The Global Water Cycle and Water Budget

Introduction

97% ocean, 2% ice, 0.01% rivers: following a water molecule through Earth's endless cycle

Water is Earth's most abundant surface compound and the foundation of all known life. Of the approximately 1.386 billion km³ of water on Earth, 97.5% is saline ocean water and only 2.5% is fresh. Of that freshwater, 68.9% is locked in ice caps and glaciers, 30.8% is groundwater, and only 0.3% flows through rivers, lakes, and streams — the surface freshwater that most land life depends on. The atmosphere holds another 12,900 km³ (3,095 cu mi) of water vapour, equivalent to a global layer just 25 mm (0.98 in) deep, yet this tiny reservoir drives the entire precipitation-evaporation cycle.

The water cycle (hydrological cycle) describes the continuous movement of water through Earth's reservoirs via evaporation, condensation, precipitation, runoff, and infiltration. Solar energy powers the cycle: roughly 86,000 km³ (20,631 cu mi) of water evaporates from the oceans each year, while 78,000 km³ (18,712 cu mi) falls back as ocean precipitation. The net transport to land — about 8,000 km³ (1,919 cu mi)/year — balances the river and groundwater discharge back to the sea. The water balance equation summarises this at any watershed scale: Precipitation (P) = Evapotranspiration (ET) + Streamflow (Q) + Change in Storage (ΔS). Over a long average, ΔS ≈ 0 and P = ET + Q.

Residence time — the average time a water molecule spends in each reservoir — varies enormously. Ocean water has a mean residence time of ~3,200 years; deep groundwater can exceed 10,000 years; glacial ice in Antarctica averages ~20,000 years (with some ice dated to 800,000 years). In contrast, soil moisture turns over in weeks, rivers in ~16 days, and water vapour in the atmosphere in just ~8–9 days. Short residence times mean rapid cycling: a water molecule evaporated from the tropical Pacific may precipitate over the Amazon within a week.

Human activities have significantly altered the water cycle. Dams store ~10,000 km³ (2,399 cu mi) of water in reservoirs — equivalent to 10% of all river discharge — delaying flow and trapping sediment. Groundwater extraction (now ~1,000 km³ (240 cu mi)/year globally) depletes aquifers faster than recharge. Urbanisation increases impermeable surfaces, raising runoff and reducing infiltration. Irrigation returns water to the atmosphere via evapotranspiration, shifting regional precipitation patterns. NASA's GRACE satellite mission (2002–2017) and GRACE-FO (2018–present) track these changes by measuring tiny variations in Earth's gravitational field caused by shifting water mass, revealing aquifer depletion in the Ganges basin, the High Plains, and the Middle East.

Key Terms