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Earth's Water and the Hydrologic Cycle

The Hydrologic cycle is a closed loop system.

Demonstration from Poland Spring

  1. Water evaporates from the oceans,
  2. where it condenses into clouds,
  3. and falls as rain (or snow, hail, etc.),
  4. and flows back into the oceans through runoff and underground aquifers.

Within the basic cycle, water is also returned the atmosphere by:

  • Transpiration from trees and plants
  • Evaporation from lakes, rivers, etc.
  • Sublimation from ice packs and snow.

Where is all the Water?

The Earth's two main reservoirs of fresh water are the great polar ice caps and the ground. All ground water, found in aquicludes and aquifers, contains trace amounts of dissolved minerals such as calcium and magnesium. Hard water has a high concentration of minerals being greater than 60 parts per million (ppm). Hard water doesn't lather well with soap, is difficult on fabrics, and can leave mineral deposits inside plumbing and boilers.


  • About 1200 cubic kilometers of water evaporates from oceans, land, plants, and ice caps every day.
  • Very little water has been created or lost over the past billion years.
  • The atmosphere holds about 12,000 cu km of water at any time.