Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/385

IRRIGATION 369 of our ordinary water-meadow irrigation is rather obscure. For we are not dealing in these grass lands with a semi- aquatic plant like rice, nor are we supplying any lack of water in the soil, nor are we restoring the moisture which the earth cannot retain under a burning sun. We irrigate chiefly in the colder and wetter half of the year, and we "saturate" with water the soil in which are growing such plants as are perfectly content with earth not containing more than one-fifth of its weight of moisture. We must look in fact to a number of small advantages, and not to any one striking beneficial process, in explaining the aggre gate utility of water-meadow irrigation. We attribute the usefulness of water-meadow irrigation, then, to the follow ing causes : (1) the temperature of the water being rarely less than 10 Fahr. above freezing, the severity of frosts in winter is thus obviated, and the growth, especially of the roots of grasses, is encouraged ; (2) nourishment or plant food is actually brought on to the soil, by which it is absorbed and retained, both for the immediate and for the future use of the vegetation, which also itself ob tains some nutrient material directly ; (3) solution and redistribution of the plant food already present in the soil occur mainly through the solvent action of the carbonic acid gas present in a dissolved state in the irrigation- water ; (1) oxidation of any excess of organic matter in the soil, with consequent production of useful carbonic acid and nitrogen compounds, takes place through the dissolved oxygen in the water sent on and through the soil where the drainage is good ; and (5) improvement of the grasses, and especially of the miscellaneous herbage, of the meadow is promoted through the encouragement of some at least of the better species and the extinction or reduction of mosses and of the innutritious weeds. To the united agency of the above-named causes may safely be attributed the benefits arising from the special form of water-irrigation which is practised in England. Should it be thought that the traces of the more valuable sorts of plant food (such as compounds of nitrogen, phos phates, and potash salts) existing in ordinary brook or river water can never bring an appreciable amount of manurial matter to the soil, or exert an appreciable effect upon the vegetation, yet the quantity of water used during the season must be taken into account. If but 3000 gallons hourly trickle over and through an acre, and if we assume each gallon to contain no more than one-tenth of a grain of plant food of the three sorts just named taken together, still the total, during a season including ninety days of actual irrigation, will not be less than 9 B> per acre. It appears, however, that a very large share of the benefits of water-irrigation is attributable to the mere contact of abundance of moving water, of an even temperature, with the roots of the grass. The growth is less checked by early frosts ; and whatever advantages to the vegetation may accrue by occasional excessive warmth m the atmo sphere in the early months of the year are experienced more by the irrigated than by the ordinary meadow grasses by reason of the abundant development of roots which the water has encouraged. Irrigation in India. The irrigation works of India may be grouped under five descriptions or classes, as follows : (I) perennial canals, works fed by risers of which the discharge at all times of the year suffices, without storage, to supply the canals ; (2) intermittent canals, works fed by rivers having an uncertain and very variable discharge, which is stored and rendered constantly available for the canals by means of reservoirs formed in the river- basins themselves ; (3) periodical canals, works fed by rivers having a supply available during the rainy season only ; (4) inun dation canals, works fed by rivers having a constant discharge of s>m3 magnitude, but fed only when the rivers are in flood; (5) tanks, works which either impound a supply from rivers or small catchment areas, or collect a supply by means of embank ments thrown across valleys or gorges. The rainfall of India is not only very irregular in its yearly distribution, but the annual amount varies much from year to year, while the annual average differs in the twenty-two " meteorological tracts " into which the empire has been divided. The following table of average annual rainfall, stated in inches, is from the Report for 1879 of the Select Committee on Indian Public Works : 1. Sind and Cuteh. 