Water Balance

A water balance is the accounting exercise that quantifies all water inputs, outputs and changes in storage within a defined system, whether a river basin, a country, a city or a company, over a specified period. Much as a financial balance sheet reveals economic health, a water balance reveals the security and sustainability of the resource, highlighting surpluses, deficits and trends that inform decision-making.

The basic equation

At its simplest, a hydrological water balance is expressed as:

Precipitation = Evapotranspiration + Runoff ± Change in storage

Inputs include precipitation, surface inflows, groundwater recharge and water imports (transfers, desalination or reuse). Outputs include evapotranspiration, surface outflows, withdrawals and losses. The change in storage (ΔS) captures the rise or fall of reservoirs, soil moisture and aquifers. A persistent negative ΔS, for example a falling water table, is a clear warning sign of overexploitation.

Why it is crucial

• Drought and resource planning: tracking the balance over time helps anticipate shortages and trigger drought-management measures before a crisis.
• Infrastructure design: reservoirs, desalination plants and aquifer-recharge schemes are sized using up-to-date balances.
• Allocation and governance: shared basins rely on agreed balances to set sustainable abstraction limits and ecological flows.

How a water balance is calculated

• In-situ observation: rain gauges, lysimeters and streamflow gauging stations measure local inputs and outputs.
• Remote sensing: satellite missions such as GRACE (groundwater and total water storage) and SMOS (soil moisture), and sensors like MODIS for evapotranspiration, extend coverage to large or poorly monitored areas.
• Hydrological models: tools such as SWAT and WEAP integrate land use, climate and demand to simulate the balance and test future scenarios.

Water balance under climate change

In many semi-arid regions, including the Mediterranean, climate change is reducing precipitation and increasing evapotranspiration, which widens the gap between supply and demand. Recalculating the balance regularly is therefore essential to keep basin-management plans realistic and to plan adaptation.

Strategies to restore balance

• Demand management: efficient irrigation, leak reduction and tiered tariffs lower withdrawals.
• Non-conventional resources: water reuse and desalination, ideally powered by renewable energy, add supply.
• Managed aquifer recharge: storing surplus water underground for use during dry periods.
• Nature-based solutions: restoring wetlands and soils to improve retention and recharge.

Related concepts

• Water stress: the pressure on resources, which rises as the balance turns negative.
• Water deficit: the gap that appears when demand exceeds effective availability.
• Blue water scarcity: compares the blue water footprint with the availability set out in the balance.
• Water neutrality: requires quantifying the balance before any offsetting.

The water balance underpins sound hydrological planning and corporate water management. At Manglai we help companies measure their water footprint and prepare their sustainability reporting. Discover how Manglai can help you.