IRRIGATION.

Otago Witness, Issue 3114, 19 November 1913, Page 20

IRRIGATION.

IN PRACTICE,

By JwiN M’Keague

[Copyright.] 111. In the last article we saw that, even under conditions much less favourable than those that exist to-day, irrigation paid enn had to be constructed to water the sides of mountains and unpromising stretches of level country ; tliat water for irrigating purposes may, with profit, be raised high above its source or level, and that if tho water supply be neglected ruin results to agriculture. These conditions the last excepted, apply to Otago, where abundant

supplies of water exist in the lakes of the province. Similar lakes have been dealt with in the irrigation system of Italy. The eastern part of the great Italian plain, within the circle formed by the great chains of the Alps, is known as the plain of Lombardy. The river Po flows across this plain from west to east. The rivers, which, having their sources in the western ranges of the Alps, flow' through the province of Piedmont into the Po, come directly from the mountains to the plain as foaming torrents. No natural means exist for arresting the rush and force ;f these torrents. It is very different in the case of the province of Lombardy. Hundreds of lakes exist among the ranges that supply the waters of the rivers which flow through Lombardy into the Po. Crossing from Piedmont into the province of Lomnardy, nud continuing in an easterly direction, the most important of these lakes are Lake Maggiore, Lake Como, Lake Isea, Lake Garda. "These lakes are situated among the ranges of the Alps arid, in nearly every ease, smaller lakes exist higher up among the ranges, and generally a series, each succeeding lake placed at a higher elevation, froms the chief receiving Basin fur the surrounding drainage system. High up, amid the eternal solitudes of the Alps, impetuous foaming torrents dash with irresistible force from cliff to crag and end the roaring turmoil of their discoloured waters in a small mountain lake. The overflow from this Jake continues its course among the mountains, until it finds a resting place in the succeeding lake at a lower elevation. In the end, the water, its impetuousness somewhat abated, finds its way into one of the chief ■u-jwco, acub it iillixl pUacumu/vci for the raging torrents, and the turbid and raging waters issue from it a subdued and placid, though rapidly flowing, river. Lake Maggiore is 40 miles in length, and about two broad. Lake Lugano is 28 miles in length, and, generally, about a mile and a-balf in breadth. Lake Maggiore is situated 638 ft and Lake Lugano 893 ft above the level of the sea.

Various smaller hikes exist high among the mountains, and discharge their waters into Lake Lugano, which, in turn, discharges its overflow' into Lake Maggiore. 'The depth of these lakes is generally about 500 ft. Lake Como is 50 miles long and one in breadth; and its greatest depth is 1928 ft. Lake Isea is 15 miles in length, and about three miles in breadth. Its greatest depth is 984 ft. Lake Garda is 32 miles in length, and its breadth varies from four to 10 miles, its greatest depth is 1915 ft. These, and an immense number of smaller lakes, with which they are connected, receive thousands of impetuous mountain streams and torrents, which, having passed through the series of lakes, issue on the Lombardy Plain subdued and purified tributary rivers, on their way to join the parent Po. Commencing at the Lombardy, the chief tributary rivers are the Ticino, Addo, Oglio, and Mincio. There are, also, many smaller rivers. As a rule, these rivers flow with a rapid curent, often through hill country and often over plains, the soil of which, unwatered, would in the parts near the mountains be light and sterile, in the lower parts clayey and parched: and those rivers generally flow' between high banks. Let us see what use the people of Lombardy have made of the waters of these rivers. The river Ticino carries the overflow of Lake Maggiore to the Po; and from both hanks of this river numerous canals lead in all directions across the plain. One of the chief of these is the Grand Canal of the Ticino. The weir is formed of masonry, concrete, and masses of dry stones, bound by strong piles and hon/juutai pallia ‘Ji v\ vruu. iu.. iniAva

of masonry and concrete are covered by a pavement of cut stone, fitted and strongly bound together. Behind the weir an immense dam has been constructed of similar masonry work. Slopes of masses of stones lead gradually from the top of the weir downwards to the bed of the river. For the first 13 miles the Grand Canal follows the course of tlie river; and when it

