In an inundation canal division in the Punjab, some dozen distributaries, varying in length from 5 to 28 miles, and with discharges ranging up to 300 c. feet per second, were dealt with as above in one season. The engineer in charge being specially desirous that sufficient water should reach the tails, reduced the sizes of some outlets too much. When an outlet of 1 or 2 sq. feet has to be reduced to a small fraction of its size it is not easy to say what the fraction shall be. Water reached the tails of all the channels in sufficient quantity, in some cases in rather more quantity than was necessary. When the irrigation register was examined, it was found that the general results were entirely satisfactory. In a small proportion of cases outlets had irrigated too little and had to be re-enlarged somewhat. After a second season hardly any changes were needed. When any silt clearance or berm-cutting seemed necessary the irrigation register again came into play. If, for instance the tail outlets, as a whole, were receiving too little water, enlargement of the upstream reaches was effected with consequent lowering of the water level there.
In the case above described the channels flowed for only five months in the year. Some of them silted a good deal but as this silting was roughly the same every year, it did not greatly affect the question of outlet sizes. On a perennial distributary of which the head reach silts during part of the year and scours during the other part, a proper distribution of supply by adjustment of outlet sizes alone may be more difficult. If the silt was frequently cleared, this would cause needless expense and interference with irrigation. In cases where the distributary is not run constantly, something can be done by attending to the regulation. When there is silt in the head reach, the discharge can be reduced and the period of flow proportionately increased. The lowered water level reduces the supplies of the upper outlets, and increases the discharges of those lower down. Moreover the periodical silting and scour are not always serious. Also it is not essential that the supply to each watercourse should be exactly the same every year. There are always good and bad seasons. It is sufficient if a watercourse is not allowed to suffer on the whole, and is never allowed to suffer much. There is no doubt that it is possible to deal satisfactorily in the above manner with very many distributaries. It is frequently reported that “difficulty is experienced in getting water to the tail.” This is owing to timidity in reducing the sizes of outlets. The suitable plan is to reduce them to such an extent as to cause a proper supply to reach the tail and then, if necessary to enlarge some. It has been already remarked that only a short length of the barrel need be altered. The cost of this is very small. The real difficulty in the case is not the impossibility of securing good results, but the impracticability, in many cases, of securing the constant attention which the procedure demands.[38]