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The total quantity of water on the earth remains essentially constant. Water moves about, changing form (vapor, liquid, solid) and location as part of the hydrologic cycle. Water is delivered to the earth as precipitation (rain or snow) and then seeps into the ground (infiltration) or travels over the ground (runoff). Some of the water moving over land or in streams and lakes is lost to the atmosphere through evaporation. In addition, plants extract water from the ground and release it to the atmosphere as water vapor (transpiration). Losses to evaporation and transpiration are referred to collectively as evapotranspiration. Water that has seeped into the soil moves along as groundwater flow and water which runs off to streams moves as stream flow.

Components of the hydrologic cycle

1. Precipitation
2. Infiltration
3. Runoff
4. Evaporation
5. Transpiration
6. Groundwater Flow
7. Stream Flow