IRRIGATION.

Otago Witness, Issue 3115, 26 November 1913, Page 17

IRRIGATION.

Bv John M’Keagge.

IN PRACTICE.

[Copyright.] IV. A further useful lesson regarding alluvial flats and the country between mountain ranges when the rainfall is deficient mav be learnt from' the case of bpain, which has always been a land of extremes. It has one of the finest of climates. Its soil, which consists chietty of alluvial flats from 3ft to 10tt deep, will grow anything, yet a scanty rain a 1 reduced unirrigated Spain to sterility and barrenness. But, irrigated, Spain produces plentifully the olive tree, the a me, the mulberry tree to feed silkworms, as well as dates, wheat, rice, cotton, and sugar-cane. The country between the mountain ranges consists chiefly of tablelands, similar to those in Otago, but about 2000 ft above sea level. These tablelands slope abruptly towards the sea. River* fed bv thousands of streams and rivulets, in deep beds, drain the mountain ranges and meander their courses to the ocean. These steep slopes do not retain moisture. When the storm is over the rivulets, the streams, and the rivers are foaming ■watercourses for a few days, and then dry up. The area of Spain is 125,000,000 acres. Of these, 100,000,000 are lit tor cultivation, or for pasture. The rainfall of Spain was not always scanty. The cutting down of the forests in Cyprus and in other countries ruined the rainfall. So it has been in Spain, which formerly had extensive forests, which were recklessly cut down, and the usual disastrous consequences to the rainfall followed. The Castilian peasant thought himself very lucky, with his ruined rainfall, if he could get a scanty wheat crop once in two or three years. Water became so scarce that the peasant in many instances preferred to give the wayfarer a drink of wine rather than of water. Primitive methods of irrigation were resorted to, without producing any noticeable amelioration of the existing parched conditions. The rivers flowed noiselessly along, and conveyed the priceless water to the ocean, while a few yards away the struggling vegetation was burnt brown, and the germinating crops that with sufficient moisture would have yielded rich and abundant harvests assumed the sickly copper hue of dried and ruined vegetation. Yet, with a sufficient water supply, no soil could surpass that of Spain in yielding plentiful harvests. Matters became so serious that the Spanish Cortes voted £1,000,003 to subsidise new irrigation works by granting loans, without interest, in sums not exceeding £20,000. The effect of that legislation was that very large areas were soon irrigated. The first of these works was the Henarcs Canal, projected by the Iberian Irrigation Company. The Henarcs is a tributary of the River Tagus. The headworks of the canal are 50 miles distant from the city of Madrid, and the irrigation canal runs along the side of the Henarcs Valley. The headworks consist of a weir across the river. The weir raises the water 20ft above the ordinary river level', and an immense bodv of water is dammed and stored for irrigation purposes. Where the water is let into the irrigating canal from this dam eight sluice gates are built into the weir. The irrigating canal is about sft deep and Bft wide at the bottom, with sloping sides. Immediately after leaving the river the canal runs into a heavy cutting 16ft deep, and enters a tunnel 3062yds long. Six miles from the headworks the canal is carried across a mountain torrent in an iron aqueduct or trough. Immediately before the water flows into this aqueduct there is a waste weir or escape, by’ which any unusual flow of water finds its way into the mountain torrent, and by this means a constant flow is secured in the aqueduct. There are 20 waterfalls along the course of the canal, and the water-power thus developed is used to drive mills. Five miles from the weir the first branch-chan-nel for irrigating the land is taken from the canal. At the point in the side of the main canal where the branch channel is led out a masonry chamber is constructed. The water fills this, passes through a masonry grating, built across the chamber to stop the boiling action of the water, which escapes oyer a bevelled iron edge into the branch irrigating channel. The canal company is bound to carry these irrigating channels to the extreme limits of the land to be irrigated, but the owners or lessees must construct, at their own expense, the small private branches that serve to distribute the irrigating water over the land. Small sluice gates are placed in the side of ts! branch channel, where the private irrigating water-courses lead the -water from the branch channel to tlie land. At the beginning of the season the land owner or lessee has to send to the irrigating company’s agent an application for water, stating the area to be irrigated, and the number of waterings required, this being regulated by the kind of tlie crop, and t-he months when the water is required. An agreement is then made to supply and to receive and pay for the water. The irrigating company carefully makes its arrangements for irrigating for the season. It employs servants, known as water-guards, to regulate the supply of the water to those who have contracted to receive it. The guard and the man to receive the water each receives a check, which states the number of waterings, the times, and the hours when the supply is to be delivered. When the time arirves, or. presenting this check to the guard, the proprietor or lessee receives the stipulated supply of water. The Spaniards soon found that irrigating the land caused the exhaustion of the soil, and that it was necessary to keep cattle for manure, and to pay special attention to the manuring o! the land. The man who receives the water has to pay a stated sum per acre, and the canal company, in return, is obliged to a sufficient

