12 THE IRRIGA 1 1ON A OE 



them, and to requiring th.e minimum of periodical repairs. While it 

 is usual for each canal to be closed entirely at its head, for a few 

 weeks, at some time in each year, to enable ordinary repairs and 

 clearances of the bed to be carried out, yet it may happen any year 

 through a failure of the rains, that the canal has to be kept in full 

 flow at that time to save the crops dependent on it. In such a case, 

 the usual annual repairs, must be postponed, perhaps for many 

 months; and the canal should be able to do its duty efficiently without 

 them. Deep and solid foundations are therefore given to all works 

 where any scour in the bed close by, is possible from any cause. 



In the southwest regions of the Panjab, and throughout the 

 province of Sindh the annual rainfall is so slight, averaging less than 

 six inches, that without irrigation no crops can be grown. Along the 

 strips of bottom land bordering the great rivers, the spring level is 

 sufficiently near the ground surface to allow of water being profitably 

 lifted from wells by bullock power- Close to the river the amount of 

 life may not be more than ten or fifteen feet, but it rapidly increases 

 with the distance from the river, and would usually exceed thirty feet 

 at a distance of three miles, the land being fairly level along a line at 

 right angles to the river throughout the width of the bottom land of 

 the river valley. With a greater lift than forty feet but little irriga- 

 tion can be done during the intense dry heat of the hot weather in the 

 plains of North India. These plains are less than 1,000 feet above the 

 sea level; and in the arid regions now alluded to, only strips of land a 

 few miles wide bordering the great rivers, can be cultivated without 

 canal irrigation. For the tracts further distant, numerous small in- 

 undation canals take out from the rivers Indus, Jhelam, Chanab, Ravi 

 and Satlaj. The largest are about fifty or sixty miles in length, have 

 bed widths up to eighty feet, can carry a full supply depth of from 

 eight to ten feet, with a discharge up to about. 2,000 cubic feet per 

 second. Some are quite small, with a length of only about ten miles, 

 and a bed width of eight or ten feet. Many of these were made and 

 were in use by the people long before the commencement of British 

 rule. They were usually very badly aligned, and crossed drainages 

 or low ground with weak embankments which often breached, so that 

 they gave great trouble. Ever since the annexation of the country 

 the engineers in charge have been busy in improving and extending 

 these; amalgamating two or more small adjacent ones by giving them 

 a common head channel. None at first had any head regulator, or 

 any head works; they were simply open ditches fed from the river, 

 and their supply fluctuated with it. But by this time most have been 

 provided with head regulators to control the discharges and keep out 

 excessive 'flood supplies which were always a source of danger. These 

 regulators cannot be built at the very commencement of a canal, as 

 they would be destroyed before long by river erosion, which may cut 

 away from a quarter to a half of a mile in width of land along a river 

 bank in one year. They have to be built at some distance off, so as 

 to be always safe from river erosion. 



(Continued next month.) 



