When accurate statistics of rainfall are required for any work, the rainfall of the tract concerned must be specially studied and local figures obtained for as many years as possible. Very frequently it is necessary to set up a rain-gauge, or several if the tract is extensive or consists of several areas at different elevations. Sometimes there is only a year or so in which to collect figures. In this case the ratio of the observed fall to that, for the same period, at the nearest station where regular records are kept, is calculated. This ratio is assumed to hold good throughout, and thus the probable rainfall figures for the new station can be obtained for the whole period over which the records have been kept at the regular station. The volumes of the British Rainfall Organisation contain a vast amount of information regarding rainfall. For a large area there should be one rain-gauge for every 500 acres, for a small area more. In the case of a valley there should be at least three gauges along the line of the deepest part--one at the highest point, one at the lowest, and one midway as regards height--and two gauges half way up the sides and opposite the middle gauge (_Ency. Brit._, Tenth Edition, vol. xxxiii.). Some extra gauges may be set up for short periods in order to see whether the regular gauges give fair indications of the rainfall of the tract. If they do not do so some allowances can be made for this. 2. =Available Rainfall.=--The area drained by a stream is called its “catchment area” or “basin.” The available rainfall in a catchment area is the total fall less the quantity which is evaporated or absorbed by vegetation. The evaporation does not chiefly take place directly from the surface. Rain sinks a short distance into the ground, and is subsequently evaporated. The available rainfall does not all flow directly into the streams. Some sinks deep into the ground and forms springs, and these many months later augment the flow of the stream and maintain it in dry seasons. The available rainfall of a given catchment area is known as the “yield” of that area.