supply for 12 waterings a year, each to consist of a sheet of water nearly 3in deep. Should the canal company be unable to supply sufficient water it is bound to refund a certain sum to the man deprived of sufficient water. When the supply runs short the water is fairly divided, and no preference is given to anyone. Here, as elsewhere experience has shown that constant irrigation causes the land to become soaked, and the water-table to come nearer the surface. Unless on open, sloping land it has therefore been found that any system of irrigation becomes injurious by waterlogging the land, unless accompanied by a system of drainage, it is easy to calculate how much land the water flowing in any canal will irrigate. The Benares' Canal had to supply a minimum of 140 cubic feet per second, but usually it supplied 300 to 400 cubic feet per second. In Spain it has been found that 1 cubic foot per second will irrigate 8 eacres each day. Therefore 300 cubic feet will irrigate 2400 acres each day. Now supposing the irrigating season lasts four months or 120 days, then 300 cubic feet per second will irrigate 2400 acres 120 times, which means that it will irrigate 288,000 acres once with a sheet of water about 3in deep. In Spain, after the advent of irrigation, the rent of land advanced from 12s an acre for land not irrigated to £3 7s per acre for irrigated land. None of the ordinary crops require more than four waterings, and allowing this, then the Henarcs irrigating canal increased the rent of the land it was capable of irrigating from £21,600 to £120,600, The cost of the canal was £120,000, and the interest on this at 5 per cent, was £6OOO. Working expenses, etc., must no added to this. The Spanish landlords raised the rents of irrigated land unduly, vet the increased yield was so great that irrigated farms were eagerly sought. Tne most recent system of irrigating is to be found in India, where watercourses are laid from the main canal, usually at distances 2 or 3 miles apart, and are constructed to run along the highest ridge of the land to which they bring the irrigating water. Small field watercourses lead from those all over the land. The water is let into them through small sluices, and the unused water from each chief watercourse passes on to' the next. In Central India, in the territory of Mairwara, which forms nart of the Aravulla hills, and through the middle regions of Rajpootana, a very arid district exists, 100 miles long by about 30 wide. Ridges of rock and low hills cut up the surface into a series of valleys. The soil contains the elements of fertility, but the rainfall is very deficient, and the slope of the surface is so great that when rain did fall it speedily flowed off. The habitable parts of the district were deficient even of drinking water. In cases of this nature, a strong masonry dam, massive enough to resist the rush of the stream, is generally constructed- Then, at varying distances along the course of the valley, embankments are constructed, and the force cf the water, the nature of the valley, and the elevation of the land to be irrigated, determine the nature of the works to be constructed. By the construction of a number of embankments the valley is. largely converted into a of reservoirs. In addition to the leading methods of damming rivers, and conveying the water. over the land bv means of such canals as that described, there are fopr additional methods or systems adopted in India—the well system, the land and reservoir system, the independent canal system, the Deltaic system. The well system is a very important source of water for irrigation purposes in India. The water is usually found at a distance from the surface that varies from 20 to 100 feet. All over India, wells having a diameter of from 4 to 6 feet are sunk and lined with brick. They are sunk on the highest land, so that their water may be easily conducted to the surrounding land. The irrigator may use the canal water to soak his land previous to ploughing, but as soon as lie sows his more valuable crops he will pay for the canal water, and leave it unused, preferring instead to provide the extra labour required to irrigate these crops from wells. One well usually will irrigate 10 or 12 acres, and many thousands of such wells are found all over India. The reservoir system is practically the same as the method already described in Central Asia. The independent canal system regards the river a«s the canal; and irrigating subcanals and watercourses are led directly from it. The Deltaic system consists of weirs in the river. They are often two or three miles in length, and of immense size and strength, with the usual sluice gates, and the other ordinary irrigating arrangements. Actual results of irrigating the land are as follow : (1) By irrigating to a depth of 4in, the land, which, nmvatered, yielded only 14 bushels of wheat per acre, yielded 26 bushels ; and allowing for charges, an increased return of £2 -an acre resulted. f2) Without irrigation, the land produced eight bushels of wheat per acre; irrigated, it produced 32 bushels, and allowing for extra cost, gave an increased return of over £3 an acre. (3) Unirrigated, the land produced four bushels of oats per acre ; irrigated, it produced 34. (4) Fifty acres, unirrigated, yielded no crop of wheat; irrigated, an adjoining 50 acres yielded 24 bushels an acre. (5) On a 350-acre farm 25 cows were kept, and the greatest profit derived from them in one year was £3BO. After irrigation over 60 cows were kept on the same area, and these yielded a gross return of over £BOO. (6) On a certain holding planted with vines, previous to irrigation the highest yield was 200 gallons per acre. The owner sunk Bft, when he came to solid rook He then sunk through 60? t of rock, and got water. The cost was £SOO. the whole of which was repaid in two years, and

