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ENSP101Chapter 17 – Water Use and Management17.1 Water Resources- Some 2 billion people, a 1/3 of the world’s population, live in countries with insufficient fresh water  Some experts estimate this number could double in 25 years- The total amount of water on our planet is immense – more than 1,404 million km3- Hydrologic cycle – the natural process by which water is purified and made fresh through evaporation and pptn. This cycle provides all the freshwater available for biological lifeo Plants play a major role in the hydrologic cycle, absorbing groundwater and pumping it into the atm by transpiration (transport plus evaporation)o In tropical forests, as much as 75% of annual precipitation is returned to the atm by plantso Solar energy drives the hydrologic cycle by evaporation surface water, which becomes rain and snow- Because water and sunlight are unevenly distributed around the globe, water resources are very uneven- 3 principal factors control these global water deficits and surpluses:o 1st – global atm circulationo 2nd – proximity to water sources influences pptno 3rd – topography- Human activity also explains some regions of water deficit- As noted earlier, plant transpiration recycles moisture and produces rain- When forests are cleared, falling rain quickly enters streams and returns to the ocean17.2 Major Water Compartments- The distribution of water often is described in terms of interacting compartments in which water resides, sometimes briefly and sometimes for eonso Residence time – the length of time a component, such as an individual water molecule, spends in a particular compartment or location before it moves on through a particular process or cycle- Oceans hold 97% of all water on eartho They contain 90% of the world’s living biomasso Oceans play a crucial role in moderating the earth’s temperatureo In tropical seas, surface waters are warmed by the sun, diluted by rainwater and runoff from the land, and aerated by wave actiono In higher latitudes, surface waters are cold and much more denseo This dense water subsides or sinks to the bottom of deep ocean basins and flows toward the equatoro Warm surface water of the tropical stratifies or floats on top of this cold, dense water as currents carry warm water to high latitudes- Of the 2% of all water that is fresh, nearly 90% is tied up in glaciers, ice caps, and snowfieldso E.g. Antarctica, Greenland- After glaciers, the next largest reservoir of fresh water is held in the ground as groundwater- Infiltration – the process of water percolation into the soil and pores and hollows of permeable rocks- Upper soil layers that hold both air and water make up the zone of aeration- Moisture for plant growth comes primarily from these layers- Lower soil layers where all spaces are filled with water make up the zone of saturation- Water table – the top layer of the zone of saturation; undulates according to the surface topography and subsurface structure- Aquifers – porous, water-bearing layers of sand, gravel, and rock below the Earth’s surface; reservoirs for groundwater o Aquifers are always underlain by relatively impermeable layers of rock or clay that keep water from seeping out at the bottomo When a pressurized aquifer intersects the surface, or if it’s penetrated by a pipe or conduit, an artesian well or spring results from which water gushes without being pumped- Recharge zone – area where water infiltrates into an aquifero Urbanization, road building, and other development often block recharge zones and prevent replenishment of important aquiferso Contamination of surface water in recharge zones and seepage of pollutant into abandoned water in recharge zones and seepage of pollutants into abandoned well have polluted aquifers in many places, making them unfit for most uses- While water can flow through limestone cavers in underground rivers, most movement in aquifers is a dispersed and almost imperceptible trickle through tiny fractures and spaceso Depending on geology, it can take anywhere from a few hours o several years for contaminants to move a few hundred meters through an aquifero Pptn that doesn’t evaporate or infiltrate into the ground runs off over the surface, drawn by the force of gravity back toward the sea- Ponds are generally considered to be small temporary or permanent bodies of water shallow enough for rooted plants to grow over most of the bottom- Lakes are inland depressions that hold standing fresh water year-round- While lakes contain nearly 100x as much water as all rivers and streams combined, they arestill a minor component of total world water supplyo Their water is much more accessible than groundwater or glaciers- Wetlands play a vital and often unappreciated role in the hydrologic cycleo Their lush plant growth stabilizes soil and holds back surface runoff, allowing time for infiltration into aquifers and producing even, year-long stream flowo When wetlands are disturbed, their natural water-absorbing capacity is reduced and surface waters run off quickly, resulting in floods and erosion during the rainy season and dry, or nearly dry, streambeds in the rest of the year- The atm is among the smallest of the major water reservoirs of the earth in terms of water volume, containing less than 0.001% of the total water supplyo It also has the most rapid turnover rateo Movement of water through the atm provides the mechanism for distributing fresh water over the landmasses and replenishing terrestrial reservoirs17.3 Water Availability and Use- The availability of water determines the location of activities of humans on earth- Renewable water supplies – annual freshwater surface runoff plus annual infiltration into underground freshwater aquifers that are accessible for human useo About 2/3 of the water carried in rivers and streams every year occurs in seasonal floods that are too large or violent to be stored or trapped effectively for human useso Stable runoff is the dependable, renewable, year-round supply of surface watero Much of this occurs, however, in sparsely inhabited regions or where technology, finances, or other factors make it difficult to use it productively- Many countries suffer water scarcity and water stresso Water scarcity – Annual available freshwater supplies less than 1,000 m3 per persono Water stress – a situation when residents of a country don’t have enough accessible, high-quality water to meet their everyday needso


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