into yield assessment?
On a DTH rig the compressor does more than drive the hammer. Air flushes cuttings and lifts water from the hole. That makes the compressor part of the first yield indication and of well development.
- Airlift during drilling: as water-bearing fractures are struck, water returns with the cuttings. Drillers often note the depth of each water strike and a rough discharge. These notes show where the water comes from, which later helps set the pump depth.
- Development by airlifting: after drilling, air is used to surge and lift water to clean fines from fractures and the area around the hole, until the discharge runs reasonably clear.
- Limits of an airlift figure: airlift discharge depends on air flow, pressure, submergence and the depth of the air line. It does not hold the water level at a controlled drawdown, so it cannot replace a pumping test.
Development and airlift work need a compressor with enough pressure to start lifting against the water column. Borewell compressors typically deliver 600–1,500 cfm at 200–400 psig (13.8–27.6 bar), and special units reach 35 bar (507.6 psig).
The three stages of a pumping test
| Stage | How it is run | What it tells you |
|---|---|---|
| Step-drawdown test | Pump at several increasing rates, each for a set period, recording water level against time | How drawdown grows with discharge; helps identify well losses and a sensible rate range |
| Constant-rate test | Pump at one steady rate for an extended period, recording water level and discharge | Whether the water level stabilises at that rate and how the aquifer responds over time |
| Recovery test | Stop pumping and record the water level as it rises back | How quickly the well recovers; an independent check on the constant-rate result |
The durations, rates and measuring intervals should be set by a hydrogeologist or the project engineer to suit the well, the planned use and any local authority requirement. The aim is to answer a decision: what rate can this well sustain for its purpose?
Plan the test before you start
- Record the well construction: depth, diameter, casing depth, screen or open-hole zones and water-strike depths from the drilling log.
- Fix a measuring datum, such as the top of casing, and use it for every water-level reading.
- Measure the static water level before pumping, after the well has rested.
- Note nearby pumping wells and other activity that could affect levels during the test.
- Choose a test pump able to cover the planned rates, with a means of measuring and controlling discharge.
- Arrange the discharge to run well away from the borewell so it does not recharge the test.
- Prepare a log sheet for time, water level and discharge during pumping and recovery.
Basic calculations, with their conditions
With all water levels measured as depths below the same datum:
- Drawdown s = pumping water level − static water level.
- Specific capacity = discharge Q ÷ drawdown s, quoted with the rate and duration of pumping.
- Pumped volume V = Q × t, valid only while the discharge is constant; for a varying discharge add up the measured interval volumes.
A worked example: a static level of 30 m and a stabilised pumping level of 50 m give a drawdown of 20 m. At 600 litres per minute, specific capacity is 30 litres per minute per metre of drawdown. Pumping steadily at that rate for 24 hours moves 864 m³. These figures illustrate the arithmetic only; they are not a forecast for any particular well or area.
Specific capacity is not a fixed property of the well. It usually falls as the rate rises and as pumping continues, which is why the rate and duration must travel with the number.
From test results to an operating rate
Choosing the operating rate is a judgement, not a single formula. A sound selection considers:
- The drawdown at the chosen rate, with a margin above the pump intake and above the main water-bearing fractures.
- Seasonal fall in water level: a test in the monsoon may overstate summer yield.
- Interference from neighbouring wells pumping at the same time.
- The pump’s required submergence and the maker’s installation rules.
- Water quality, sand content and the risk of drawing fines.
- The demand pattern: continuous pumping is different from a few hours a day with recovery between.
Many practitioners select a rate below the maximum tested rate to leave headroom for dry seasons and ageing of the well. The selected rate should be written down with the water-level limits that would trigger a review.
Connecting the well to the pump system
The production pump is sized on the selected flow and the total head: the pumping water level, the lift to the delivery point, the required pressure at the outlet and the friction losses in the riser and pipeline. Total borehole depth is not the head. Set the pump depth with reference to the water-strike depths and the expected seasonal pumping level, not simply near the bottom of the hole.
Monitoring the well in service
| What to record | Why it matters |
|---|---|
| Static water level before the day’s pumping | Shows seasonal and long-term trends |
| Pumping water level at steady running | Early warning of falling yield or a clogging well |
| Discharge or meter reading | Confirms the pump is delivering the selected rate |
| Pump running hours | Links production to energy use and maintenance |
| Sand or turbidity observations | Signals over-pumping or a need for redevelopment |
If the pumping level falls towards the pump intake, reduce the rate or the running hours and review. When the static level recovers in later seasons, the earlier rate may become feasible again, but confirm it with readings rather than assuming it.
Redevelopment and the compressor’s second role
Over years, fines, scale or biological growth can reduce a well’s yield. Redevelopment by airlifting and surging with a high-pressure compressor can often restore part of the lost capacity. The same planning applies: record the levels and discharge before and after, so the effect can be judged on evidence.
How Reddy Compressors helps
We supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts. For yield-related work this means:
- High-pressure compressors for drilling, development and airlifting, for sale or hire from our own fleet.
- Service and repairs so the rig’s compressor holds its pressure and flow through development work.
- Spare parts identified from the nameplate and confirmed for fit before quoting.
