水力学考博专业英语

水力学考博专业英语 LESSON 1 WATER In the winter of wet years the streams ran full-freshet, and they swelled the river until it sometimes raged and boiled bank full, and then it was a destroyer. The river tore the edges of the farm lands and washed whole acres down; it toppled barns and houses into itself, to go floating and bobbing away. It trapped cows and pigs and sheep and drowned them in its muddy brown water and carried them to the sea. Then when the late spring came the river drew in from its edges and the sand banks appeared. And in the summer the river didn't at all run above ground. There were dry years too... The water came in a thirty-year cycle. There would be five or six wet and wonderful years when there might be nineteen to twenty-five inches of rain, and the land would shout with grass. Then would come six or seven pretty good years of twelve to sixteen inches of rain. And then the dry years would come, and sometimes there would be only seven or eight inches of rain. The land dried up... And it never failed that during the dry years the people forget the rich years, and during the wet years they lost all memory of the dry years. It was always that way. East of Eden, Iohn Steinbeck, 1952 J ohn Steinbeck wrote the above words 50 years ago to describe the hydrology of the Salinas Valley in northern California. In doing so he revealed an attitude toward water that was held by many in the early part of the twentieth century. Water was always assumed to be available and no worried about its longevity until it seemed threatened. We perhaps have a more realistic attitude today and know that we must preserve and protect our precious and limited natural resources, including water. Although our intentions toward preserving the environment may be good, we sometimes act without full consideration of all possible outcomes. In 1990, Congress passed the Clean Air Act. To reduce the mass of smog-creating chemicals released by vehicles, gasoline sold in certain urban areas was required reformulated, starting in 1992, so that it contained at least 2% oxygen. At the time there were only two chemicals considered practical to add to gasoline, ethanol and methyl tertiary butyl ether (MTBE). At that time, no one knew if MTBE posed any potential health risks if ingested, but its high solubility in water was known. In addition, it was well known that many gasoline retailers had leaking underground storage tanks. By 1996, about 100 million barrels of MTBE were used to formulate gasoline in the United States (Andrews 1998). Reformulated gasoline contains 10% MTBE. While air quality has improved in urban areas where MTBE is used in reformulated gasoline, not surprisingly we now find that ground water in some areas has been contaminated with it. Most chemicals found in gasoline degrade rather quickly in the earth, but not MTBE; it is persistent as it resists biodegradation. As of 2000 there are still no federal drinking-water standards for MTBE; the toxicity is still being evaluated. Yet, legislation was passed a decade ago that could reasonably have been expected to result in the release of MTBE into ground water. In the spring of 2000 the Environmental Protection Agency (EPA) decided to phase out the use of MTBE in gasoline due to ground-water contamination. The lesson to be learned here is even the best of intentions can have unanticipated and extremely undesirable consequences on our limited water resources. Water is the elixir of life; without it life is not possible. Although many environmental factors determine the density and distribution of vegetation, one of the most important is the amount of precipitation. Agriculture can flourish in some deserts, but only with water either pumped from the ground or imported from other areas. Civilizations have flourished with the development of reliable water supplies—and then collapsed as their water supplies failed. This book is about the occurrence of water, both at the surface and in the ground. A person requires about 3 quarts (qt) or liters (L) of potable water per day to maintain the essential fluids of the body. Primitive people in arid lands existed with little more than this amount as their total daily consumption. A single cycle of an older flush toilet may use 5 gallons (gal) (19 L) of water. In New York City the per capita water usage exceeds 260 gal (1000 L) daily; much of this is used for industrial, municipal, and commercial purposes. For personal purposes, the typical American uses 50 to 80 gal (200 to 300 L) per day. Even greater quantities of water are required for energy and food production. In 1995, the total off-stream water use in the United States was estimated to 402 billion gallons (1520 billion liters) per day of fresh and saline water. This does not include water used for hydroelectric power generation and other in-stream uses, but does include water used for thermoelectric power plant cooling. Fresh-water use in 1995 included 77.5 billion gallons (290 billion liters) per day of ground water and 263 billion gallons (995 billion liters) per day of surface water (Figure 1.1). Per capita fresh-water use was 1280 gal (4850 L) per day. Consumptive use of water, that is, water evaporated during use, was about 81 billion gallons (300 billion liters) per day (Solley, Pierce, & Perlman 1998). FIGURE 1.1 Trends in fresh ground and surface water withdrawals and population in the United States. Total water use in the United States peaked in 1980 and has declined since then. The estimated total water use in 1995 was 2% less than in 1990 and 10% less than in 1980. Water use for public water supply has shown a continual increase since 1950 due to increasing population. Public water supply (40.2 billion gallons in 1995) accounts for 10% of total water use in the United States. The largest uses of water are for cooling of electric power generation facilities and for irrigation. Although it had generally been assumed that economic growth results in increased water use, from 1975 to 1995 per capita water use in the United States actually declined by 25%. This can be attributed to increased conservation of water (Wood 1999). As an example, toilets now sold in the United States can use no more than 1.5 gal. (6 L) per flush. An account of the water supply of the world would reveal that saline water in the oceans accounts for 97.2% of the total. Land areas hold 2.8% of the total. Ice caps and glaciers hold 2.14%; ground water to a depth of 13,000 feet (ft) [4000 meters (m)] accounts for 0.61% of the total; soil moisture, 0.005%; fresh-water lakes, 0.009; rivers, 0.0001%; and saline lakes, 0.008% (Feth 1973). More than 75% of the water in land areas is locked in glacial ice or is saline (Figure 1.2).