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Leon 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 earth’s 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 earth's 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 eential for cleanline 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6.5～7.5万加仑。
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 eential 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3.5～5.5 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.Carele 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。
Leon 18 evapotranspiration 腾发量
Evaporation is the transfer of water from the liquid to the vapor state.Transpiration is the proce 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 procees, 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 preure at the surface and vapor preure in the overlying air(Dalton ’s law).In still air, the vapor-preure 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 eentially the evaporation of water from the leaves of plants.Rates of transpiration will, therefore, be about the same as rates of evaporation from a free water surface if the supply of water to the plant is not limited.Estimated free water evaporation may, therefore, be aumed to indicate the potential evapotranspiration from a vegetated soil surface.蒸腾从本质上是指水分从植物的叶表面的蒸发，因此，如果植物供水不受限制，蒸腾速率大约等于自由水面的蒸发率。因此，估计的自由水面蒸发量可以假定代表有植物的土壤表面的潜在腾发量。
The total quantity of transpiration by plants over a long period of time is limited primarily by the availability of water.In areas of abundant rainfall well distributed through the year, all plants will transpire at about the same rates and the differences in total will result from the differences in
the length of the growing seasons for the various species.Where water supply is limited and seasonal, depth of roots becomes very important.Here, shallow-rooted graes wilt and die when the surface soil becomes dry while deep-rooted trees and plants will continue to withdraw water from lower soil layers.The deeper-rooted vegetation will transpire a greater amount of water in the course of a year.The rate of transpiration is not materially reduced by decreases in soil moisture until the wilting point of the soil is reached.整个长时期的植物蒸腾总量主要受水的可获得量限制。在雨量充足且年内分布均匀的地区，所有植物都将以大致相同的速度蒸腾，蒸腾总量的不同是由于各种植物生育期长短不同而引起的。对于供水受限制和季节性（降雨）的地方，植物根系的深度是很重要的。在这样的地区，当表土变干时，浅根系的草凋萎后便死亡，而根深的树木和作物将继续从深层土壤中吸取水分。深根植物在一年蒸腾过程中将蒸腾掉大量的水分。在土壤含水量达到凋萎点之前，植物的蒸腾速度实质上不会因土壤水分的减少而减少。
Evapotranspiration, sometimes called consumptive use or total evaporation describes the total water removed from an area by transpiration and by evaporation from soil, snow, and water surfaces.An estimate of the actual evapotranspiration from an area can be made by subtracting measured outflow from the area(surface and subsurface)from the total water supply(precipitation, surface and subsurface inflow, and imported water).Change in surface and underground storage must be included when significant.腾发量有时被称为耗水量或总蒸腾量，表示某一区域植物蒸腾、土壤蒸发、雪蒸发和自由水面蒸发的总水量。某一区域上的实际腾发量可以从本区总的供水量（包括降雨、地面和地下流入量以及引入的水量）减去从本区实测流出量（地表和地下流出量）来进行估算。当地表和地下储存量的变化显著时必须包括在腾发量之内。
Several attempts have been made to relate evapotranspiration to climatological data through simple equations such as用最简单的方程表示腾发量与气象数据之间的关系，人们已经进行了许多尝试，如：
Where: is the consumptive use in feet and is the sum of the growing season maximum temperatures le 32ºF.With in centimeters and temperatures in degrees Celsius, Eq.(18.1)becomes式中： 表示耗水量，单位是英尺； 代表植物生长季节最高温度减去32ºF的累计值。当 以厘米为单位，温度由ºC表示时，方程(18.1)变为
Such formulas agree fairly well with average values of annual evapotranspiration over a period of years, but it clear that the evaporative proce is too complex to be well defined by a simple temperature function.这些公式与多年某一阶段的年平均腾发量吻合良好。但由于蒸发过程的复杂性，很显然，我们不可能用简单的温度函数去很好的定义腾发量。
As indicated before, the potential evapotranspiration from an area can be estimated from the free water evaporation.Actual evapotranspiration equals the potential value as limited by the available moisture.On a natural watershed with many vegetal species, it is reasonable to aume that evapotranspiration rates do vary with soil moisture since shallow-rooted species will cease to transpire before deeper-rooted species.A moisture-accounting procedure can be established using the continuity equation
如前所述，某一区域的潜在腾发量可以用自由水面腾发量来估算。实际腾发量等于有效水量受限时的潜在腾发量。对于一个有多种植物的自然流域，假设腾发速率确实随土壤水分的变化而变化是合理的。这是因为浅根植物将在深根植物前停止蒸腾。我们可以用连续方程建立水量平衡方程：Where: is precipitation;is surface runoff;is subsurface outflow;is actural
evapotranspiration;is the change in moisture storage.式中： 表示降雨量； 表示地表径流量； 表示地下流出量； 表示实际腾发量； 表示水分储存的变化。is estimated as（可用下式估算：）
Where: is the computed soil moisture storage on any data and is an aumed maximum soil moisture content.A moisture-accounting procedure of this type may be used to calculate runoff as well as estimate evapotranspiration.式中： 表示任何计算的土壤水分储存量； 表示一假定的最大土壤含水量。这个典型的水量平衡方程可用于计算径流量和估算腾发量。
