—The chief improvements which have been under consideration during recent years are three in number. The first is increased economy of water in its actual use in the fields; the second is reduction of the losses by absorption in the channels; and the third is distribution by means of modules.

Regarding the first, it has long been known that the ordinary methods of laying on the water are more or less wasteful. In California, when the water instead of being applied to the surface of the ground, is brought in a pipe and delivered below the ground level, the duty is increased from 250 to 500 acres. In India a field is divided, by means of small ridges of earth, into large compartments. The water is let into a compartment and gradually covers it. By the time the further side is soaked the nearer side has received far too much water. Frequently the water for a compartment, instead of being carried up to it by a small watercourse, is passed through another compartment and this adds to the waste. Also the number of waterings given to a crop is often 5 or 6, when 4 would suffice. Experiments made on the Upper Bari Doab Canal, by Kennedy, showed that the water used in the fields was nearly double what it might have been. The 53 c. ft. shown in Chapter 1, Art. 4, as reaching the fields, were used up when 28 c. ft. would have sufficed. It is not certain that the waste is generally quite as much as the above. It is possible that the restricted supplies might have given smaller yields of crops. More recent experiments made by Kanthack on the same canal give the needless waste as about 25 per cent. The field compartments ought, according to Kennedy, to be 70ft. square, the small branch watercourses being 140ft. apart. It would be better to have still smaller compartments, but this would be rather hard on the people.

At one time Government issued orders, in Northern India, that compartments of 1296 square feet were to be used, and that, otherwise, increased water rates would be charged, but the orders were never enforced. They were thought to press too hardly on the people. Extreme measures for enforcing economy in the use of water in any country are likely to be introduced only when they become absolutely necessary owing to the supplies of water being otherwise insufficient.

2. Reduction of Losses in the Channels.

—For several years experiments have been going on in the Punjab as to the effect of lining watercourses with various materials. The following conclusions have been arrived at[51]:—

I. Ordinary Unlined Trenches.

(a) The rate of absorption varies greatly, and this is due probably to unequal fissuring of the upper layers of the soil.

(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.

(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.

In cases where it is not desired to incur much expenditure, it may be a good plan to construct watercourses to a cross section somewhat larger than that ultimately desired. The silt deposited on the bed and sides forms, in most cases, a more impervious lining than the original soil. The same plan can be adopted in the tail portion of a distributary. In a larger channel there would be less certainty that any deposit would take place unless short lengths, at frequent intervals, were excavated to the true or ultimate section, so as to form weirs and spurs; and even these might not stand.

In Italy, in cases where the water naturally contains lime in suspension, the beds of canals have become gradually watertight by the deposit of lime in the channel.[53] In some cases lime has been artificially added. It appears that a considerable period of time is necessary for the process.

3. Modules.

—A module is an appliance which automatically gives a constant discharge through an aperture, however the water level on either the upstream or downstream side of the aperture may fluctuate. In an old and simple form of module there is a horizontal orifice in which works loosely a tapering rod attached to a float. The water passes through the annular space surrounding the rod. If the water level rises, the rise of the float brings a thicker part of the rod to the orifice and reduces the annular space. In another kind of module the water is discharged through a syphon. If the water level alters, the syphon moves in such a way that the head, or difference between the levels of its two ends, remains the same. The great objections to modules are that they are liable to get out of order or to be tampered with. A module recently invented and patented by Gibb[54] has no movable parts, and is not liable to these objections.

A few years ago the question of the desirability of using modules for the outlets of distributaries in India was raised. The opinions of a large number of the senior canal engineers were called for and considered, and since then the subject has been thoroughly discussed. There are certain inherent difficulties in the way of moduling the outlets of a distributary. Owing, for instance, to rain further up the canal, or to the closure of a distributary owing to a breach in it, the canal supply may increase, and it may be necessary to let more water into the distributary under consideration. Under the present system any excesses of water are automatically taken by the outlets. If all outlets were rigidly moduled they would discharge no more than before the excess supply came in, and the excess supply would all go to the tail of the distributary, and, most likely, breach the banks. To get over this difficulty, the module has to be so arranged that when the water level in the distributary rises to a certain “maximum limit” the module ceases to act as such, and the discharge drawn off from the distributary increases as the water level rises. Again, the discharge of the distributary may at times be considerably less than its full supply. In order that, in such a case, the outlets towards the tail of the distributary may not be wholly deprived of water, it has to be arranged so that when the water level in the distributary falls below a certain “minimum limit” the modules cease to act as such, and draw off supplies which are less the lower the water level. Such supplies are not in proportion to the full supplies of the outlets. It will, however, be shown presently that low supplies need seldom be run. When a distributary, say the upper reach, contains silt, the water level corresponding to a given discharge is higher than before, and care has to be taken that the maximum limit is high enough. At the same time the minimum limit must be so low that it will not be passed when the silt scours out. The difference between the maximum and minimum limits is called the “range” of the module.

