Chapter 10 Water: Hydrologic Cycle and Human Use By: Spencer Tungate A vital resource • Water is a vital resource that is essential to life. Of the 1.4 billion cubic kilometers of water that covers Earth’s Surface, only 2.5% of this is fresh water- water with a salt content less than .1% Developed and Developing Countries • Developing countries, in contrast to developed ones, are struggling with water issues. 1.1 billion people still lack access to safe drinking water and 2.6 do not have access to adequate sanitation services. Hydrologic Cycle • Earth’s water cycle is also known as the hydrologic cycle. This cycle involves evaporation, condensation, and precipitation. Terms of Hydrologic Cycle • Water vapor- water molecules in a gaseous state- is the results from evaporation. Also, humidity is the amount of water vapor in the air (usually measured in relative humidity). • Condensation is facilitated by aerosolsmicroscopic liquid or solid particles that attract water vapor. The processes of evaporation and condensation create purification of water. Hadley Cells • Convection currents create Hadley cells- the system composed of rising and falling air. These cells spill dry air to the north and south of the equator, creating desserts (Sahara dessert). Also, rain shadow effects occur in mountainous areas. This effect creates a dry region downwind of a mountain range. Groundwater • Water that has percolated through the water (becoming gravitational water) eventually collects on an impervious layer of rock or dense clay, creating groundwater. This groundwater’s upper surfaces are referred to as the water table. If the water does not percolate, it is instead capillary water and returns to the atmosphere through evapotranspiration. Water as a resource • Worldwide, the largest consumptive use for water is for irrigation (70%), second is for industry (20%), and third is for direct human use (10%). 37% comes from groundwater sources and 63% comes from surface waters (lakes, streams, reservoirs). Dams • Dams provide many benefits, such as providing electricity and diverting water for irrigation and other uses. However, many ecological concerns have been expressed over the building of dams, such as the damage to estuaries. However, dams such as the Three Gorges Dam, are making headway by capturing runoff and aiding flooding. Salt problems • Saltwater intrusion is contaminating many freshwater sources. Desalination plants are being built for this intrusion, however, these plants and technologies are very costly. Sustainability • To help provide water for many parts of the world many steps must be taken. Using less water, such as in drip irrigation, will help water sources to be diverted to more vital things. Also, tapping more groundwater can be a major source for freshwater. Municipal systems must be made that practice sustainability, such as the WaterSense commode. Laws • Clean Water Act- Allows EPA to develop programs and rules to carry out its mandate for oversight of the nation’s water quality, but not quanitity. • Many Millennium Development Goals are targeting freshwater to impoverished countries.