Lesson 1 importance of water 水的重要性Water is best known and most abundant of all chemical compounds occurring in relatively pure form on the earths surface. Oxygen, the most abundant chemical element, is present in combination with hydrogen to the extent of 89 percent in water. Water covers about three fourths of the earths surface and permeates cracks of much solid land. The polar regions are overlaid with vast quantities of ice, and the atmosphere of the earth carries water vapor in quantities from 0.1 percent to 2 percent by weight. It has been estimated that the amount of water in the atmosphere above a square mile of land on a mild summer day is of the order of 50,000 tons.在地球表面以相对纯的形式存在的一切化合物中，水是人们最熟悉的、最丰富的一种化合物。在水中，氧这种最丰富的化学元素与氢结合，其含量多达89%。水覆盖了地球表面的大约3/4的面积，并充满了陆地上的许多裂缝。地球的两极被大量的冰所覆盖，同时大气也挟带有占其重量0.1%2%的水蒸气。据估计，在温暖的夏日，每平方英里陆地上空大气中的水量约为5万吨。All life on earth depends upon water, the principal ingredient of living cells. The use of water by man, plants, and animals is universal. Without it there can be no life. Every living thing requires water. Man can go nearly two months without food, but can live only three or four days without water.地球上所有的生命都有赖于水而存在，水是活细胞的基本组分（要素）。人类、植物和动物都得用水。没有水就没有生命 。每一种生物都需要水。人可以接近两个月不吃食物而仍能活着，但不喝水则只能活三四天。In our homes, whether in the city or in the country, water is essential for cleanliness and health. The average American family uses from 65,000 to 75,000 gallons of water per year for various household purposes.在我们的家庭中，无论是在城市还是农村，水对于卫生和健康来说都是必不可少的。美国家庭的年平均用水量达6.57.5万加仑。Water can be considered as the principal raw material and the lowest cost raw material from which most of our farm produces is made. It is essential for the growth of crops and animals and is a very important factor in the production of milk and eggs. Animals and poultry, if constantly supplied with running water, will produce more meat, more milk, and more eggs per pound of food and per hour of labor.水可以被认为是最基本的和最廉价的原料。我们的农产品，大部分都是由它构成的。水是农作物和动物生长的要素，也是奶类和蛋类生产的一个很重要的因素。动物和家禽，如果用流动的水来喂养，那么每磅饲料和每个劳动小时会生产出更多的肉、奶和蛋。For example, apples are 87% water. The trees on which they grow must have watered many times the weight of the fruit. Potatoes are 75% water. To grow an acre of potatoes tons of water is required. Fish are 80% water. They not only consume water but also must have large volumes of water in which to live. Milk is 88% water. To produce one quart of milk a cow requires from 3.5 to 5.5 quarts of water. Beef is 77% water. To produce a pound of beef an animal must drink many times that much water. If there is a shortage of water, there will be a decline in farm production, just as a shortage of steel will cause a decrease in the production of automobiles.例如，苹果含87%的水分，苹果树就必须吸收比苹果多许多倍的水分；土豆含75%的水分，那么种植每英亩土豆就需要若干吨水；牛奶含水量为88%，为了生产每夸脱牛奶，母牛需要3.55.5 夸脱的水；牛肉含77%的水，为生产1磅牛肉牛必须饮用许多磅水。如果缺水，就会使农产品减产，就像缺乏钢会引起汽车产量下降一样。In addition to the direct use of water in our homes and on the farm, there are many indirect ways in which water affects our lives. In manufacturing, generation of electric power, transportation, recreation, and in many other ways, water plays a very important role.水除了直接为我们的家庭和农场利用外，它还以许多间接的方式对我们的生活产生影响。在制造、发电、运输、娱乐以及其他许多行业，水都起着很重要的作用。Our use of water is increasing rapidly with our growing population. Already there are acute shortages of both surface and underground waters in many locations. Careless pollution and contamination of our streams, lakes, and underground sources has greatly impaired the quality of the water which we do have available. It is therefore of utmost importance for our future that good conservation and sanitary measures be practiced by everyone.我们对水的利用随人口的增长而迅速增加。在许多地方，无论地面水或地下水都已经严重短缺了。由于任意污染河流、湖泊和地下水源，已经大大地损害了人们能够利用的水的水质。因此，人人有责对水采取保护措施和卫生措施，这对于我们人类的未来是极端重要的。Lesson 18 evapotranspiration 腾发量Evaporation is the transfer of water from the liquid to the vapor state. Transpiration is the process by which plants remove moisture from the soil and release it to the air as vapor. More than half of the precipitation which reaches the land surfaces of the earth is returned to the atmosphere by the combined processes, evapotranspiration. In arid regions evaporation may consume a large portion of the water stored in reservoir.蒸发是水从液态变成气态的过程。蒸腾是植物将从土壤中吸收的水分以汽体的形式释放到大气中去的过程。到达地面的降雨有一半以上以蒸发与蒸腾相结合的形式腾发，回到大气中去。在干旱地区，蒸发可以消耗掉大量的水库存水。The rate of evaporation from a water surface is proportional to the difference between the vapor pressure at the surface and vapor pressure in the overlying air (Dalton s law). In still air, the vapor-pressure difference soon becomes small, and evaporation is limited by the rate of diffusion of vapor away from the surface. Turbulence caused by wind and thermal convection transports the vapor from the surface layer and permits evaporation to continue.水面蒸发率与水表面水气压与上层空气大气压的差成比例（道尔顿定律）。在静止的空气中，气压差不久就变得较小，蒸发受水面水汽弥散速率的限制。由风引起的扰动和热对流将表层的水变为水汽，（从而）使蒸发得以持续下去。Transpiration is essentially the evaporation of water from the leaves of plants. Rates of