托福考试写作例文1
发布时间：2013/7/17 21:41:04 来源：城市网学院 编辑：admin
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.
