The water cycle also called the hydrologic cycle is taught in grade school science class but is worth a review in regards to a changing climate and a growing population. Where water falls, where and how it moves by wind patterns over mountains and deserts, watersheds, rivers, lakes and marshes that run to the sea are part of a process. Evaporation, condensation, precipitation, the cycle, a circle, unbroken.
Conversations about precipitation, the rain and snow or lack thereof are topics common to all people around the globe. Record breaking weather events have been experienced on every continent recently and the water conversation is an important one to have. Changes in rain/snow patterns are affecting metropolitan, industrial and agricultural pursuits and ultimately means transformation of the earth.
The near future is going to challenge communities on what they do to mitigate water issues. Some towns will flood while others have the wells run dry. How to achieve sustainability and security will depend on the water shed of any given region. Understanding the hydrologic cycle is fundamental to making intelligent decisions for water sustainability. Now is the time to plan as we face the alarming fact that the physical supply of water is limited.
The hydrologic cycle describes the movement of water on, above and below the surface of the earth. The sun drives the water cycle but there are nine physical processes that comprise the movement of water in that continuum.
Evaporation occurs when the physical state of water is changed from a liquid state to a gaseous state. Solar radiation, air temperature, vapor pressure, wind and atmospheric pressure affect the amount of natural evaporation that occurs in an area. Human activity such as heated buildings can also cause evaporation of water that settles on a building.
Condensation is the process by which water vapor changes it’s physical state from vapor to a liguid. Water vapor condenses on small airborne particles to form dew, fog or clouds. Condensation is brought about by cooling of the air or when the amount of vapor in the air reaches a saturation point.
Precipitation is the process when water particles fall from the atmosphere and reach the ground. Clouds release precipitation in two ways, the coalescence process or the ice-crystal process. Coalescence happens when a water drop reaches a critical size and gravity takes hold causing it to fall. Along the downward path other smaller water particles will adhere to the original water drop increasing size. The second type of precipitation occurs in cloud formations with freezing temperatures. When ice crystals develop within the cloud other water droplets will condense and freeze on the crystal until it reaches a critical size and will drop as snow flakes or ice pellets.
Interception is the process of interrupting the movement of water as it travels towards streams. When rain first begins, the water striking leaves and other organic material spreads to a thin layer until reaching maximum storage capability, additional storage is accomplished by growing drops along the edges until surface tension is exceeded and the drops fall to the ground. The water layer on organic surfaces and the drops of water clinging to the edge are freely exposed to evaporation.
Infiltration is the physical process when water moves through the boundary area where atmosphere interfaces with the soil. Water transfer is related to the porosity and permeability of the soil. The texture and structure of the soil, the moisture content and soil composition all affect the infiltration.
Percolation is the movement of water through the soil and it’s layers by gravity and capillary action. The prime moving force of ground water is gravity. Voids and fissures in geologic formations which transmit water from one location to another are called aquifers.
Transpiration is the biological process where water inside of plants is transferred to the atmosphere as water vapor through numerous individual leaf openings and water-releasing cells (stomata). Only a small portion of the water that plants absorb are retained in the plant. Transpiration is affected by sunlight and varies greatly on plant species and soil conditions.
Runoff is the flow from a drainage basin or watershed that appears in surface streams. The flow is made up of precipitation that falls directly on the stream, surface runoff that flows across the land and through channels, subsurface runoff that infiltrates surface soils and moves latterly towards the stream and groundwater runoff from deep percolation that migrates through soil horizons. When each of the component flows enter the stream, they form the total runoff.
Storage happens at three basic locations in the planetary water cycle. Water is stored in the atmosphere, on the surface of the earth and stored in the ground. Water stored in the atmosphere can be moved relatively quickly from one place on the planet to another. Surface storage occurs in oceans, lakes, reservoirs, and glaciers. Underground storage occurs in the soil, in aquifers and in the crevices of rock formations.
Although the water cycle is continuous it can be erratic. The uneven distribution of water in geographic and geologic areas plus the movement of water over time can cause extreme phenomena such as floods and drought. Fundamental thermodynamics and climate models suggest that dry regions will become drier and wet regions wetter in regards to a warming planet.
Now is the time to educate yourself, your family and neighbors about future water issues facing your region. Now is the time to ask questions of your community leaders about what plans are being developed for water sustainability.