---
title: "Underground and Overhead Tank Water Balance: Count Transfers Once"
description: Reconcile underground and overhead tank storage using clear water-balance boundaries, verified transfers and separate water-service records.
---

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# Underground and Overhead Tank Water Balance: Count Transfers Once

TownEve Editorial Team · October 1, 2026

How do you reconcile a housing society's underground and overhead tank levels without double-counting water? Draw a separate accounting boundary around each tank or hydraulically connected group, record verified transfers between those boundaries, and count external municipal, borewell and tanker receipts only when they enter the society-wide boundary. Pumping water from an underground tank (UGT) into an overhead tank (OHT) changes where water is stored; it does not create a second water receipt.

Two different questions, two different boundaries

A tank-level dashboard shows storage at monitored points. A water balance asks which water volumes crossed a defined boundary during a stated period. For an individual UGT, incoming municipal or tanker water and outgoing pump transfers can change storage. For an individual OHT, pump transfers and resident draw can change storage. For a whole-society boundary containing both tanks, UGT-to-OHT pumping is an internal transfer. Its volume should cancel rather than inflate the society's total external supply.

## Simple two-tank equations

Let ΔUGT be UGT closing stored volume minus opening stored volume for the same period. Let I be independently established external inflow into the UGT, T the independently established UGT-to-OHT transfer, and X any other independently established UGT outflow. Then ΔUGT = I - T - X, subject to measurement uncertainty and any additional paths.

Let ΔOHT be the corresponding OHT storage change, D the independently established OHT distribution outflow and Y any other OHT outflow. Then ΔOHT = T - D - Y, subject to uncertainty and other paths.

Adding the equations gives ΔUGT + ΔOHT = I - X - D - Y. The transfer T cancels. These equations are accounting identities under their stated boundaries; tank-level observations alone do not measure all the individual flows.

## Worked example, hypothetical only

At the start of a matched interval, the UGT stores 80 kL and the OHT stores 20 kL. During the interval, an independently measured 30 kL of external water enters the UGT, an independently measured 25 kL is pumped to the OHT, and an independently measured 15 kL leaves the OHT for distribution. Assume no other paths and negligible measurement error for this illustration. Closing UGT volume is 80 + 30 - 25 = 85 kL. Closing OHT volume is 20 + 25 - 15 = 30 kL. Combined storage rises from 100 to 115 kL, a 15 kL increase. The whole-system balance is 30 kL external inflow minus 15 kL distribution. Counting the 25 kL pump transfer as additional external supply would incorrectly report 55 kL of intake. These figures are invented to illustrate the arithmetic, not actual TownEve customer measurements.

## When transfer flow is not metered

If the pump transfer T is unmetered, two tank traces can show when the UGT fell and the OHT rose. They cannot uniquely determine the gross transfer while municipal inflow, resident draw or other flows occur simultaneously. Even equal-and-opposite storage movements do not prove the transfer volume if other paths were active. A verified pump-off or no-inflow observation can sometimes narrow an investigation, but it does not automatically provide a measured transfer or resident consumption figure. Log the unknown rather than forcing the equations to close by inventing a flow.

## A practical boundary map

For each storage asset record: tank ID, service category, physical capacity, configured geometry, dead-water or minimum usable level, monitoring coverage, source connections, pump inlet and outlet, overflow, bypass, distribution lines and known isolation points. Identify which flows are inside the chosen reporting boundary and which cross it. Confirm physical connectivity with the facilities team. A diagram based only on labels in a dashboard can miss a shared manifold, bypass or an independent compartment.

## Service categories and compartments matter

Drinking, domestic and treated/recycled water should be reported separately when their uses, treatment and distribution networks differ. Do not treat a rise in a treated-water tank as a replenishment of drinking-water reserve. A divided UGT may contain hydraulically independent chambers; one sensor cannot be assumed to represent all compartments. Connected chambers may equalize, but the actual connections, valve positions and sensor placement require site validation. If an OHT serves only one tower or service, do not count inaccessible storage elsewhere as available to it.