the value of the land raised from £8 to £SO an acre. (7) The owner of a farm of parched land raised water by pumping into a receiving box, and was thereby enabled to irrigate his land. Previous to irrigation he could not be sure of any crop. With irrigation, he was able to grow hops, the produce of each acre of which was worth from £IOO to £l2O. (8) The owner used his land for grazing purposes for three years, and could not make a living. For another three years he tried grazing and cropping, but could get a yield of only six bushels per acre of wheat, if he 'got a crop at all. With the advent of irrigation he was able to get 40 bushels of wheat and 60 bushels of oats per acre off the same land. (9) The owner spent £IOOO on irrigation works on his farm. In one vcar he sold over £IOOO worth of produce off his land, £444 of that sum being derived from the fruit grown on 30 acres. Previous to irrigation his land was worth £2 10s an acre, and after irrigation it was worth £7 10s an acre. It has been found that while too much water ruins the crop, within desirable limits the yield is in direct proportion to the watering. The practical results given below set forth the yields per acre of different parts of farms, and are selected from amoncrst manv such cases:

The general experience in growing these crops lias been that thorough cultivation must go hand in hand with irrigation, and that the water must be applied by such methods and at such times as will avoid caking the soil, the most favourable times being the late evening and early morning. It has also been found that plant life, with irrigation, is best promoted by a deep seed-bed, with the. soil thoroughly and finely cultivated. All kinds of crops, have been successfully cultivated under intelligent cultivation; and all kinds have been mined by over-irrigation. It has further been found in practice that the imperfect irrigation of large tracts by one irrigator leads to very unsatisfactory results.

Acres Inferior Best irrigated. Unirrigated. irrigation. irrigated. 30 no crop 15 bushels 32 bushels no 7 bushels 22 bushels 45 bushels r,G no crop 12 bushels 30 bushels 300 no crop 18 bushels 38 bushels 18 no crop 16 bushels 34 bushels GO 2 bushels 21 bushels 42 bushels '27 4 bushels 17 bushels 42 bushe's GO no crop 18 bushels 34 bushels 250 2 bushels 16 bushels 38 bushels 00 no crop 16 bushels 38 bushels 66 4 bushels 19 bushels 32 bushels 80 3| bushels 14 bushels 3fi bushols 60' no crop 16 bushels 42 bushels