Water Cycle

WATER CYCLE

Water Cycle, also known as hydrologic cycle, is a process that is constantly recycling the Earth's supply of water. This is important because humans, animals, and plants all need water to survive. It is controlled by the sun, which produces energy in the form of heat. This heat energy causes the water in the world's oceans, lakes, etc. to warm and evaporate. As the water is heated, it changes its phase from liquid to gas. This gas is called water vapor and this process is called evaporation. When plants give off water vapor, it’s called transpiration. When water evaporates, it rises into the cooler air, collects, and forms clouds. There, the water vapor molecules cool down and change back into liquid water. This is called condensation. As more and more water vapor cools into the clouds, the water droplets that form the clouds become larger and larger. When the swirling winds in the atmosphere can no longer hold them up, the droplets fall from the sky and precipitation is the term for the falling, condensed water molecules, which come down as rain, snow, sleet, or hail depending on conditions in the atmosphere. When water falls to the Earth, the water seeps into the soil because of the force of gravity. This seeping is called infiltration. Or the water flows over the land and into bodies of water, such as rivers and lakes.

Evaporation (the importance)

The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling. Without the cooling effect of evaporation the greenhouse effect would lead to a much higher surface temperature of 67 Ð'oC, and a warmer planet.

Most of the solar energy warms tropical seas. After evaporating, water vapor rises into the atmosphere and is carried by winds away from the tropics. Most of this vapor condenses as rain in the Intertropical convergence zone, also known as the ITCZ, releasing latent heat that warms the air. This in turn drives the atmospheric circulation. (http://en.wikipedia.org/wiki/Water_cycle)

Impacts of Temperature changes in Water Cycle

Increasing temperature increases the rates of evaporation and ice melting, and causes sea level to rise. Like in Florida for example, where sea level rose to 4.7m (Noting that during the last glaciations sea level was a full 100 m lower than it is today). Severe droughts, like in the Sahel in Africa, and glacier melting like in the French Alps and Alaska are caused by small changes in the geographical distribution of water that are in turn caused by changes in temperature. Moreover, there will be an increased river flow of freshwater from land to the ocean, and especially to the Arctic Ocean, as more and more of the ice caps on land melt. This flow of water will place a less dense, freshwater "cap" on the surface water of the ocean, and could prevent sinking of cold, salty water ("deep water formation") that drives ocean currents. (http://www.globalchange.umich.edu/globalchange1/curr ent /lectures/kling /rainforest/rainforest.html)

Human Impacts on Terrestrial Water Cycle

Water cycle plays a central role in the climate, ecology, and biogeochemistry of the planet. Increasing historical evidence for the influence of greenhouse warming on recent climate, and modeling projections into the future, highlight changes to the land based water cycle as a major global change issue (Houghton et al. 1995,Watson et al. 1996, SGCR 1999). Disturbance of the water cycle has received significant attention with respect to atmosphere exchanges, plant physiology, net primary production, and the cycling of major nutrients (Foley et al. 1996, Sellers et al. 1996, McGuire et al. 1997).

Major Agents of Anthropogenic Change and its Impact on the Global Water Cycle

The expansion of global water use requires the stabilization of continental runoff and diversion of water from one part of the hydrologic cycle to another. These changes can be translated into particular storage changes as well as reductions or increases in natural hydrological fluxes. The following are the major agents of anthropogenic change and their potential impacts on the terrestrial water system.

Aquifer mining

In most dry regions, where surface water is insufficient, mined fossil groundwater is the main water resource for irrigation and other uses. Natural rates of water recharge to the aquifer are very low. Therefore, when water is extracted from such groundwater pools, most will be transferred from the aquifer to the atmosphere. This transfer represents both a depletion of continental storage and at least a temporary increase in atmospheric water vapor through the enhanced evaporative flux.

Surface water diversion and changes in internally draining lake volumes

Diversion of surface waters for irrigation in the internally draining basins of arid regions