Thursday, September 22, 2011

Water, Water Everywhere, How Much is there to Drink?

All the water that ever was or will be on earth is here right now. More than 97% of the Earth’s water is within the in oceans. The remaining 2.8% is the water within the land masses. The land masses contain all the fresh water on the planet. Of the land surface water, 77% is contained in icecaps and glaciers and for all practical purposes is inaccessible in the short run. The remaining fresh water is stored primarily in the subsurface as ground water with a tiny fraction of a percent of water is stored as rivers and lakes.

The water on earth never rests, it is constantly moving within the hydrologic cycle along various complex pathways and over a wide variety of time scales. Water moves quickly through some pathways -rain falling in summer may return to the atmosphere in a matter of hours or days by evaporation. Water may travel through other pathways for years, decades, centuries, or more—the groundwater stored in the Wasia aquifer in Saudi Arabia fell from the atmosphere as rain thousands of years ago.

Water enters the atmosphere through evaporation and exits as precipitation -rain or snow. Typically, water remains in the atmosphere as vapor for about 10 days and it is this time that allows water to move from the oceans to the land mass. Then condenses and becomes rain, snow, or mist. The pattern of precipitation changes over time and causing or responding to changes in the climate of the planet. A falling raindrop might evaporate, or perhaps be taken up by a blade of grass or other plant. The rain drop might fall on the ground and form a puddle or run off across hard packed soil or pavement. This water will likely evaporate, infiltrate the soil or travel to a stream and ultimately flow to an ocean; at any point along this journey water can evaporate and start again. The average time for water to go from rain to stream flow to oceans ranges between 16 and 26 days. Mankind has interrupted the flow of streams and rivers to the oceans by diverting water for irrigation and building reservoirs, thus slowing or interrupting its flow to the ocean.

Not all surface water flows to oceans. Some lakes and wetlands have no surface drainage. They lose water to evaporation and to groundwater. Water moves much more slowly in the subsurface than in the atmosphere or on land surface. Water that infiltrates the soil can remain in the unsaturated zone where it is returned to the atmosphere by evaporation or plant transpiration or the water it can discharge to the surface in a channel becoming surface flow; or it can begin the longer journey- traverse the unsaturated zone and recharge an underlying aquifer. The water that remains in the unsaturated zone typically remains in the subsurface less than a year. Infiltrated water that travels to the saturated zone, and becomes recharge for the aquifer spends much more time in the subsurface. The time that it takes for water to travel through the entire thickness of the unsaturated zone varies tremendously. It can take mere hours to travel through thin unsaturated zones in humid regions to millennia, for thick unsaturated zones in arid regions. The types of soil and rock, the amount of overburden and ground cover and the thickness of the unsaturated zone determines the travel time.

The available supply of fresh water is limited to that naturally renewed by the hydrologic cycle or artificially replenished by the activities of mankind. The recharge rate, the amount of natural replenishment, varies with weather and can exceed water demands during unusually wet periods or fall far below demands during drought periods. Despite conservation the need for water continues to grow planet wide with the growth of human population and the development of emerging economies.

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