- Advice on the air side of airlift and development work. Pumping-test design, interpretation and pump selection remain with your hydrogeologist, pump supplier or project engineer.
We are an independent multi-brand specialist (ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others), not an authorised dealer. We have tie-ups with all major and reputed brands. Our service teams cover Telangana and Andhra Pradesh and we supply across India from Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017. Phone +91 99590 60718 or +91 92468 88812.
What to send us
- Whether you need air for drilling, development, airlifting or redevelopment.
- Hole diameter, depth, casing depth and the depths of water strikes.
- Static water level, if known.
- Your rig and hammer, and the compressor you use now.
- Site location and dates.
Frequently asked questions
How is the yield of a borewell measured?
An early indication comes from the airlift discharge at the end of drilling. A reliable measure needs a pumping test: a step-drawdown test at increasing rates, a constant-rate test at one steady rate, and a recovery test after pumping stops. Water levels and discharge are recorded against time from a fixed datum, then interpreted to select a sustainable operating rate.
Is the airlift yield the same as the pumping yield?
No. Airlift discharge depends on the compressor’s flow and pressure, the depth of the air line and submergence, and it does not hold the water level at a controlled drawdown. It shows that the well produces water and where it comes from. The pump should be chosen from a proper pumping test, not from the airlift figure.
What is specific capacity of a borewell?
Specific capacity is the discharge divided by the drawdown, for example litres per minute per metre. It must always be quoted with the pumping rate and duration, because it usually falls as the rate increases and as pumping continues. It helps compare wells and track a well’s condition over time, but it is not a fixed property.
Why does my borewell yield drop in summer?
The static water level falls in the dry season, so the same pumping rate draws the level closer to the pump intake. Neighbouring wells may also pump harder. Fines or scale can clog fractures over time. Record static and pumping levels through the year and reduce the rate or hours when the pumping level approaches the pump intake.
Can a compressor be used to clean or redevelop a borewell?
Yes. Airlifting and surging with a high-pressure compressor is a common way to remove fines and restore part of a well’s lost capacity. The compressor needs enough pressure to lift against the water column at the air-line depth. We sell and hire suitable borewell compressors; record water levels and discharge before and after to judge the result.
How deep should the submersible pump be set?
Set it with reference to the expected lowest pumping water level, the main water-strike depths and the maker’s submergence requirement, not just near the bottom of the hole. Your pumping-test results and seasonal readings guide this. Pump selection and setting depth should be confirmed by your pump supplier or engineer.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Pumping test: Drawdown, specific capacity and pumped volume
Work out drawdown, specific capacity and pumped volume from a pumping test, with every level measured from the same datum.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Static water level below datum (m): 30
- Pumping water level below the same datum, stabilised (m): 50
- Discharge: 600
- Discharge unit: L/min
- Pumping duration at constant discharge (h): 24
Result
- Drawdown: 20 m
- Discharge: 600 L/min = 10 L/s = 36 m³/h
- Specific capacity: 30 L/min per m of drawdown, at 600 L/min after 24 h
- 1.8 m³/h per m
- 0.5 L/s per m
- Pumped volume at constant discharge: 864 m³ = 8,64,000 litres
Method. Drawdown s = pumping water level − static water level (both as depths below the same datum, e.g. top of casing). Specific capacity = discharge Q ÷ s, quoted with the rate and the pumping duration, because it usually falls as the rate rises and as pumping continues. Pumped volume V = Q × t, valid only while the discharge is constant; for a varying discharge add the measured interval volumes. 1 L/s = 3.6 m³/h = 60 L/min.
What to take from it. Choose the operating rate from the constant-rate and recovery tests with headroom for the dry season and nearby pumping, not from the airlift figure or the maximum tested rate.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Borewell air-lift start pressure and submergence
Estimate the pressure a compressor needs to start an air-lift (development, testing or redevelopment), the submergence ratio, and the effect of hot intake air on the mass of air delivered.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Submerged length of the air line below standing water (m): 60
- Lift from the water level up to the discharge point (m): 40
- Hose and fitting losses (bar): 0
- Compressor rated pressure, to check margin (bar g; 0 to skip): 0
- Compressor intake temperature (°C): 45
Result
- Air-lift start pressure: 5.89 bar g = 85 psig = 6 kg/cm² (60 m × 0.0981)
- Submergence ratio: 60% of the air-line length below the discharge point
- Air mass at this intake temperature: -7.9% compared with 20 °C intake air
Method. Start pressure ≈ submerged length of the air line × 0.0981 bar per metre of fresh water (10 m ≈ 0.98 bar ≈ 14.2 psi) + hose and fitting losses. The submerged length is measured from the water level at the time of starting to the air-line foot; once lifting begins the working pressure usually falls. Submergence ratio = submerged length ÷ (submerged length + lift from the water level to the discharge point); about 60% is commonly quoted as a target, and lower submergence lifts water poorly. Free air delivery is a volume at inlet conditions, so hot intake air carries less mass: mass ratio = (20 + 273.15) ÷ (intake °C + 273.15).
What to take from it. Choose a compressor with clear margin above the start pressure, and record the datum and submerged length with the result.