water cycle

Water Cycle

The water cycle has no starting point. But, we'll begin in the oceans, since that is where most of Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe; cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snow packs in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with stream flow moving water towards the oceans. Runoff, and ground-water seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as ground-water discharge, and some ground water finds openings in the land surface and emerges as freshwater springs. Over time, though, all of this water keeps moving, some to reenter the ocean.
























cd7.e2bn.net/.../cd7/website/BluePlanet.htm








Watersheds and their complex network of flow paths regulate the movement of water and nutrients to aquatic ecosystems. Overland and subsurface (soil and ground water) flow paths transport nitrogen (N) and phosphorus (P) with differing levels of efficiency; P transport occurs via surface flow paths almost exclusively and N transport can utilize both overland and subsurface flow paths.





















www.geo.msu.edu/geogmich/MIwatershed.html

Floods are a natural consequence of stream flow in a continually changing environment. Floods have been occurring throughout Earth history, and are expected so long as the water cycle continues to run. Streams receive most of their water input from precipitation, and the amount of precipitation falling in any given drainage basin varies from day to day, year to year, and century to century.


Weather patterns determine the amount and location of rain and snowfall. Unfortunately the amount and time over which precipitation occurs is not constant for any given area. Overall, the water cycle is a balanced system. Water flowing into one part of the cycle (like streams) is balanced by water flowing back to the ocean. But sometimes the amount flowing in to one area is greater than the capacity of the system to hold it within natural confines. The result is a flood. Combinations of factors along with exceptional precipitation can also lead to flooding. For example, heavy snow melts, water saturated ground, unusually high tides, and drainage modifications when combined with heavy rain can lead to flooding.


www.calstatela.edu/dept/geology/G158.htm



Bibliography


http://www.kidzone.ws/WATER/


http://library.thinkquest.org/C003603/english/flooding/causesoffloods.shtml