ENS 201 Chapter 17, Spring 2008

Water Use and Management Chapter 17 Outline • Hydrologic Cycle • Major Water Compartments • Water Availability & Use • Freshwater Shortages • Increasing Water Supplies • Water Management & Conservation WATER RESOURCES • Water, liquid & solid, covers more than 70% of world’s surface Hydrologic (Water) Cycle • Describes the circulation of water as it:  Evaporates from land, water bodies, & organisms (esp. transpiration from plants)  Enters the atmosphere  Condenses & precipitates back to the earth’s surfaces  Moves underground by infiltration or overland by runoff into rivers, lakes & seas • Water cycle is driven by solar energy & gravity Cunningham - Cunningham - Saigo: Environmental Science 7th Ed. 19_02 Hydrologic cycle Slide number: 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evaporation and transpiration 71,000 km3 Comet Condensation Ice and snow Water vapor Precipitation 111,000 km3 Infiltration Groundwater Flow Ocean Lake Runoff 40,000 km3 Precipitation 385,000 km3 Evaporation 425,000 km3 Condensation Water vapor Onshore transport 40,000 km3 Hydrologic Cycle Major Water Compartments • Oceans • Glaciers, ice & snow • Groundwater • Rivers and Streams • Lakes and Ponds • Wetlands • The Atmosphere MAJOR WATER COMPARTMENTS • Oceans  Contain more than 97% of all liquid water in the world & 90% of world’s living biomass  Moderate earth’s temperature • Glaciers, Ice, & Snow  About 90% of fresh water (fresh water = 2.4% of all water) is tied up in glaciers, ice caps & snowfields - Glaciers  Are rivers of ice flowing downhill very slowly  Occur at high latitudes or high altitudes Major Water Compartments • Groundwater: second largest reservoir of fresh water  Infiltration - Process of water percolating through the soil & into fractures & spaces of permeable rocks  Zone of Aeration - Upper soil layers that hold both air and water  Zone of Saturation - Lower soil layers where all spaces are filled with water  Water Table – Top of Zone of Saturation Groundwater Groundwater • Aquifers - Porous layers of sand, gravel, or rock lying below the water table => saturated with water • Recharge Zone - Area where water infiltrates into an aquifer  Recharge rate is often very slow • Artesian Well - Pressurized aquifer intersects the surface => water flows without pumping Groundwater Groundwater Depletion • In many areas, groundwater is being removed faster than it can be replenished • Urbanization, road building, & other development projects, often block recharge zones & prevent replenishment of aquifers • Contamination of surface water in recharge zones & seepage of pollutants have polluted aquifers in many places WATER AVAILABILITY & USE • Renewable Water Supplies are made up of surface runoff plus infiltration into accessible freshwater aquifers • Human water use has been increasing about twice as fast as population growth over the past century Water Use By Sector • Agriculture claims about 69% of total water withdrawal  In many developing countries, agricultural water use is extremely inefficient & highly consumptive - 60-70% of the water withdrawn for agriculture in developing countries is wasted • Industry accounts for about 25% of all water use  Cooling water for power plants is the largest industrial use of water Growth of Global Water Use, 1900-2000 FRESHWATER SHORTAGES • In 2000 an estimated  1.5 billion people lack access to an adequate supply of drinking water  3 billion lack acceptable sanitation • A country where consumption exceeds 20% of available, renewable supply is considered vulnerable to water stress FRESHWATER SHORTAGES • Globally water supplies are abundant but unevenly distributed • In many developing countries, the capital needed to manage (collect, store, purify & distribute) water is unavailable • If present trends continue, about 2/3 of the world’s population will live in countries experiencing water shortages by 2025 FRESHWATER SHORTAGES • The reasons for freshwater shortages are many:  Natural  Human-made - Too many people - Human practices that minimize infiltration & recharge of groundwater - Contamination or pollution - Bad management of available water resources including waste of these resources Water: A Precious Resource • Currently, 45 countries (mostly in Africa or Middle East)  Are considered to have serious water stress  Cannot meet the minimum essential water requirements of their citizens, which is1,000 m3 of water/person/year) • More than two-thirds of world’s households have to retrieve water from outside the home • Sanitation levels decline when water is expensive INCREASING WATER SUPPLIES • Seeding Clouds with hygroscopic salts => formation of condensation Nuclei • Desalination  Most common methods are - Distillation (evaporation & recondensation) - Reverse osmosis (forcing water through a semipermeable membrane)  Three to four times more expensive than most other sources of fresh water Increasing Water Supplies • Dams, Reservoirs:common to trap runoff • Canals, Aqueducts, tunnels, & underground pipes: transfer water from areas of excess to areas of deficit Environmental Costs of water projects (Storage & Transport)  Upset of the natural balance of streams, lakes, estuaries, & terrestrial ecosystems  Losses of ecosystem & wildlife habitat  Displacement of People - E.g.: three Gorges Dams on the Yangtze River in China will force relocation of over a million people  Evaporation, Leakage, Siltation of the reservoirs behind dams (siltation: deposition of silt when water flow is slowed)  Loss of Free-Flowing Rivers WATER MANAGEMENT & CONSERVATION • Watershed:  All the land drained by a stream or river  Retaining vegetation & ground cover in a watershed helps hold back rainwater & lessens downstream flooding  Watersheds protection & flooding reduction: - Retaining crop residue on field - Minimizing plowing & forest cutting on steep slopes Domestic Conservation • Estimates suggest that many societies could save as much as half of current domestic water usage without great sacrifice or serious change in lifestyle • Largest domestic use is toilet flushing • Significant amounts of water can be reclaimed & recycled  Purified sewage effluent Typical US Household Water Use Price Mechanisms & Water Policy • Growing recognition that water is a precious & finite resource has changed water policies & encouraged conservation • Charging a higher proportion of real costs to users of public water projects has helped encourage conservation