In Gibb’s module the above conditions can be complied with. The module is placed outside the bank of the distributary. The water is drawn off from the distributary by a pipe, whose lower edge is at the bed level of the distributary, and delivered from the module into the watercourse through a rectangular aperture at a higher level than that of the pipe. It is possible that, owing to the high level of the aperture, some rolling silt which would otherwise have passed out of the distributary may remain in it. The height of the aperture also prevents the watercourse from drawing off any water at all when the water level of the distributary falls below a certain level, but this objection is not important. An escape weir or notch is provided so that when the water level in the distributary rises to the maximum limit some water overflows into the watercourse. On the whole it appears that all difficulties can be got over, though a good deal of care and precision is necessary in fixing the exact height of the maximum and minimum limits.

The difficulties under consideration will all be reduced if some of the outlets on a distributary are left unmoduled, and this is desirable on other grounds. When the supply is normal, i.e. between the maximum and minimum limits, and all modules are working, the supply entering the distributary must be regulated with great precision. The outlets draw off a certain supply. If less than this enters the distributary the tail outlets must go short. If more enters there will be a surplus at the tail, though it can probably be disposed of, because the tail water will rise above the maximum limit. For short periods, say an hour or two, no trouble arises because the distributary acts as a reservoir, the water level rising to take in any excess supply, and falling to allow for a deficiency. At the tail the rise and fall may be hardly perceptible. But if the supply were deficient for a whole night the tail outlets would certainly go short. This could theoretically be remedied to some extent by letting in an excess supply for a short time and causing the water level at the tail to rise above the maximum limit, but in practice no such system of compensation could be worked. The very fact of the tail outlets having gone short for a night would not be known. The proper method of preventing any such troubles as those under consideration is to leave some of the outlets on the distributary un-moduled.

It has been more than once mentioned that there are periods when a distributary is run, not full, but about three-fourths full. If that were done in the case of a distributary whose outlets were mostly moduled, the water level would probably be below the minimum limit, and the modules would not be acting as such. The outlets would not, under these circumstances, obtain their proper proportionate supplies. This difficulty can, no doubt, be got over by running the distributary full for short periods at a time instead of three-fourths full for longer periods. The people, when once they understood the case, could arrange to use the water in greater volume for two days instead of in smaller volume for three. If this arrangement comes into force it will not be necessary to design distributaries—see Chapter III, Art. 4—so as to have a good command when three-fourths full supply is run.

On nearly every distributary there are some watercourses whose command is bad, and it has been stated (Chapter II, Art. 9) that in an ordinary unmoduled distributary the sizes of the outlets in such cases should be extremely liberal. To module any such outlet would cause a lowering of the water level in the watercourse and would interfere with the irrigation. Such outlets should not be moduled. Again, there are some few outlets which are not submerged, i.e., there is a free fall into the watercourse. The discharge does not depend on the water level in the watercourse, and it is not affected by any enlargement or clearance of it. It depends only on the water level in the distributary. This water level, if most of the outlets are moduled, will be fairly constant. Such outlets need not be moduled, and they should not be moduled unless the other unmoduled outlets in the reach concerned are sufficiently numerous, and perhaps not even then, because moduling involves some expense.

A distributary generally has some falls which divide it into reaches. Immediately upstream of a fall the water level for a given discharge is not affected by the silting or scouring of the channel. Any outlets near to and upstream of the fall are less subject than others to variation in discharge, and are suitable for non-moduling in case a sufficient number of unmoduled outlets is not otherwise obtainable.

Regarding the watercourses at the extreme tail of a distributary it has been pointed out (Chapter III., Art. 7) that in an ordinary case they should not be left without masonry outlets, because they may then lower the water level and so unfairly reduce the supply of any watercourse, even though upstream of them, which has such an outlet. But any outlets near the tail of a distributary can suitably be left unmoduled because of the difficulty of ensuring that the supply at the tail shall be exactly what is needed.

Gibb’s modules have been tried on various distributaries in the Punjab and found to give good results. It is believed however that in only one case has a whole distributary been moduled. The distributary is a large one, its length being 35 miles. It appears that the discharge reaching the tail of the distributary is not constant but varies, as was to be expected, when the head discharge varies for any length of time. The command on the distributary is good. There is nothing to show that matters would not have been improved, and money saved, by leaving some of the outlets without modules.

It has been remarked above, that at the downstream end of a reach ending in a fall, the F.S. level of a distributary is not affected by silt. At the upstream end of the reach it is affected. There are thus two gradients, one flat, and one steep. It appears to have been decided in one case in the Punjab, that the minimum limit of supply for the module should be about half an inch below the flat line and the maximum limit ·3 feet above the steep line. In many cases a greater range would be required,[55] say a foot.

In Chapter III. Art. 7, the case of a distributary without modules but with the outlets carefully adjusted, was considered. The question to be decided in each case is whether such an arrangement is preferable to moduling some of the outlets. This turns largely on the amount of attention which would be bestowed on the case. In view of the difficulty of securing such attention and of the trouble of constantly making alterations in a certain number of outlets, it is probable that moduling will in many cases be considered preferable.

The question of moduling the heads of distributaries has also been considered in the Punjab. For minor or small distributaries modules are feasible. For a large distributary a module would be expensive and it appears that the present system of regulating is preferable.

Kennedy’s “Gauge Outlet,” which is a kind of semi-module is described in Appendix K. It is being tried in the Punjab.