Estimation of the available rainfall is necessary chiefly in cases where water is to be stored in reservoirs for town supply or irrigation. The ratio of the available to the total rainfall depends chiefly on the nature and steepness of the surface of the catchment area, on the temperature and dryness of the air, and on the amount and distribution of the rainfall. The ratio is far greater when the falls are heavy than when they are light. Again, when the ground is fairly dry and the temperature high--as in summer in England--nearly the whole of the rainfall may evaporate; but when the ground is soaked and the temperature low--as in late autumn and winter in England--the bulk of the rainfall runs off. In the eighteen years from 1893 to 1900 the average discharge of the Thames at Teddington, after allowing for abstractions by water companies, was in July, August, and September 12 per cent. of the rainfall--6·9 inches--in its basin, and in January, February, and March 60 per cent. of the fall which was 5·9 inches. The total fall in the year was 26·4 inches. Some rivers in Spain discharge, in years of heavy rainfall, 39 per cent., and in years of light rainfall 9 per cent. of the rainfall (_Min. Proc. Inst. C.E._, vol. clxvii.). The discharge of a river is not always greatest in the month, or even the year, of greatest rainfall. The table opposite gives some figures obtained by comparison of rainfall figures and stream discharges. The case of the area of 2208 acres near Cape Town is described in a paper by Bartlett (_Min. Proc. Inst. C.E._, vol. clxxxviii.), and it is shown by figures that part of the rainfall in the rainy season went to increase the underground supply which afterwards maintained the flow in the dry season.