Water harvested from a watershed and conducted to ponds for storage can substantially increase available water for garden irrigation, livestock watering, aquaculture and other domestic needs. The circulation of water in its various forms around the earth is called the hydrologic cycle. Water may be harvested effectively by man at certain points in the hydrologic cycle. An understanding of how water circulates around the earth will assist in the selection of an appropriate technology for harvesting it. The sun heats water causing it to evaporate from the earth's surface.
Water returns to the earth in the form of rainfall, snowfall, sleet, dew and hail.
The higher the temperature of an air mass, the more water vapor it can carry. As air masses cool, water vapor changes to liquid forming droplets that fall of their own weight. The most important source of air moisture is water which evaporates from the oceans, but water may also evaporate from other open bodies of water and from the ground. For example, a corn field may transpire up to 7,000 to 10,000 gallons of water per hectare per day into the atmosphere. Some precipitation runs overland by natural drainage channels where it eventually enters rivers, lakes and oceans.