托福写作 7、8课程

　　例二：
　　READING (Time-3 minutes)
　　Irrigation, the artificial watering of land for agriculture, uses water from a number of sources: direct rainfall.
　　例二：
　　READING (Time-3 minutes)
　　Irrigation, the artificial watering of land for agriculture, uses water from a number of sources: direct rainfall, direct stream flow, water stored in lakes and reservoirs, high-quality groundwater, brackish surface water, and even seawater. Water for irrigation is diverted from rivers and lakes or pumped underground. Different crops have different irrigation requirements, so there are many forms of irrigation and types of irrigation technology.
　　Various methods of surface irrigation deliver water to a field directly from a canal, well, or ditch. The surface technique of flooding large fields is widely used because of low capital costs and long tradition. Furrow irrigation, practiced since ancient times, involves digging numerous U-or V-shaped open furrows through irrigated land and introducing water into them from a channel at the top of a field. As with other surface techniques, water collects into ponds on the field. In surface-pipe irrigation, the water is piped to the field and distributed via sprinklers or smaller pipes.
　　Border irrigation is a type of surface irrigation that involves flooding land in long parallel strips separated by earth banks built lengthwise in the direction of the slope of the land. Water flows from the highest point in the field to the lowest. Basin irrigation is similar to border irrigation but includes earth banks constructed crosswise to those used for border irrigation, dividing a field into a series of basins that can be separately irrigated.
　　LISTENING AND WRITING
　　(narrator)．
　　Now listen to part of a lecture on the same topic.
　　(Professor)
　　One thing that really concerns water resource analysts is how much water agriculture uses. Agriculture uses a lot of water, more than all other water-using sectors of society. One of our greatest concerns is the very high use of water by irrigation. This is because, in most cases, the water used for irrigation can’t be used afterward for other purposes, such as water supply for homes or industry.
　　Some forms of irrigation use water more efficiently than others. The efficiency of water use varies by region, crop, agricultural practice, and technology. The least efficient types of irrigation are the surface methods. Your reading really didn’t go into this, but think of how much water it takes for a traditional surface method like field flooding. It takes a lot of water to flood a field. The water collects into ponds or basins, but then most of it either evaporates into the air or passes down through the soil into groundwater. This means that, in lots of places, less than half of all the water applied to a field is actually used by the crop. The rest is lost to evaporation or to groundwater. All of the flooding methods generally waster a lot of water-water that could otherwise be used for other purposes.
　　Fortunately, there are several irrigation technologies that are more efficient than the poorly controlled and highly wasteful flooding methods. They range from sprinkler systems to drip irrigation. In sprinkler systems, water is sprayed over crops, and this provides an even distribution of water. New precision sprinkler technologies have greatly improved our ability to deliver water exactly when and where it’s needed. However, sprinkler systems are also a form of surface irrigation, and just as in other surface methods, some of the water is still lost to evaporation.
　　(Narrator)