The Global Water Balance
The global water balance accounts for all the water stored in and exchanged among the oceans, atmosphere, land, ice, and groundwater, expressing the conservation of mass that constrains the entire hydrological cycle.
Definition
The water balance is the application of conservation of mass to a region or to the whole Earth, stating that over a period the inputs equal the outputs plus the change in storage; the global water balance quantifies the reservoirs and fluxes of the entire hydrological cycle.
Scope
This topic covers the water-balance equation applied from catchment to global scale, the sizes of the major reservoirs and the fluxes between them, and the residence times of water in each store. It uses the budget as an accounting framework rather than detailing the individual processes covered in neighboring topics.
Core questions
- How is the water-balance equation written and applied at different scales?
- How large are the major water reservoirs, and how is water distributed among them?
- What are the principal fluxes linking ocean, atmosphere, and land?
- How long does water reside in each reservoir?
Key concepts
- Conservation of mass
- Water-balance equation
- Reservoirs and storage
- Residence time
- Precipitation, evaporation, runoff fluxes
- Renewable freshwater
Key theories
- Water-balance equation
- For any control volume, precipitation equals evapotranspiration plus runoff plus the change in storage; closing this budget at catchment and global scales is the central accounting tool of hydrology.
- Global water budget estimates
- Observational and model-based estimates quantify the global stocks and the precipitation, evaporation, and runoff fluxes, framing how much fresh water is renewed each year and where it resides.
Clinical relevance
The water balance underlies estimates of renewable water resources, the closure of catchment studies, the detection of long-term storage changes such as groundwater depletion and ice loss, and the evaluation of climate and land-surface models against observed fluxes.
History
The water-balance concept grew from 19th- and 20th-century catchment studies that compared rainfall, runoff, and evaporation; global budget estimates were refined through the 20th century and, from the late 20th century onward, by satellites, reanalysis, and gravimetric measurement of large-scale water-storage change.
Key figures
- S. Lawrence Dingman
- Kevin E. Trenberth
- Taikan Oki
Related topics
Seminal works
- dingman2015
- oki2006
- trenberth2007
Frequently asked questions
- Why is the water balance useful if it is just bookkeeping?
- Because conservation of mass is exact, the balance lets hydrologists estimate one unknown flux or storage change from the others; for example, basin evapotranspiration can be inferred from measured precipitation and runoff.
- Where is most of the Earth's water?
- The vast majority is salt water in the oceans; fresh water is a small fraction, most of it locked in ice and deep groundwater, leaving only a tiny share in the lakes, rivers, soil, and atmosphere that supply most human and ecological needs.