emerges upon the plain its channel is led sometimes through excavations, sometihies on a level with the plain, and sometimes between embankments. From the sides of the main canal, through sluice gates, 120 small canals are spread over the face of the country. From these again, through small shutters, innumerable field watercourses leave hardly a foot of soil unwatered. Along the course of the main canal hydrometers, eight in number, show the height of the water necessary for the full supply of the district in which the water gauge has been erected. The hydrometer, or water measurer, consists of a white marble slab, embedded in a pillar of granite and graduated. The lower part of the canal is subject to constant filling up, and has to be closed twice a year for purposes of cleaning. The repairs to the main canal cost £6O a mile each year. Water guards are appointed, who look after the protection of the outlets; see to the distribution of the irrigating water, according to a pre-arranged plan; regulate the depth of the canal; and guard against any fraudulent proceedings by the irrigators. The canal supplies motive power to 200 mills. It supplies 1851 cubic feet of water per second to the 120 small canals which lead from its banks; and altogether it irrigates 93,440 acres, at a cost which ranges from 3s to 17s an acre. After allowing for loss of water by filtration, evaporation, etc., this gives 62 acres as the area irrigated by each cubic foot per second. Where the Grand Canal terminates the canal of Pavia branches off, and leads from Milan to Pavia. In its course there are 12 masonry locks of cut stone, fitted with sluice gates. It is carried over or flows through 75 aqueducts and syphons. It cost £14,000 a mile. Twenty-five small canals or irrigating channels lead from ite sides through sluice gates, and in all directions. The capacity of the canal is 200 cubic feet per second, and it irrigates 9500 acres at a cost of from 4s to 9s an acre.

The canal of Bereguards is one of the numerous canals that are taken from the side of the Grand Canal, It leads from the right bank through a large sluice-gate inserted in a simple weir of masonry, along the banks of the canal. The country it irrigates slopes greatly, and, in its course, it has 11 locks to regulate the current. Eighteen outlets are_taken from the canal which has a capacity of 156 cubic feet per second, and irrigates 10,400 acres. The Cavo Canal is 150 miles in length, and costs £I6OO a mile. It costs from £2O to £25 a mile each year to keep it in repair. Innumerable small canals, sluices, and distributing channels lead from the canal all over the country. The annual cost of irrigating is about 4s 6d an acre, but the cost varies with the nature of the soil, and the kind of crop grown. The water given to each irrigator is measured as it passes on to his land. 'The two chief canals on the river Adda, which flows across the plain from Lake Como to the Liver To, are the Canal Muzzo and the Cana) Martesana. The Canal Muzzo is 24 miles in length, and in that short distance, following the slope of the plain, it falls no less than 230 ft. Locks and sluice gates regulate the flow of the current. During the first four miles of its course the canal is carried along the face of its high bank, which bounds the river on the right, and sometimes its current, during that distance, flows between walls of solid masonry, which had to bo built for that purpose. 'The head-works of the canal consist of (1) a great weir and dam on the river; (2) an escape weir, 767 ft in length; (3) four regulating escapes with 42 sluices; (4) a regulating head ; (5) a regulator with six sluices to regulate the supply of water to the lower portion of the canal. Along its difficult course six hydrometers, similar to those along the course of the Grand Canal regulate the canal supply to the respective districts. Seventy-five outlets are taken from the sides of the canal, and conducted over aqueducts and through syphons over a most difficult country, which is thoroughly irrigated by these means. The canal supplies water power for 127 mills. Its capacity is 1175 cubic feet per second, and it irrigates 182,500 acres.

I he Martesana was a most difficult one to construct, and its cost shows clearly the great value placed on irrigating water by those practically acquainted with the subject. A great weir and dam have been built across the River Adda. The weir, which is of masonry, is 879 ft in length. Three regulating escapes, with 22 sluices, regulate the flood or surplus water in the river. These head works are at Trezzo; and for nearly six miles the canal at first is carried along the bank of the river at a height of 60it above the stream. The canal channel is sometimes cut through solid rock. In its further course towards Milan it is carried across a long aqueduct, stippoi fed on arches each of 64f1 span. For 28 miles the banks of the canal are protected by masonry walls along its course, most extensive dykes, dams, escapee, weirs, and sluices regulate the flow of its water, and even at times admit fresh supplies into the canal from fresh streams. Eighty-five small canals or irrigation outlets are taken through sluices from its sides, and lead by similarly difficult passages in all directions. The canal supplies the motive power for 50 mills. Its capacity is 981 cubic feet per second, and it irrigates 58,900 acres. Between the rivers Ticino and Adda the total area of cultivable land is 520,000 acres, and of this 471,000 acres are irrigated at an average cost of 4s an acre.

