Borewell production variance: finding why output changed

What production variance means for a borewell

Every borewell supply, whether it feeds a factory, a gated community, a farm or a village scheme, is run against some expectation: so many cubic metres a day, so many hours of pumping, a tank that should be full by morning. When the tank is not full, the first reaction is often that the well is failing. Sometimes it is. More often the difference comes from fewer pumping hours, a power cut, a meter read at a different time, a leaking delivery line or simply lower demand.

Variance analysis is the discipline of putting a number on the difference and then explaining it in parts. It does not replace a proper yield test or a hydrogeological review. It tells you which of those, if any, is actually needed. A well that is producing its normal rate for fewer hours needs an electrical or control investigation, not a new borewell.

For drilling contractors and rig owners the same thinking applies to the drilling side: compare planned metres drilled per shift with actual metres, and split the difference into penetration rate and productive hours before deciding that the compressor, hammer or bit is at fault.

How do you set a fair comparison basis?

A variance is only meaningful when the forecast and the actual result describe the same thing. Before calculating anything, write down:

• The forecast version that was actually issued, with its date and the assumptions it used (rate, hours per day, days in the period).
• The volume boundary: wellhead discharge, tank inflow or usable water after losses. These can differ significantly on a long delivery line.
• The period and its exact start and finish times. A meter photographed at 8 a.m. one day and 11 a.m. the next does not represent 24 hours.
• Any restriction or revised duty introduced during the period, and the date it took effect.
• Units, and any meter rollover, replacement or reset. An unexplained totaliser jump can create a variance far larger than any physical change.

If the forecast assumed one rate for the whole month but a lower operating rate was approved halfway through, compare each half against its own basis. Otherwise the analysis reports an approved decision as a performance loss.

How to calculate absolute and percentage variance

With actual volume V_a and forecast volume V_f, the absolute variance is V_a − V_f. The percentage variance is 100 × (V_a − V_f) / V_f, provided the forecast is not zero. State the sign convention every time. In this convention a negative value means the well delivered less than forecast. It does not, on its own, mean the result was bad: if demand fell, lower production may be exactly right.

As an illustration only, a forecast of 2,000 m³ against a measured 1,760 m³ gives −240 m³, or −12%. The arithmetic tells you the size of the gap. It does not tell you the cause. Calling it a “12% loss of well capacity” before checking rate, hours, demand, downtime, meter quality and the delivery boundary is a common and expensive mistake.

Splitting the gap into rate and runtime effects

For a pumped well, volume is the product of average rate and pumping hours. Forecast volume is Q_f × H_f and actual volume is Q_a × H_a. One exact way to divide the total variance is:

• Rate effect = (Q_a − Q_f) × H_f
• Hours effect = Q_a × (H_a − H_f)

The two effects add up to the total variance. Other conventions allocate the interaction term differently, so state which one you use and keep it the same from month to month.

Illustrative example: the forecast is 10 m³/h for 200 hours (2,000 m³). The well actually averaged 9 m³/h for 190 hours (1,710 m³). The rate effect is (9 − 10) × 200 = −200 m³. The hours effect is 9 × (190 − 200) = −90 m³. Together they explain the full −290 m³.

This is an accounting explanation. Finding out why the rate dropped and why hours were lost is the engineering task that follows.

Classifying lost pumping hours by cause

“Not running” is not a cause. Use an event log that separates the reasons a pump was stopped:

• Pump or motor fault, including dry-run or overload trips.
• Grid power unavailable or voltage outside the starter’s limits.
• An authorised groundwater restriction or reduced duty.
• Tank full, with the system correctly on standby.
• Planned maintenance.
• Missing data, where the state is simply unknown.

A healthy system responding to low demand should never appear in a reliability report as a failure. Equally, avoid counting the same hours twice: if a power cut happens while the pump is already stopped for maintenance, adding both durations exaggerates the lost opportunity. Decide how overlapping events are handled, keep the original event times and show the chosen allocation openly in the report rather than burying it in a spreadsheet formula.

Where the site relies on a standby generator, record generator run hours alongside pump hours. A gap between the two often explains an hours effect quickly.

Check the measured volume before diagnosing the well

Many negative variances turn out to be measurement problems. Before anyone touches the pump or the borehole, check:

1. Meter type, size, condition and installation (straight pipe runs, air in the line, partial filling).
2. Scaling and units on any flow transmitter or logger.
3. Totaliser continuity: rollovers, replacements, resets and clock corrections.
4. Whether the metering point has moved, for example from wellhead to tank inlet.
5. Volume against runtime: does the implied average rate make physical sense for the installed pump?
6. Agreement with any independent measurement, such as a timed tank-level rise, after reconciling the boundaries.

Keep estimated intervals visible. If part of the actual volume was estimated, say how and for what share of the period. An estimate made using the forecast rate cannot prove that the actual rate matched the forecast.

