FORMS OF IRRIGATION Basin are two main forms of irrigation. In the more ancient system the area to be served is surrounded by embankments, and is thus called a "basin"; into it a canal leads the waters from the river until a depth of 3 ft. or 4 ft. is attained. In Egypt many of the basins are of great size, some covering as much as 50,000 BC. ; the water is run off after a time and the ex posed area is sown with crops which need no further watering to bring them to maturity. The system obviously originated in the fact that a river can be easily made to give a supply once a year during its flood period, whereas enormous expense would be re quired to make it do so at other times. It happens, also, that the production of food crops follows the flood season in rivers like the Nile ; very naturally, therefore, the system devised was one which took advantage of a river's flood season.
Fig. 2 will serve to explain this system of irrigation, the firm lines representing canals, the dotted lines embankments. It will be seen, beginning on the east or right bank of the river, that a high level canal from an upper system divides into two. The right branch waters all the desert slopes within its reach. The left branch passes by a syphon aqueduct, under the main canal of the system, taken from the river close at hand (and therefore at a lower level). This left branch irrigates the high lands bordering on the river. In years of very favourable flood this high level canal may not be wanted at all ; irrigation could be done from the main canal, and with this great advantage, that the main canal water would carry with it much more silt than would be got from the tail end of the high level canal, which left the river perhaps 25 m. further upstream. The main canal flows freely
over the areas C and D and if the flood is good, over B and a part of A. It is carried round the next desert point, and to the north becomes the high level canal. The masonry works required for this system are a syphon to pass the high level under the main canal near its head, bridges fitted with sluices where each canal passes through an embankment, and an escape weir at the tail of the system, just south of the desert point, to return the water to the river.
The diagrams (figs. 2 and 3) show a perennial canal system. Only the main canals and branches are shown as such a system may extend over any distance up to 200 m. or even more. This plan also shows how efficient drainage should be provided be tween the canals. The more detailed canalization from the branches is shown in fig. 4.