The Paver Oglio flows from Lake Isca to the Po. From the left bank of the Oglio 10 canals irrigate 136,430 acres, and cavil v,uuic fv»ov w v» uter per cvconci ini* gates 89 acres. The length of these canals varies from two to 20 miles. Six canale lead from the Mella, and irrigate 36,000 acres. Four canals lead from the Clisio, and irrigate 74,750 acres. The River Mincio is the escape line for the surplus waters of Lake Garda. In the vicinity of the lake there are beautiful tracts of hill country. The banks of the

river are high,' and . chiefly formed of gravel. It- is not necessary to describe the canals, beyond saying that they are carried over the surface of a most difflcult country, along hill sides, over gorges, and that aqueducts, and cuttings, and embankments, and syphons are the common •order of the day. The inclination of the district is generally steep, and most irregular. Hundreds of canals lead from the rivers, and cover, the face of the country, including the hills and the mountain slopes with a perfect network of irrigating canals, and watercourses. In Lombardy alone 15,118 cubic feet of water per second are drawn from the rivers. All over the plain a substratum of water exists, as in Canterbury. This supply is tapped, often by wells 300 ft deep, and is highly valued for irrigating winter meadow beds. Ihe supply from this source is very considerable. The area of the land irrigated ’n the districts which lie between these rivers *is 1,061,292 acres. The smallest stream or rivulet among the hills is used for irrigating, and the whole irrigation system of the province is linked together. The area of the province is 5,248,000 acres, or with the additional district of Verona 6,000,000 acres. Very little of this is left unculLvated, hardly one acre out of 15. The upper part of tile plain chiefly consists oi deep grave] beds, overlaid by a layer of light sand. Over 40,000,000 tons of water are distributed over the plain every, day. The temperature during the irrigating season is from 70 to 75 per cent Fahrenheit. By these means this sandy plain has been converted, as in the case of the adjacent plain of Piedmont, into land which yields crops of astonishing abundance, and supports one of the densest populations in Europe. The increased value of the agricultural produce bardy which results from the use of irrigatino- water is £560,000 a year, repiesenting a capital of '£14,000,000. Throughout the valley of the Po, which includes the provinces of Piedmont and. Lombards , 1,600,000 acres are irrigated, and the increased rental alone is £900,000 a year, besides the great increase in agricultural produce, and the resulting prosperity, happiness, and contentment to an industrious population. But the disastrous effects consequent on the uncontrolled flow of water are Well shown in the Italian province of Tuscany. This province is situated at a distance from the Equator that ought to ensure a delightful climate, and, given good agricultural conditions, crops and fruits of the most desirable kinds. Yet, long stretches of the coast districts of Tuscany are noted hotbeds of malaria and general unhealtbiness on these very coats, which look upon the blue, placid waters of the Mediterranean, the Etruscans •• of old left cities and earthworks and towers and ail such records of a very advanced civilisation. Two of these districts have been noted specially for their malaria and general unhealthiness. One of these is the last 60 miles of the course of the River Chiana. At some distance inland a low range of hills runs parallel with the coast. Through openings in these hills rivers flow from the inland districts, and, being uncontrolled as they approach the sea, spread over the adjacent lands and convert them into pestilential swamps. The • second district is the great marsh Castiglione, which has been for centuries a centre *of miasma. These districts have not only been most unhealthy, but their malarial exhalations have poisoned the air in the surrounding districts for long distances. All the short rivers which flow from the inland districts through these hills had originally a clear channel to the Mediterranean * Sea. But in nearly every case the river channel was blocked up close to the sea. The waters then spread over the country, and converted it into a dismal, disease-breeding swamp. The blocked-up water is often sufficient to form shallow lakes. The first step in the reclaiming of such districts is to restore the original communication with the sea. If the swamp or lake be of considerable depth, a weir with j sluices across the restored channel admits s of the water being allowed to escape or to be dammed up at will. As soon as any sediment in the the dammed water, has settled, the purified water is allowed to escape; and by admit trig fresh sediment-bearing water into the dam, and allowing the sediment

again to settle, in time the bed of the lake is raised, and, bit by bit, cultivation encroaches further and further towards the original river-bed until at last the water is compelled, to flow within a well-defined channel. Cultivation takes the place of the swamp, and the home of malaria and disease, and untimely death gives place to the well-cultivated plain. In this way and by these means both the districts we have referred to have been reclaimed, and their former unhealthy conditions replaced by the favourable conditions of life and living that existed there in bygone ages. From the facts in connection with irrigation in Lombardy we learn the following facts :—, (1) Irrigation, in conjunction with manuring, can convert a sandy and sterile district into a fruitful agricultural one. (2) Short of barren, inaccessible mountain tops, the roughest country can be successfully irrigated. (3) The most impetuous mountain torrent can be controlled, and its waters used for irrigation purposes. (4) Water, , whether derived from a natural flow or from irrigation channels, if uncontrolled, will create malarial swamps and act as diseasebreeding centres, the exhalationsfrom which will act most prejudicially on the climate of the surrounding districts. (5) All such swamps may be converted into healthy and prosperous districts. These districts and canals have been selected from many others, as they appear to the writer specially to suit the conditions of Otago, with its lakes and streams, and short rivers and streams, and hills and mountain slopes. In short, the same effect may be produced in the province

of Otago, as if the area of the province, under existing conditions, were doubled.