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Water Resources Objectives Review the hydrologic cycle Describe the important sources of water and the major ways we use it Appreciate the causes and consequences of water shortages around the world Debate the merits of proposals to increase water supplies and manage demands Hydrologic cycle Describes the circulation of water as it: Evaporates from land, water, and organisms Plants lose water through transpiration Enters the atmosphere Condenses and precipitates back to the earth’s surfaces Moves underground by infiltration or overland runoff into rivers, lakes and seas Hydrologic cycle Solar energy drives the hydrologic cycle by evaporating surface water Evaporation - Changing liquid to a vapor below its boiling point Only water evaporates - salt and contaminants stay behind Hydrologic cycle Humidity - Amount of water vapor in the air Warm air can hold more water than cold air Saturation point When a volume of air contains as much water vapor as it can hold at a given temperature Hydrologic cycle Condensation is when air exceeds saturation point and water becomes liquid or ice again Condensation Nuclei - Tiny particles that facilitate condensation Smoke, dust, sea salt, spores, volcanic ash Hydrologic cycle Oceans account for 86% of total evaporation Ninety percent of water evaporated from the ocean falls back on ocean as rain Remaining 10% is carried by prevailing winds over continents Hydrologic cycle When water falls on land, some is incorporated into biological tissues and a large share seeps into the ground to be stored All water makes its way back downhill to oceans Water is distributed unevenly on earth Regions of plenty and regions of deficit Principal factors that control global water deficits and surpluses Global atmospheric circulation Prevailing winds Topography Human activity Global atmospheric circulation Creates areas low rainfall about 20° to 40° north and south of the equator Prevailing winds Areas far from oceans - in a windward direction are usually relatively dry Topography Mountains act as cloud formers and rain catchers Air sweeps up the windward side of a mountain, pressure decreases, and the air cools Eventually saturation point is reached, and moisture in the air condenses Rain falls on the mountaintop Cool, dry air descends and warms, absorbing moisture from other sources Rain shadow Mt Waialeale windward side receives 12 m/year, while the leeward side receives 46 cm Human activity Removal of plants causes desertification Plants recycle moisture, produce rain, and slow water flow to streams Oceans Together, oceans contain more than 97% of all liquid water in the world Average residence time of water in the ocean is about 3,000 years Residence time is the amount of time a water molecule spends in the ocean before entering the hydrologic cycle again Glaciers, ice and snow 2.4% of world’s water is fresh 90% in glaciers, ice caps, and snowfields Now, Antarctic glaciers contain nearly 85% of all ice in the world Groundwater Second largest reservoir of fresh water Infiltration - Process of water percolating through the soil and into fractures and permeable rocks Groundwater Aquifers - Porous layers of sand, gravel, or rock lying below the water table Artesian - Pressurized aquifer intersects the surface Water flows without pumping Groundwater Recharge Zones - Area where water infiltrates into an aquifer Recharge rate is often very slow Presently, groundwater is being removed faster than it can be replenished in many areas Lakes and ponds Ponds are generally considered small 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 Lakes contain 100 times as much water as all rivers and streams combined Great Lakes 20% of the world's supply of surface water 95% of the North American supply of surface freshwater Rivers and streams Precipitation that does not evaporate or infiltrate into the ground runs off the surface, back toward the sea 16 largest rivers in the world carry nearly half of all surface runoff on earth Amazon carries 10 times the volume of Mississippi River Wetlands Play a vital role in hydrologic cycle Lush plant growth stabilizes soil and retards surface runoff, allowing more aquifer infiltration Disturbance reduces natural water-absorbing capacity, resulting in floods and erosion in wet periods, and less water flow the rest of the year Atmosphere Among the smallest water reservoirs Contains < 0.001% of total water supply Has most rapid turnover rate Provides mechanism for distributing fresh water over landmasses and replenishing terrestrial reservoirs Water-Rich and Water-Poor Countries Water availability usually measured in terms of renewable water per capita Highest per capita generally found in countries with moist climates and low population densities Drought cycles Every continent has regions of scarce rainfall due to topographic effects or wind currents Water shortages have most severe effect in semi-arid zones where moisture availability is the critical factor in plant and animal distributions U.S. seems to have 30 year drought cycle Climatic changes such as global warming may alter cycles Droughts Land use exacerbates the effects of drought Dust bowl of 1930’s The dust bowl Occurred in the 1930s in the Great Plains Overgrazing and prolonged drought left the ground bare 1934 winds produced dust storms that stripped about 9 million acres of topsoil Dust clouds reached as far as Washington DC Quantities of water used Human water use has been increasing about twice as fast as population growth over the past century Americans use 1,300 gallons per person per day, while Haitians use 8 gallons per person per day Quantities of water used Worldwide, agriculture claims about 70% of total water withdrawal In many developing countries, agricultural water use is extremely inefficient and highly consumptive Worldwide, industry accounts for about 25% of all water use Cooling water for power plants is single largest industrial use Domestic water use Worldwide, domestic water use accounts for about one-fifth of water withdrawals American water use A precious resource Currently, 45 countries, most in Africa or Middle East, are considered to have serious water stress, and cannot meet the minimum essential water requirements of their citizens More than two-thirds of world’s households have to retrieve water from outside the home Sanitation levels decline when water is expensive Depleting groundwater Groundwater is the source of nearly 40% of fresh water in the U.S. On a local level, withdrawing water faster than it can be replenished leads to a cone of depression in the water table On a broader scale, heavy pumping can deplete an aquifer Ogallala Aquifer, which once held more water than all the freshwater lakes, streams, and rivers on earth, has fallen dramatically Depleting groundwater Withdrawing large amounts of groundwater in a small area causes porous formations to collapse, resulting in subsidence Sinkholes form when an underground channel or cavern collapses Saltwater intrusion can occur along coastlines where overuse of freshwater reservoirs draws the water table low enough to allow saltwater to intrude Increasing water supplies Seeding clouds Condensation nuclei Towing Icebergs Cost Desalination Three to four times more expensive than most other sources Increasing water supplies Dams, reservoirs, canals and aqueducts Common to trap excess water in areas of excess and transfer it to areas of deficit Environmental costs Upsets natural balance of water systems Ecosystem losses Loss of wildlife habitat Reservoir size Water quality Dams, reservoirs, canals and aqueducts Displacement of people Three Gorges Dams in China is forcing relocation of over a million people Evaporation, leakage, siltation Evaporative losses from Lake Mead and Lake Powell on the Colorado River is about 2 billion m3 per year Dams slow water flow, allowing silt (nutrients) to drop out Loss of free-flowing rivers Watershed management Watershed - All the land drained by a stream or river Watershed management Retaining vegetation and ground cover helps retard rainwater and lessens downstream flooding Retaining crop residue on fields reduces flooding Minimizing plowing and forest cutting on steep slopes protects watersheds Domestic conservation Estimates suggest many societies could save as much as half of current domestic water usage without great sacrifice or serious change in lifestyle Low-volume shower heads Efficient dishwashers and washing machines Landscape choices Waterless or low-volume toilets
