The next thing to be calculated is the “run-off,” _i.e._ the probable proportion of the rainfall which will at once run off. This may be less than the proportion which will eventually become “available,” because some of it may go to feed the underground supply from which springs are fed. The proportion running off a small area in a short time would, under most circumstances, be rather difficult to estimate, but in the case under consideration, only the probable maximum figure is required. This occurs when the ground is saturated. Under these circumstances the ratio of the run-off to the total fall may be somewhat as follows:-- Steep rocky hillsides ·70 to ·90 Ordinary hills ·50 ” ·75 Undulating country ·40 ” ·50 Flat country ·30 ” ·35 The figures can be increased when the surface is specially hard or frozen, and decreased when it is soft, sandy, covered with woods or vegetation, or cultivated. Whether or not the above procedure is necessary in its entirety depends chiefly on the size of the proposed work and on the degree of inconvenience likely to arise from any wrong estimation of the discharge. In designing syphons to carry torrents across the Upper Jhelum Canal in the Punjab, the discharge from a catchment area of ·79 square miles was found to be about 4000 cubic feet per second. This is at the rate of about 5000 cubic feet per second per square mile, and is equivalent to a run-off of 7·8 inches in an hour. The catchment area was among low hills, not far from the Himalayas, and the declivities of the rills were very steep. The superintending engineer, Mr R. E. Purves, states[18] that the discharge observations were reliable, and that falls of rain of an inch in ten minutes occur not infrequently, even though the fall in twenty-four hours might not exceed 2 or 3 inches. In order to account for the discharge in the case under consideration, it would at first seem to be necessary to assume not only that a fall at the rate of 7·8 inches per hour had occurred, but that the whole of it had run off. It is not, however, necessary to assume quite so much. The ground being saturated, the rain falling in a period of five minutes might be reaching the discharge site with little loss. A suddenly increased fall at the rate of 6 inches per hour might then occur, and the water travelling more quickly and with hardly any loss, would overtake that already passing the site. This case seems to show that for a very small catchment area the whole of the fall, and more, must be allowed for.