=167. Available Portion of Rainfall or Run-off of Watersheds.=—If the public water-supply is to be drawn from a stream where the desired rainfall records exist, it is necessary to know what portion of the rainfall, either in the driest or in other years, may be available. This is one of the departments of the hydraulics of streams for which much data yet remain to be secured. The watersheds or areas drained by some streams, like the Sudbury River of the Boston, and the Croton of the New York water-supply, have, however, been studied with sufficient care to give reliable data. The amount of water flowing in a stream from any watershed for a given period, as a year, is called the annual “run-off” of the watershed, and it is usually expressed as a certain percentage of the total rainfall on the area drained. For certain purposes it is sometimes more convenient to express the run-off from the watershed as the number of cubic feet of water per second per square mile of area. Table VII, taken from Turneaure and Russell, exhibits run-off data for a considerable number of streams in connection with both average and minimum rainfalls. TABLE VII. STATISTICS OF THE FLOW OF STREAMS. -----------------+---------+-------+----------------+----------------- | Area | | Average Yearly.|Year of Minimum Stream. | Drained,| | | Flow. | Square | Years.+-----+-----+----+-----+------+---- | Miles. | |Rain,|Flow,|Per |Rain,|Flow,|Per | | |Inch.|Inch.|Cent|Inch.|Inch.|Cent -----------------+---------+-------+-----+-----+----+-----+-----+---- Sudbury | 75.2 |1875-97|45.77|22.22|48.6|32.78|11.19|34.1 Cochituate | 18.87|1863-96|47.08|20.33|43.2|31.20| 9.76|31.3 Mystic | 26.9 |1878-96|43.79|19.96|45.6|31.22| 9.32|29.8 Connecticut |10,234 |1871-85|44.69|25.25|56.5|40.02|18.25|45.6 Croton | 338 |1870-94|48.38|24.57|50.8|38.52|14.54|37.8 Upper Hudson | 4,500 |1888-96|39.70|23.36|59.0|33.49|17.46|52.2 Genesee | 1,060 |1894-96|39.82|12.95|32.5|31.00| 6.67|21.5 Passaic | 822 |1877-93|47.08|25.44|54.0|35.64|15.23|42.7 Upper Mississippi| 3,265 |1885-99|26.57|4.90 |18.4|22.86| 1.62| 7.1 =================+=========+=======+=====+=====+====+=====+=====+==== | | | Average for |Average for June | Area | |December to May.| to November. Stream. | Drained,| | | | Square | Years.+-----+-----+----+-----+-----+---- | Miles. | |Rain,|Flow,|Per |Rain,|Flow,| Per | | |Inch.|Inch.|Cent|Inch.|Inch.|Cent -----------------+---------+-------+-----+-----+----+-----+-----+----