chapter 17 notes
Water shortages wand water stress affect at least a third of the worlds population. Water resources are expected to be a major source of regional and international conflict in coming decades.
Distribution of water around the globe depends mainly on climate factors, including high pressure zones and prevailing winds, and topography. Human activities such as deforestation, also affect regional water supplies.
The hydrologic cycle is the movement of water between the ocean, atmosphere, land, and living things. All water on land originates as evaporation, mostly from oceans, which produces rain and snow. Major water compartments involved in the hydrologic cycle include oceans; glaciers, ice, and snow; groundwater; lakes, rivers, and wetlands; and the atmosphere. Residence time in compartments ranges from thousands of years in the ocean to minutes or days in the atmosphere.
Aquifers are porous rock formations that hold water. Water enters aquifers through recharge zones. A confined aquifer is one whose saturated layers are capped (overlain) by impermeable rock layers.
Pumping water from an aquifer produces a cone of depression, often drying out shallower wells. Risks of overpumping aquifers- extracting water faster than it is recharged- include subsiding ground levels, sinkholes, and saltwater intrusion, as well as depletion of water supplies.
Water withdrawal refers to all water taken for use. Consumption refers to water lost to direct use, usually through evaporation or seepage in the ground. Degradation is water contamination by pollutants, salts, or heat, which reduces it utility for later uses.
Worldwide, two-thirds of water withdrawn is used for agriculture. Agriculture accounts for 85 percent of water consumption, mainly through evaporative losses and seepage from unlined canals. Improved irrigation methods are beginning to reduce some consumptive losses.
Industrial and domestic water uses are increasing, although not as fast as agricultural use. Consumption and degradation have fallen somewhat due to conservation and increased efficiency in households and industries.
Water stress occurs when consumption exceeds 20 percent of available, renewable water supplies. At least 45 countries have serious water stress and cannot supply minimum essential water needs for citizens.
Many strategies have been attempted to increase available water supplies. Desalination and water diversions are the principal methods that increase supplies in arid regions. Dams and reservoirs are controversial. They provide essential power and irrigation, but they also have great environmental, economic, and social costs, including ecosystem losses, displaced human populations, and water loss through evaporation.
Water conservation is often the cheapest and most effective way to increase water supplies. Watershed management involves coordinated planning to improve resource allocation and reduce water loss. Efficient household appliances, such as toilets, shower heads, and laundry machines, have greatly reduced per capita consumption in many cities. Dry landscaping is required in some southwestern communities of the United States. Drip irrigation and other agricultural practices can reduce irrigation demands. Water policy- laws regarding water rights and use- is a key factor in conservation.
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