(b) The rate of absorption in the three hottest months averaged ·0571 feet per hour, or more than double the rate (·026) in the three coldest months. The difference is ascribed to the greater viscosity of the water when cold.
(c) The average losses with canal water were ·0315 feet per hour, or 8·75 c. feet per second per million sq. feet.[52] With well water the figures were ·1096 and 30·5. The conclusion is that the silt in canal water reduces the losses by more than two-thirds.
[52] This loss of 8·75 c. ft. per second was in water only about a foot deep. This confirms the conclusion arrived at in [Chapter I, Art. 4], that the depth of water is not a factor of much importance.
(d) With canal water the average loss decreased by 40 per cent. (from ·0491 to ·0293) in about four years. This was no doubt due to the effect of the silt. With well water the loss at the end of four years (·2293) was nearly four times as great as at first (·0591). This may have been due to removal of the finer particles of soil by the water, but the experiments were made at only one place, and were not conclusive.
II. Lined Trenches.
(e) With trenches lined with crude oil ¹⁄₁₆ inch thick, or with Portland cement ¹⁄₁₆ inch thick, or with clay puddle 6 inches thick, the “efficiency ratios,” as compared with unlined trenches, are respectively about 4·0, 5·7 and 5·7, the age of the lining being four years. The efficiency ratio is the inverse of the loss. Thus with an efficiency ratio of 3 the loss in the lined trench is 33 per cent. of that in the unlined trench.
(f) The efficiency ratio in the case of oil may diminish at the rate of 10 per cent. per annum, but in the case of cement and clay puddle it tends to increase rather than to decrease.
Assuming that the efficiency ratios are only 3·0, 4·5 and 4·5, and that the loss in an unlined channel is 8 c. feet per second per million sq. feet, the saving in water by using channels lined with oil, cement and puddle respectively would be 5·33, 6·25 and 6·25 c. feet per second. The average duty of the water at the canal head is about 242 acres, and the average revenue per acre is Rs 3·93. The revenue from 1 c. ft. of water at the canal head is thus Rs 950. Only about half the water reaches the fields ([Chapter I., Art. 4]), and the revenue from 1 c. ft. of water which reaches the fields is about Rs 1900. The mean of the above two sums is Rs 1425. If 6 c. ft. of water per second could be saved the revenue would be increased by Rs 8,550 per annum.
The cost of lining a million square feet of channel with oil, cement and puddle is estimated at Rs 30,000, Rs 27,500 and Rs 35,000 respectively. Allowance has to be made for the fact that watercourses flow intermittently, and that a lined channel gives no saving when it is not in flow, also that extensions of canals might have to be undertaken in order to utilise the water saved. After making these allowances it is estimated, in the paper above quoted, that the saving effected by lining a million square feet with oil, cement or puddle represents the interest on a capital sum of Rs 69,300, Rs 81,250 and Rs 81,250 respectively, or 2 or 3 times the sums sunk in constructing the linings.
Hitherto the experiments have been carried out on a moderate scale, but extensive operations are now being undertaken on the Lower Chenab Canal, and possibly on others.