Methods of Irrigation

The methods by which irrigation is applied to the land should depend, under ideal conditions, on individual land features such as the slope of the land, the crops to be irrigated, the nature of the water-supply and the ability of the soil to absorb and hold water.

There are four general methods of applying water: 1) by flooding, thus wetting all the 1and surface; 2) by furrows, thus wetting only part of the ground surface; 3) by sprinkling, in which the soi1 is wetted with а spray; 4) by subirrigation, in which the soil is wetted on1y а little if at all, but in which the subsoil is saturated. The first three methods come under the general heading of surface irrigation.

Flood irrigation generally requires large streams or canals, gentle topography(ground slopesshould usually be nogreater than three per cent) and careful levelling of the land. In theory, it should be possible to ensure that every part of the area to be irrigated absorbs the predetermined amount of water; but in practice, although all parts usually receive an adequate amount, some receive too much. For this reason, flood irrigation is more suited to close-growing crops like rice.

Furrow irrigation is а method by which water is run in furrows, normally made by cultivating between crop rows. The earth is thrown up into ridges between the furrows and the seeds are planted in the centre of the ridges. Furrow irrigation is very common because it is adaptab1e to а great variety of land slopes and soi1 textures and can be used with either large or small streams of irrigation water.

Difficulties may arise with the use of furrow irrigation on unsuitable soils. If the soil is very pervious, the water running along the furrows may sink vertically into the soil without ever reaching the centre of the ridges where the seeds are sown. On the other hand, the soil may be so impervious that the water does not reach the centre of the ridge and the seeds do not germinate anyhow. А more general disadvantage of furrow irrigation is that to ensure that the whole of the irrigation area receives enough water, it is almost always necessary to overwater some parts.

Sprinkler or spray irrigation applies water to the surface of the soi1 in the form of а spray, and is а form of artificial rain. А common type of sprinkler head is the revolving one which distributes water radially. This form of irrigation has advantages over other surface irrigation. It can be adapted for use on almost a11 types of soil, especially sandy soils which absorb water rapidly. It enables erosion to be controlled on steep land and can often be used on soils that are too shallow too steep or too rolling to be irrigated by surface methods. Sma11 streams of water can be used efficiently with this method, and it is adaptable to all the major crops with the exception of those, 1ike rice, that require standing water. А uniform application of water is made possib1e, and the amount and timing of the irrigation can be easily controlled. Land is not needed for irrigation structures, and so larger areas аге available for cropping.

There are, however, а number of disadvantages of sprinlkler systems. The water distribution is easily affected by the wind, which may disturb the pattern of wetting so that some parts gettoo much water and some too little.

I. Найдите в тексте слово, которому соответствует следующее словарное определение:

The moving of soi1 in ргерагing land for irrigation to produce а leve1 or uniformly sloping surface

Date: 2015-12-11; view: 661