The Hydrologic Cycle or water cycle is the global cycling of water. The cycle consists of different phases that occur both regionally and globally. The different phases are: Evaporation
: In this phase of the cycle liquid water is transformed into gas (water vapor) due to heating. Heating gives water molecules enough kinetic energy to escape the liquid into the atmosphere. Evaporation occurs over the oceans, lakes, reservoirs, rivers, streams and wet ground. Evaporation is faster the warmer and drier the conditions are. Condensation
: This is the opposite of evaporation. It is the transformation of water vapor into liquid water. Just as water evaporates into the atmosphere when it warms up, it will condense out of the atmosphere when the water vapor cools down. This is why you see moisture form on cold drinks on hot humid days. As water vapor rises it generally cools and condenses to form clouds. Transport
: Both water vapor and clouds move through the atmosphere on air currents. Great distances can be traveled from sites of evaporation. Precipitation
: Condensation of water vapor to form clouds occurs when the water vapor condenses on small particles in the atmosphere. This forms water droplets or snow flakes or hail stones depending on the conditions. After reaching a large enough size the water falls from the cloud as precipitation either as rain, sleet, snow, or hail. Runoff
: After a rainstorm on land the water takes different routes. Runoff refers to the surface flow of water to nearby receiving waters such as streams, rivers, lakes, ponds, and storm drains. If there is a lot of rain, there can be a lot of runoff that causes rivers and stream to increase their volume and cause flooding. Another route water takes is called infiltration
. Infiltration is the flow of water into the ground. It is the process of the ground soaking up water from the surface. The amount of infiltration that occurs depends on how dry and porous the ground is. Dry and/or porous ground can soak up a lot of water. Wet and/or non porous ground does not soak up much water which leads to greater runoff. In heavily developed areas with a lot of hard asphalt surfaces there is much more runoff than infiltration leading to greater runoff and risk of flooding. There are many environmental concerns regarding runoff in developed areas. Runoff can wash off and carry many pollutants such as motor oil, pesticides, fertilizers, and litter into local waters that then carry the pollution down stream and eventually coastal waters. Another pathway for water to take to local water bodies is subsurface flow
. After water infiltrates into the ground it can flow below the surface too. Eventually water that falls as rain or snow finds its way back to the ocean where the cycle begins again. Also some water that infiltrates into the ground percolates deeper and becomes part of the groundwater. Groundwater
: This is the water that is beneath our feet underground. In some places the groundwater level or water table is deep and in some places it is shallow. Groundwater is an important source of drinking water for many communities and so it is important to protect its quality from contamination from pollution. Groundwater is not static but moves. Groundwater can well up in some places as springs and provide water to streams, ponds, lakes and even coastal waters. Transpiration
: This is the process of water vapor being lost from the leaves of plants and water being continuously replaced by the plant from water in the ground though the root system. The transpiration rate from forested areas can be tremendous and contributes to the humidity and rainfall to nearby areas. This is why places that clear cut forests become much drier.
When thinking of the water cycle it is important to remember that water is always moving and flowing both through us and around us as it proceeds through the cycle. Thus our activities that impact the environment are transported through water and impact the greater environment. When we litter, fertilize excessively, use pesticides carelessly, dig up the land, or clear cut the sediment, plastic, and chemicals, etc will find their way to streams, rivers, lakes, and coasts and threaten wildlife, drinking water and our quality of life. So do what you can to use things wisely and protect our water supplies and habitats.
For more information check out these resources: The Hydrological Cycle Water cycle - Wikipedia, the free encyclopedia The Water Cycle: Summary, from USGS Water Science