ENSP101 Chapter 17 Water Use and Management 17 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 life o 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 o Solar energy drives the hydrologic cycle by evaporation surface water which plants 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 circulation o 2nd proximity to water sources influences pptn o 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 ocean 17 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 eons o 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 earth o They contain 90 of the world s living biomass o Oceans play a crucial role in moderating the earth s temperature o In tropical seas surface waters are warmed by the sun diluted by rainwater and runoff from the land and aerated by wave action o In higher latitudes surface waters are cold and much more dense o This dense water subsides or sinks to the bottom of deep ocean basins and flows toward the equator o 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 snowfields o 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 Aquifers porous water bearing layers of sand gravel and rock below the Earth s surface topography and subsurface structure reservoirs for groundwater o Aquifers are always underlain by relatively impermeable layers of rock or clay that keep water from seeping out at the bottom o 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 aquifer o Urbanization road building and other development often block recharge zones and prevent replenishment of important aquifers o 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 spaces o Depending on geology it can take anywhere from a few hours o several years for contaminants to move a few hundred meters through an aquifer o 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 are still a minor component of total world water supply o Their water is much more accessible than groundwater or glaciers Wetlands play a vital and often unappreciated role in the hydrologic cycle o Their lush plant growth stabilizes soil and holds back surface runoff allowing time for infiltration into aquifers and producing even year long stream flow o 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 supply o It also has the most rapid turnover rate o Movement of water through the atm provides the mechanism for distributing fresh water over the landmasses and replenishing terrestrial reservoirs 17 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 use o 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 uses o Stable runoff is the dependable renewable year round supply of surface water o 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 stress o Water scarcity Annual available freshwater supplies less than 1 000 m3 per person o Water stress a situation when residents of a country don t have enough accessible high quality water to meet their everyday needs o Water stress is most likely to occur in poor countries where the per capita renewable water supply is low The highest per