Investigating a negative variance in the right order

Work from the cheapest and most likely explanations to the most expensive:

1. Confirm the comparison basis and the meter record.
2. Separate demand-driven standby from genuine lost hours.
3. Review power availability, trips and control settings.
4. Check the delivery system for leaks, partly closed valves and changed outlets.
5. Compare pumping water level and discharge with earlier records at a similar rate and season.
6. Only then consider pump wear, intake blockage, borehole fouling or a change in the aquifer.

A falling pumping water level at a constant rate may indicate seasonal decline, interference from nearby wells or well deterioration. A falling rate with a stable water level points more towards the pump, motor or delivery head. Telangana and Andhra Pradesh both see strong seasonal swings in water level between the monsoon and the pre-monsoon summer, so compare like with like.

When the evidence points to a clogged or silted borehole, air development or redevelopment using a compressor may be considered by the drilling contractor. That is a decision for an experienced driller, with the right compressor pressure and flow for the depth and casing, not a routine fix.

Treating positive variance with the same care

Above-forecast production is not automatically good news or proof that the well has improved. It may reflect a longer pumping period, a meter error, an unauthorised rate increase or a deliberately conservative forecast. Investigate it in the same way. Sustained pumping above an approved rate can draw down the water level faster and bring the pump closer to its intake, which raises the risk of dry running and sand ingress.

Turning variance into a production-control decision

The output of a monthly review should be a short decision, not a large spreadsheet. Typical dispositions are:

Record what was decided, on what evidence and by whom, and carry any revised forecast forward as a new version rather than overwriting the old one. That way next month’s variance is calculated against what was actually agreed.

How Reddy Compressors helps

Reddy Compressors has served industry from our office at Tarnaka, Secunderabad, since 2002. On the borewell side our work is the air: we supply all types, all brands and all models of borewell compressors, including new machines, used and refurbished units, rentals, service and spare parts. Typical borewell duty runs from 600–1,500 cfm at 200–400 psig (13.8–27.6 bar), with special high-pressure units up to about 35 bar (507.6 psig).

We sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and other makes as an independent multi-brand specialist, not as an authorised dealer. Our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India.

For rig owners who want the same variance thinking on the drilling side, we can help you set up simple records of compressor running hours, delivery pressure, discharge temperature and service intervals, so that a drop in metres drilled per shift can be traced to the compressor, the hammer, the ground or lost hours. On our own hire jobs we change oil and service every 300 running hours, and we keep those records for the machine.

What to send us

• Compressor make, model, serial number and a nameplate photo.
• Rated and actual working pressure and flow, and the hammer and bit sizes in use.
• Recent running hours, service history and any fault or shutdown messages.
• Typical drilling depth, ground conditions and site location.
• Whether you need sale, hire, service or spare parts, and the expected duration.

Frequently asked questions

How do I calculate borewell production variance?

Subtract the forecast volume from the measured volume for the same period and metering point. Divide by the forecast and multiply by 100 for a percentage. Then split the gap into a rate effect and an hours effect using average flow and pumping hours. The percentage shows how big the difference is, but the split shows where to start looking for the cause.

Why is my borewell giving less water than before?

Common causes are fewer pumping hours because of power cuts or trips, a meter or timing error, leaks in the delivery line, pump wear and a seasonal fall in water level. A clogged or silted borehole is also possible. Check runtime and metering first, then compare the pumping water level with earlier readings taken at a similar rate and season.

Does lower borewell output always mean the well is drying up?

No. A lower monthly volume often comes from lost pumping hours, lower demand or a measurement problem rather than the aquifer. If the pump delivers its usual rate at a stable water level, the well is probably fine. A steadily falling pumping water level at a constant rate is the stronger sign of aquifer decline, interference or well deterioration.

What records should I keep for a borewell pump?

Keep daily meter readings with exact times, pumping hours, stop reasons, pumping water level where it can be measured, power interruptions, maintenance work and any change in pump or settings. Record meter replacements and resets. These few items are enough to separate rate, hours and measurement effects when output changes, and they make any later investigation much quicker.

Can a compressor be used to clean or redevelop an old borewell?

Air development with a compressor is a recognised method that drilling contractors use to clear fines and silt from a borehole. The right pressure and flow depend on depth, water level, casing and well design, so it should be planned and carried out by an experienced driller. We can supply, rent and service suitable borewell compressors for this kind of work.

Do you rent borewell compressors outside Hyderabad?

Yes. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spare parts across India, with project-based site support where it is feasible. Tell us the site location, depth, pressure and flow you need and the expected hire period, and we will advise on a suitable machine.

Related pages

• Borewell and DTH compressor library
• Production and availability forecasting
• Borewell yield testing and production duty
• Data quality flags and correction records
• Borewell compressor rentals and hire
• Borewell compressor service and repairs