98 chapter 3 Landscapes Fashioned by Water 98 chapter 3 Landscapes Fashioned by Water Sculpturing Earth's Surface Groundwater
figure 3.33 A wintertime eruption of Old Faithful, one of the world's most famous geysers. During a typical eruption it emits as much as 45,000 liters (almost 12,000 gallons) of hot water and steam. (Photo by Art Director & Trip/Alamy) The most common method for removing groundwate well, a hole bored into the zone of saturation. Wells s small reservoirs into which groundwater migrates ar which it can be pumped to the surface. The use of wel back many centuries and continues to be an important: of obtaining water. By far the single greatest use of this \ the United States is irrigation
for agriculture. More thar cent of the groundwater used each year is for this purf dustrial uses rank a distant second, followed by the i used by homes in cities and rural areas. The water-table level may fluctuate considerat ing the course of a year, dropping during dry seas< rising following periods of precipitation. Therefon sure a continuous supply of water, a well must p< below the water table. Whenever a substantial am water is withdrawn from a well, the water table aro well is lowered. This effect, termed drawdown, d< with increasing distance from the well. The result pression in the water table, roughly conical in shape as a cone of depression (figure 3.34). For most si mestic wells, the cone of depression is negligible. E when wells are used for irrigation or for industrial p the withdrawal of water can be great enough to crea wide and steep cone of depression that may subs lower the water table in an area and cause nearby wells to become dry. Figure 3.34 illustrates this situc surrounding rock. At the bottom of the chamber, the water is under great pressure because of the weight of the overlying water. This great pressure prevents the water from boiling at the normal surface temperature of 100°C (212°F). For
example, at the bottom of a 300-meter (1000-foot) water- filled chamber, water must attain a temperature of nearly 230°C (450°F) before it will boil. The heating causes the water to expand, with the result that some is forced out at the surface. This loss of water reduces the pressure on the remaining water in the chamber, which lowers the boiling point. A portion of the water deep within the chamber quickly turns to steam and causes the geyser to erupt. Following the eruption, cool groundwater again seeps into the chamber, and the cycle begins anew. Sculpturing Earth's Surface Groundwater In most wells, water cannot rise on its own. If waf encountered at 30 meters (100 feet) depth, it remai level, fluctuating perhaps a meter or two with sea; and dry periods. However, in some wells, water ris times overflowing at the surface. The term artesian is applied to any situation groundwater rises in a well above the level where • dally encountered. For such a situation to occur, two ( must exist (figure 3.35): (1) Water must be confi
Pressure surface (level to which water will rise)Water tank Well Water is pumped into • tank
You've reached the end of your free preview.
Want to read all 41 pages?
- Spring '19
- Tom Pope