## Storage volume versus usable volume

Use configured tank dimensions and the appropriate level-to-volume geometry to estimate stored volume. A displayed usable percentage can exclude configured dead-water height, so confirm whether the operational zero already excludes unavailable storage before subtracting it again. Volume conversion depends on correct tank geometry, level readings and installation assumptions. This is a stored-volume calculation, not a direct flow-meter reading or a certified water-quality measurement.

## Monthly reconciliation table

Boundary | Opening storage | Closing storage | Independently measured inflow | Independently measured outflow | Internal transfers | Unknown paths

UGT domestic | \[kL, timestamp\] | \[kL, timestamp\] | \[municipal, borewell, tanker by evidence\] | \[metered transfer if available\] | \[outgoing to OHT\] | \[unmetered\].

OHT domestic, Tower A | \[kL, timestamp\] | \[kL, timestamp\] | \[metered transfer if available\] | \[metered distribution if available\] | \[incoming from UGT\] | \[unmetered\].

Whole domestic system | \[sum of accessible relevant tanks\] | \[same basis\] | \[external receipts only\] | \[external distribution and other exits\] | \[cancel internal transfers\] | \[unmetered\].

Use identical start and end times for the same balance. Keep separately identified meter totals, supplier-declared receipts and estimated volumes rather than silently combining them as measured values. A pump log may confirm operation timing without measuring transferred kilolitres.

## Six checks before interpreting a mismatch

1\. Are the start and end timestamps aligned across UGT, OHT and source meters?

2\. Were tank dimensions, dead-water configuration, sensor freshness and units verified?

3\. Is the transfer route known, and is transfer volume independently measured or only assumed?

4\. Were external receipts counted once, and were internal transfers excluded from whole-society intake?

5\. Are all relevant overflow, bypass, maintenance and distribution paths represented?

6\. Could simultaneous inflow and outflow explain why the observed net level movement differs from an assumed gross flow?

A remaining discrepancy is an investigation item, not automatic evidence of a leak or excessive resident consumption.

## What TownEve contributes today

TownEve currently supports contactless ultrasonic UGT and OHT monitoring, historical levels, configured level-based email alerts, role-based visibility, telemetry-freshness monitoring and stored-volume calculations based on configured tank geometry. This lets operators see storage at multiple points, reconcile changes across defined boundaries and investigate transfer timing. Pump-transfer volume, incoming-source flow, resident demand and physical leakage use the corresponding measurement or investigation evidence. Direct pump-state monitoring and remote pump operation are in development and testing. A sensor-assisted controlled valve-isolation investigation has been tested and continues to be refined; automatic flow measurement is a separate capability.

## Frequently asked questions

If the UGT loses 10 kL while the OHT gains 10 kL, did the pump move exactly 10 kL? Not necessarily. Simultaneous external inflow or distribution draw can change either tank's net storage.

Should a tanker unloaded into a UGT be counted again when that water reaches an OHT? No. The receipt is external once; pumping is an internal transfer.

Can two level sensors replace a flow meter? They provide storage observations, but cannot generally separate concurrent inflows and outflows without additional verified conditions or instrumentation.

Does the OHT's percentage represent the whole society's reserve? No. It represents a configured storage boundary and may exclude other tanks or inaccessible supply.

What if the UGT has multiple chambers? Validate hydraulic connections and sensor coverage; report independently operating compartments separately.

## Related resources and CTA

Request a TownEve water-visibility assessment to map tank boundaries, monitoring coverage and the additional measurements needed for defensible flow reconciliation.

Explore further

## Understand your water system. Plan the next step.

Browse more practical resources or discuss your property's monitoring needs.

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TownEve · CyberGrove Solutions Private Limited  
Water monitoring for residential communities.

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