2S 



THE TROPICAL AGRICULTURIST. 



[July i, 1884. 



an acre, provided that this sum does not exceed 7J 

 per cent on the cost of the work, and in some 

 districts this charge is paid in kind. 



Under this Act, which is still in force, most of 

 the irrigated districts have converted their annual 

 payments into this fixed charge ; in other words 

 for this small payment of one rupee per acre, the 

 Ceylon Government construct and maintain the irrig- 

 ation works, receiving only in addition the ordinary 

 ten per cent of the value of the paddy crop. 



The small amount of this water-rate is remarkable, 

 and very exceptional. (v) On the Ganges, Jumna, and 

 Baree-Doab canals, the mean water-rate per acre irrig- 

 ated is stated to be R2.33, and in Madras it was 

 fixed by Government E6 per acre, even although the 

 ryots were willing to pay as much as RIO. 



Taking the yield of paddy on unirrigated land at 

 ten bushels per acre, and assuming the increased 

 yield on irrigated land, including a second crop per 

 annum over at least half the acreage, at the moderate 

 increase of threefold, it appears good policy for the 

 Government to expend E50 per acre on irrigation 

 work, as the return will yield fully 5 per cent 

 interest, independent of the valuable indirect benefits 

 of in ci eased prosperity in the district. 



Thus fixed charge ... ... Rl 



Tax on increase of 20-10 bushels paddy 



at Rl ••■ 2 



Total per acre ... ... R3 



Thus leaving 50 cents per acre to meet cost of main- 

 tenance of works, beyond the interest 5 per cent on 

 R50. (w) See Report of the Assistant Govenment 

 Agent, Matara, hereafter quoted.) 



Rainfall.— The rainfall is the most important item 

 as regards irrigation works. The minimum in Ceylon 

 is on the sea coast at Mannar, where for seven years, 

 ending 1S77, it averaged 32 inches per annum ; the 

 maximum is at Padupolla about 2,000 feet above 

 the sea, where the mean for nine years ending in 

 1SS0 was 224 inches, the maximum in 18S0 being 

 272 inches. In the coffee districts the rainfall varies 

 from SO to 140 inches per annum ; in the irrigation 

 district of the Eastern Province it ie about 60 inches, 

 and in that of the Southern Province it is some 

 ho inches. 



Rainfalls from 9 inches to 12 inches during twenty- 

 four hours are not unf requent all over the island, and 

 a fall of 18 inches in a day has occasionally been 

 registered. 



In September 1872, a flood occurred in the neigh- 

 bourhood of Colombo, covering an area estimated at 

 700 square miles, (x) over which an average of 8 inches 

 of rain fell per day for four consecutive days. 



During the north-east and south-west monsoons, 

 each of which lasts about four months, the ground 

 becomes so saturated that all the rainfall may bo 

 assumed to run off, and as many of the aucient 

 tanks in India and Ceylon have been breached from 

 insufficient waste weir or overflow, called in Ceylon 

 the "spill," it became of great importance to determ- 

 ine the length of "spill" required for each tank. 



To afford a margin of safety, I assumed a rainfall 

 of 1 inch per hour, and regulated the length of the 

 waste weir, so that with this fall the water should 

 not rise more than 2 feet over it. 



Thus 1 inch of rain per hour per acre equals 60-1 

 cube feet per minute, and as with a head of 2 fee? 

 605 cube feet are discharged per foot lineal of weir 



i) Proceedings of lust, of C. E., vol. xxxiii. pp. 3S6, 418, 

 423 ; also Eoorkee, " Treatise of Oivil Engineering," 2nd 

 edition, vol. ii. art. 680. 



to Ceylon Administration Report, Southern Province, for 

 1879, p. 138. 



x See Report of Flood Commission, Sessional Paper u. 

 of 1873, page 7. 



it follows that a foot of weir is sufficient for 10 

 acres ; in other words, each acre of watershed should 

 have one tenth of a foot lineal of waste weir. 



The practical difficulty in adopting this rule arose 

 from the 1 ibour and expense of ascertaining, in a 

 country frequently flat and covered with thick jungle, 

 the approximate area of the water-shed. Owing to 

 the sudden rise and fall of the rivers or streams 

 leading into the tanks, both occurring in the course 

 of a few hours, it is generally impracticable to gauge 

 the streams with any certainty, especially as the 

 officer in charge has. other tanks to supervise, and 

 before ha could be called to any one the flood had 

 probably subsided. A margin to meet these contin- 

 gencies is therefore given to the length of waste weir. 

 It is satisfactory to add that of the many tanks 

 restored during the last ten years only four have 

 suffered from insuflicieut waste weir ; that three of 

 these casualties occurred during an extraordinary 

 flood, (y) such as had not been known for fifty years; 

 and that in one case there had been a rainfall of 42 

 inches during the month, and of 12 incites during 

 the twenty-four hours preceding the breach in the daiu. 

 Rain gauges are kept at each tank, and the height 

 of the water over the sill and under or over the 

 waste weir is registered daily, but a certain distrust 

 exists as to the accuracy of the measurements taken 

 by the native overseers. 



The mean evaporation in Colombo for the last 

 eleven years has averaged '212 inches per day, the 

 mean degree of humidity of the air during that period 

 was 75, complete saturation being 100. 



Quantity of Water Required per Crop. — It is 

 calculated in Madras that tor paddy cultivation a 

 cubic yaj d of water is required for every square yard 

 of land to be irrigated constantly throughout the year. 

 This quantity, which equals cube yards 4,S40, or say 

 cube yards 5,000 per acre, is generally considered 

 sufficient. 



In the Southern Province of Ceylon, where the 

 drought is by no means so continuous as in India, 

 the tanks being replenished during the dry season, 

 half this quantity is ample for one crop, and two crops 

 are usually raised there per annum, the " yala " crop 

 being the largest. * 



Three special waterings are usually given, the suffici- 

 ency of the first and last waterings being the most 

 important, as in default of the first watering the crop 

 will not germinate, and for want of the last it will not 

 mature. 



In the Eastern Province, where the rain, which falls 

 chiefly during the north-east mousoon, is about 60 in. 

 per annum, the depth of water required for a crop is 

 said to be about 36 in., and as only one crop per 

 annum is grown in this district, the yield on irrigated 

 land is from twice to two and a-half times that pro- 

 duced on unirrigated land. This return would, how- 

 ever, pay five per cent, on au expenditure of fully 

 R30 per acre irrigated, and the market value of land 

 irrigated as compared with that Miiirrigated has in- 

 creased in the same proportion as the yield. (Report 

 of the Government Agent, Eastern Province, hereafter 

 .quoted.) 



In the Southern Province it requires 3 inches of 

 water to prepare the ground for sowing, then about 10 

 inches during the first two months, and finally about 

 11 inches more to mature the crop, say 24 inches in 

 all, but of this quantity 12 inches is usually supplied 

 by rainfall. 

 Village Tanks in the North Central Province.— 

 •The earthwork required for village tanks is done by 

 the cultivators themselves, each man working, without 

 remuneration, for 30 days per annum until the part- 



y See Sessional Paper rvi. of 1878, pp. 9 and 11. 



