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Modified Agricultural Practices Since agriculture accounts for nearly 70 percent of the world’s fresh water withdrawn from r

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问题     Modified Agricultural Practices
    Since agriculture accounts for nearly 70 percent of the world’s fresh water withdrawn from rivers, lakes, and underground aquifers for human use, the greatest potential for conservation lies with increasing irrigation efficiency. By reducing irrigation by 10 percent, we could double the amount available for domestic water worldwide. This can be done by converting to water-conserving irrigation systems; taking the poorest and steepest lands out of production; switching to less-thirsty crops (which may require changes to government subsidies for certain crops); implementing proper agricultural land drainage and soil management practices, and reducing fertilizer and pesticide use.
    Typically, governments provide water to large commercial farmers at greatly subsidized rates, decreasing the need for conservation and promoting wasteful practices. This has led to widespread use of wasteful irrigation systems. Studies show that just 35-50 percent of water withdrawn for irrigated agriculture actually reaches the crops. Most soaks into the ground through unlined canals, leaks out of pipes, or evaporates before reaching fields. Although some of the water lost in inefficient irrigation systems returns to streams or aquifers where it can be tapped again, water quality is invariably degraded by pesticides, fertilizers and salts. This is in fact another way that commercial agriculture "uses" water: by polluting it so that it is no longer safe to drink. In areas where commercial agriculture is prevalent, runoff from farms has poisoned water supply with dangerous levels of toxics.
    Poorly planned and poorly built irrigation systems not only harm water quality, but can also irreparably harm the crop-growing capability of the land through salinization. Especially in arid areas, salts that occur naturally accumulate in irrigated soils. Poorly drained irrigation water can pollute water supply, and raise the groundwater table until it reaches the root zone, waterlogging and drowning crops. Globally, some 80 million hectares of farmland have been degraded by a combination of salinization and waterlogging.
    Switching to conserving irrigation systems has the biggest potential to save water used for agriculture (experts say drip irrigation could potentially save 40-60 percent of water now used for agriculture). The most common water-conserving irrigation systems are some form of drip irrigation (also called micro-irrigation). Conventional sprinklers spray water over crops, not only irrigating more land than is needed to grow the crop but also losing much to evaporation. Drip irrigation, however, supplies water directly to the crop’s root system in small doses, where it can be used by the plant’s roots. Water is delivered through emitters that drip water at each plant, or perforated piping, installed on the surface or below ground. This keeps evaporation losses low, at an efficiency rate of 95 percent.
    Although by 1991 some 1. 6 million hectares were using drip irrigation worldwide, this is still less than one percent of all irrigated land worldwide. Some countries have made drip irrigation a serious national priority, such as Israel, which uses drip irrigation on 50 percent of its total irrigated area. But clearly it is the exception, and most dry countries have a long way to go.
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答案 改良的农业耕作 从全世界河流、湖泊和地下蓄水层汲取的供人类使用的淡水中，有近70％是用于农业，因此节水的最大潜力在于提高农业灌溉效率。减少10％的灌溉用水，我们就能够使世界各地的家庭用水量增加一倍(翻一番)。我们可以改用节水的灌溉系统，停止耕种(停止使用)最贫瘠、最陡峭的土地，再种植耗水量较低的植物(这可能会要求改变政府对某些农作物的津贴额)；采取适当的农业土地排水及土壤管理办法；减少化肥和杀虫剂的使用。 通常，政府以很高的津贴优惠向大户的商业性农民供水，这就削弱了节水的必要性，鼓励了浪费行为。这种做法已导致农民广泛使用浪费水资源的灌溉设备。研究表明，农业灌溉用水中只有35％~50％真正用在农作物上。大部分的水通过没有铺衬里的灌溉渠道浸入土壤，或从水管泄漏出来，或在到达农田之前就蒸发掉了。尽管从低效的灌溉系统中损失的部分水，会重新返回可再次利用的(可再次抽取的)河流或地下蓄水层中，但其水质因杀虫剂、化肥和盐分的渗入却不可避免地下降了。这实际上就是商业性农业“使用”水的另一种方式：对水造成污染，使水无法再安全饮用。在商业性农业盛行的地区，从农田中流出的水已经污染生活供水，使得水中的有毒物质达到危险的程度。 计划不周或建造不好的灌溉系统不仅会破坏水质，还会由于土壤碱化而损害土壤的作物生长能力，这是不可复原的。尤其是在干旱地区，天然生成的盐会在灌溉过的土壤中越积越多。排放不畅的灌溉用水会污染供水，提高地下水位，直到地下水位达到作物的根区，浸透土壤，淹死农作物。全球约有8000万公顷农田因土壤碱化和浸透而退化。 转而使用节水的灌溉系统可以最大限度地节省农业用水(专家声称，水滴灌溉有可能节省40％～60％现在所用的农业用水)。最常使用的节水灌溉系统是某种水滴灌溉系统(也称作“微灌溉”)。传统的洒水装置在农作物上方喷水，不仅灌溉了农作物生长所需的土地范围以外的土地(不仅使得到灌溉的土地超出了农作物生长所需的土地范围)，而且使得大量水分蒸发(而且由于水分蒸发而损失了更多的水)。然而，滴灌是直接向植物的根系提供少量的水(一点一点地提供水分)，植物的根部能够吸收到这些水。水通过滴灌装置传递到每株植物，或通过安装在地表或地下的穿洞管道进行传递。这将水分蒸发的损失控制在较低量，从而用水效率提高到95％。 尽管到1991年之前，世界各地已有约160万公顷农田采用水滴灌溉，但这仍不到全世界灌溉土地面积的1％。一些国家已正式将水滴灌溉列为全国重点推广事项(全国优先考虑的事项)，例如以色列总灌溉面积的50％都采用水滴灌溉。但是很明显，这只是个特例，大多数干旱的国家还有很长的一段路要走。
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