9 2. Punjab plains 22 3. Hyderabad and South Deccan 25 4. North Deccan plateau 28 5. Khandcish and Berar 29 6. Rajputana and Gujerat 32 7. Carnatic 34 8. Northern Circavs 36 9. Upper Ganges plains, North- West Provinces ...38 10. Central India and Nerlmdda 44 11. Central Provinces (iouth) 49 12. Western riengn] r>C> 13. Western Himalaya (>5 14. Lower Ganges plains , G8 15. Pegu 7(1 16. Assam and Kast Bengal 96 17. Bay Islands 108 18. Malabar and Ghats, 112 19. Eastern Hima ayii 144 20. Concan and Ghats 145 21. Tenasserim 173 22. Arakaii 193 The following statistics of the irrigated acreage in different Indian presidencies and provinces belong generally to the years 1877-8, but are in several directions imperfect. Averages are in many cases not yet available. Of course the figures here given must be received with due reserve, since the areas irrigated vary much, from year to year, according to the season ; while, as new works are brought into action, great additions to the irrigable acreage are suddenly made. Acres Irrigated. Annual Rainfall. 5 265 320 35 inclvs Bombay 20.786 24 Sind 1 267 054 9 Bengal 360,304 50 North-West Provinces and Oudh.... Punjab. 1,461,42!) 1,S20 124 40 18 The annual average rainfall refers only to that of the irrigable areas, and is a very rough approximation. Irrigation in Italy, France, and Belgium. In Italy the practice of irrigating meadows .and crops has been long followed, and is carried out in some parts by means of a complex and costly system of canals. Tb.3 extent of lands irrigated was in 1878 : Lombardy 678,000 hectares Piedmont 443,000 ,, Yenetia 74,000 Emilia 96,000 hectares. Other provinces 214,000 ., Rice is extensively grown in artificially irrigated lands in the basin of the Po. The produce in rough grain oscillates between 30 and 50 times the weight of seed sown ; if official reports maybe trusted, a hectolitre and a half of seed rice will yield from 45 to 75 hectolitres. During the four years in which a field is in rice the annual crop, beginning at 70 hectolitres, sinks successively to 65, 50, and 40. About 42 hectolitres of cleaned rice is the general average yield. In some parts of Italy the system of winter irrigation, with which we are familiar in England, is carried out upon meadows in which the Lolium italicum abounds. This is the case in many of the valleys of Lombardy and in the neighbourhood of Padua. The cuttings of grass are about six in the year, but where certain sewage waters from towns are mingled with the natural water supply eight or even nine cuttings are not unusual. The average yield of hay in these meadows when irrigated with clean river water is about 14,000 kilograms annually, or twice the amount obtained from per manent pasture in the sams district. The cuttings begin as early as the end of February, the heaviest amount being obtained in the May cutting, and the lightest in that of October. In France irrigation has met with increasing favour of late years. Since 1875 there have been Government competitions for prizes for the best examples of irrigated farms. In 1879 there were competitors from eight departments of France, two departments, those of the Basses Alpes and Hautcs Alpes, in which the areas irrigated amount respectively to 8500 and 20,000 hectares, furnishing no less than seventy-two. There are many canals in these departments. Other important irrigation works are to be found in Provence, Dauphine, and Languedoc. The valley of the Iscre near Grenoble affords a good illustration of how a devastating torrent may be turned into a source of continual fertility, 3000 hectares of useful land having now been conquered from floods and reclaimed. In the Roussillon district the irrigated area has been doubled between 1820 and 1880, and exceeds 25,000 hectares. One farmer, M. Frangois Coste, whose grandfather was ruined by having to pay 2 francs per hectare for a rugged mountain farm, now obtains from 18 hectares of the same land no less than 125,000 kilos of hay, or 6000 to 7000 kilos per hectare, a fair yield even for the average meadows of the north of France. In the Pyrenees Orientales there are canals which have been constructed since 1850, aud which now water over 6000 hectares. It is scarcely necessary to say that in some lands irriga tion without any application of manure has been unremunerative, but that with manure the natural produce has been raised from 7000 to 16,000 kilos of hay per hectare. There appear to have been some instances where that terrible vine scourge, the Phylloxera, has been entirely eradicated by autumnal submersion of the roots of the affected plants. Irrigation XIII. 47