There is an obvious connection between the duty of water and the total depth of the water, known in India as “delta,” given to the fields. Calculations are much simplified, while still being accurate enough for all practical purposes, by assuming that the number of seconds, (86,400) in a day is twice the number of square feet, (43,560) in an acre. Assuming this to be the case a discharge of 1 c. ft. per second for a day gives 2 acre-feet, i.e., it will cover an acre of ground to a depth of 2 feet in a day; and in six months it will cover 100 acres to depth of 3·65 feet. In Northern India the year is divided into two halves in each of which a crop is grown and the duty is calculated for each crop. In this case, if the flow of a canal has been continuous, a duty of 100 acres per cubic foot of its mean discharge per second, corresponds to a total depth of 3·65 feet over the area irrigated. Generally the flow in the half-year has not been continuous. In other countries, and in India on canals other than the perennial canals, the periods of flow vary a great deal. The duty cannot be calculated from delta or _vice versa_ until the period of flow is stated. The daily gauge-readings and daily discharges corresponding to them, having been booked, the discharges are added up. The total, divided by the number of days on which the canal has been running, gives the average daily discharge. Suppose that during the “kharif” or summer crop which is considered to last from 1st April to 30th September or 183 days, the canal was closed for 13 days and that the total of the daily discharges on the remaining 170 days comes to 850,000 c. ft. per second. The average daily discharge is 5,000 c. ft. per second. Suppose the kharif area irrigated to be 500,000 acres, the kharif duty is 100 acres. To find delta the total of the daily discharges has to be multiplied by the number of seconds in a day and divided by the number of square feet in an acre (these figures are, as already stated, very nearly in the ratio of two to one) and divided again by the number of acres irrigated. Thus, in the above case, delta is very nearly 850,000 × 2/500,000 or 3·4 feet. For comparing the results of one canal or one year with another, delta is the more convenient figure to take. As soon as the areas irrigated by the canals are known for any crop, the Chief Engineer of the province issues a statement of the value of delta for each perennial canal and compares them with those for previous years. The value of delta for the Punjab canals ranges from 3 to 4 feet for the kharif and from 1·8 to 2·1 feet for the “rabi” or winter crop. Individual canals vary greatly, the worst having nearly twice as high a figure as the best. The differences are due to the causes already mentioned.