Hydrologic Cycle

The hydrologic cycle is the flow of water through ecosystems, to the atmosphere, and back to ecosystems.  Because water carries dissolved and particulate matter, the hydrologic cycle connects ecosystems to each other.

©2006 WGBH Educational Foundation. All rights reserved. Adapted from Hydrologic Cycle by NASA's Observatorium. Teachers' Domain Collection funded by the National Science Foundation and The William and Flora Hewlett Foundation.

To understand the hydrologic cycle, one must think about “pools” and “fluxes”.  A pool within the water cycle refers to any storage area for water in any form.  Pools include: oceans, lakes, rivers, streams, marshes, bays, ice sheets, glaciers, clouds, and ground water.  Fluxes are the transfer processes that are involved in the cycle.  Evaporation and precipitation are the major transfer processes that move water from the surface to the atmosphere (evaporation) and from the atmosphere to the surface (precipitation).  Whereas evaporation converts liquid water into water vapor, condensation is the opposite process.  Water vapor in the atmosphere condenses to form liquid water within clouds and eventually becomes precipitation.  Surface runoff moves water across land, while infiltration and percolation moves water down into the soil.  Transpiration is evaporation of water from plant surfaces; it moves soil water back into the atmosphere. 

Solar energy drives the hydrologic cycle, and physical processes are the most important ones in this cycle.  However, water does cycle through the biological community.  Organisms gain the water they need from the food they eat and/or from water available in their environment.  Water leaves living things either in biomass, in wastes, or as water vapor as evaporation occurs. 

Beavers are infamous for their alteration of water flow in their environment, but they are nothing compared to humans.  Humans alter the water cycle directly and indirectly; locally and globally!  Examples of direct effects on pools and fluxes would include – overuse of groundwater supplies for irrigation, building huge dams, pollution of pools, and paving large areas with concrete that alters runoff patterns.  Examples of indirect effects on the water cycle would include – global warming, bringing about changes in evaporation and precipitation patterns, and deforestation, bringing about reduction in transpiration rates leading to fewer clouds and less precipitation and bringing about changes in runoff, infiltration and percolation rates.