Components of the Global Hydrological Cycle

Components of the Global Hydrological Cycle

The hydrological cycle consists of a collection of reservoirs (each having a particular mass of water substance) and movement (fluxes, measured in units of mass or volume per unit time) of water substance between these reservoirs. The following are reservoirs of the global hydrological cycle:

  1. Global oceans
  2. Ice masses
  3. Continental seasonal snow
  4. Surface fresh water
    • Lakes
    • Rivers
    • Marshes and wetlands
  5. Subsurface water
    • Soil moisture
    • Permafrost
    • Ground water
        • - Deep aquifers
        - Volcanic water
  6. Biospheric water
  7. Atmospheric water vapor
  8. Clouds
Figure 7 gives the relative sizes of the land, ocean, and atmospheric reservoirs of the global hydrological cycle and the processes (evaporation, precipitation, and runoff) that lead to interchanges among these reservoirs. The ocean is obviously the largest reservoir, followed (by a factor 40 smaller) by land, and followed by an additional factor of 2500 by the atmosphere.

Precipitation over land is larger than evaporation, meaning that considerable water evaporated over oceans eventually falls on land and returns to the ocean by way of runoff through rivers.

By dividing the reservoir volume by its loss rate, we get an estimate of the residence time or timescale of H2O in each reservoir. Figure 8 describes the largest reservoirs and the typical residence times, which range from thousands of years for oceans to about a week for the biosphere.

*Figure 6a is theb0481, from the The Ship Collection of the NOAA Photo Library.

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