The Hydrological Cycle
The hydrological cycle is the continuous circulation of water among the atmosphere, land surface, and oceans through evaporation, precipitation, runoff, and storage, conserving water globally while redistributing it in space and time.
Definition
The hydrological cycle is the system of pathways and reservoirs through which water moves and is stored across the Earth, governed by conservation of mass: over any region and period, precipitation equals evapotranspiration plus runoff plus the change in storage.
Scope
This area covers the fluxes and stores that make up the global water cycle: precipitation as the input to land, evaporation and transpiration as the principal return to the atmosphere, infiltration that partitions water at the soil surface, and the mass-balance accounting that links these fluxes to changes in storage. It treats the cycle at scales from a soil column to the whole globe, while detailed channel and aquifer processes are developed in the surface-water and groundwater areas.
Sub-topics
Core questions
- What are the principal fluxes and reservoirs of the global water cycle, and how large are they?
- How does the water-balance equation express conservation of mass for a region?
- What controls how much precipitation returns to the atmosphere versus running off or recharging storage?
- How is the hydrological cycle changing under a warming climate?
Key concepts
- Precipitation, evaporation, transpiration, runoff
- Reservoirs and residence times of water
- Water-balance equation
- Evapotranspiration partitioning
- Infiltration and storage
- Renewable freshwater flux
Key theories
- Conservation of mass (water balance)
- Over a defined control volume and time period, the water balance states that inputs (precipitation) equal outputs (evapotranspiration plus runoff) plus the change in storage; this accounting underlies nearly all quantitative hydrology.
- Global water cycle partitioning
- Estimates of the global cycle quantify how the roughly fixed global precipitation is partitioned among ocean and land, and how continental precipitation divides into evapotranspiration and runoff, framing the renewable freshwater available to society.
Clinical relevance
The water cycle sets the renewable freshwater supply that sustains ecosystems, agriculture, and water supply; understanding its fluxes underpins drought and flood forecasting, water-resource planning, and assessment of how climate change alters precipitation and evaporation.
History
Quantitative understanding of the water cycle grew from early observations that rivers are fed by rainfall (Perrault and Mariotte in the 17th century), through the measurement of evaporation and precipitation, to modern global estimates from satellites, reanalysis, and land-surface models that close the budget at continental and global scales.
Key figures
- S. Lawrence Dingman
- Wilfried Brutsaert
Related topics
Seminal works
- dingman2015
- brutsaert2005
- oki2006
Frequently asked questions
- Is water created or destroyed in the hydrological cycle?
- No. The cycle conserves mass: water changes phase and location but the total amount is effectively constant on human timescales, which is why the water-balance equation is so central to hydrology.
- Why does only a fraction of precipitation become streamflow?
- Much of the precipitation falling on land returns to the atmosphere through evaporation and plant transpiration, and some goes into soil and groundwater storage; only the remainder becomes runoff, with the split depending on climate, vegetation, soils, and topography.