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49 Irrigation in Afghanistan The dry summer conditions necessitate irrigation of farm lands by means of streams which get water from the rivers or directly from the melting snow. This type of irrigation is generally confined to the high mountain districts. The inhabitants of these districts have shown remarkable skill in conducting water along the ledges. Another system, very ingenuous and ancient, is known by the name of "Karez". It means conducting water through underground canals for the purpose of irrigation. This system is in vogue in various parts of the country, chiefly at the foot of mountains and in broad valleys surrounded by hills. The spot, where the water is to issue, must always be at the foot of a slope extending to a hill, and the ground must be examined to ascertain whether there are springs, and in what direction they lie. In the selection of a site for a Karez the excavators are guided solely by experience and have no scientific appurtenances for assistance. A site is generally taken along the bank of a hill torrent or when such a torrent debouches from the mountains into the valley. Note is also taken of the appearance of the moisture and of the presence of moisture-loving shrubs and grasses. The next step is to dig one or two trial wells, and if they prove successful, a succession of wells are dug and connected by tunnels until the water reaches the surface. When a Karez is once constructed, its wells are of no further use, except to allow a man to descend occasionally to clear out the channel. The distances between the wells varies from 50 to 100 yards. Sometimes these tunnels have been pushed back as much as twenty miles.