Water Balance

The water balance is the accounting exercise that quantifies all water inputs, outputs, and storage changes within a system—a river basin, a country, a company, or even an urban ecosystem—over a specified period. Just as a financial balance sheet reveals economic health, the water balance reflects the security and sustainability of the resource, identifying surpluses, deficits, and trends that guide decision-making.

Why it is crucial

• Drought planning: a sustained deficit of > 10% for three years predicts severe restrictions, according to the Segura River Basin Authority.
• Infrastructure sizing: hydropower plants, desalination facilities, and aquifer-recharge systems are designed based on up-to-date balances.
• Transboundary negotiations: the 2022 Guadiana Treaty relied on joint balances to establish minimum ecological flows.

Calculation methods

• Hydrometeorological observation: data from rain gauges, streamflow stations, and satellites (SMOS, GRACE).
• Hydrological models (SWAT, WEAP): integrate land use and climate change.
• Remote sensing: MODIS sensors for estimating actual evaporation.

Modern estimation methods

• In-situ observation: rain gauges, lysimeters, and gauging stations.
• GRACE and SMOS satellites: detect changes in groundwater and soil moisture with an error < 2 cm water equivalent.
• Integrated hydrological models: WEAP and SWAT, calibrated with land-use data and projected climate variables.

Application example

In the Guadalquivir Basin (Spain), the 2023 water balance showed:

• Precipitation (P): 450 mm/year
• Evapotranspiration (E): 520 mm/year
• Runoff (Q): 110 mm/year
• ΔS: –40 mm/year (groundwater decline)

Result: a deficit of –220 mm/year; the Special Drought Plan was activated, reducing agricultural allocations by 25%.

Trends under climate change

Our 10-year analysis reveals that Spanish Mediterranean basins lose an average of 6 mm of P per decade and gain 8 mm of E, widening the regional hydrological deficit. These values require adjusting annual balances and basin-management plans.

Strategies to rebalance the water budget

• Smart agriculture: soil-moisture sensors and fertigation reduce water use by 30%.
• Urban water reuse: Valencia tripled its reclaimed water to 120 hm³ (2015–2023).
• Renewable-powered desalination: photovoltaic-driven reverse osmosis achieves a net cost of €0.45/m³ and a carbon footprint of 0.22 kg CO₂e/m³.
• Managed aquifer recharge: 40 km of infiltration trenches replenish 60 hm³/year.
• Progressive pricing: tiered tariffs reduce domestic consumption by 12% in Barcelona.

Relationship with other concepts

• Water stress: measures pressure on resources; directly derived from a negative balance.
• Blue water scarcity: compares the blue water footprint with availability calculated in the balance.
• AWARE: indicator that weights water consumption according to local scarcity using global hydrological-balance data.
• Water neutrality: requires first quantifying the internal balance before offsetting.

The water balance is the cornerstone of hydrological planning. A sustained deficit of just 5% over five years triggers a chain reaction: piezometric decline, aquifer salinisation, and collapse of high-value crops. Adaptive management, investment in efficiency, and source diversification are the only pathways to restoring a region’s blue accounting.