IN all questions relating to water-power schemes, some estimate of the water available is of first importance. Most schemes involve the storage of water in reservoirs, the size of which depends on the proposed draw-off compared with the run-off from the gathering ground in a dry period. Undoubtedly measurements of stream flow afford the best hydro-logical data upon which to base calculations. Such data are, however, often too short to give the average over a long period or the flow in the all-important dry period. Thus to a large extent calculations have to be based upon measurements of rainfall, compared where possible with gaugings of stream flow. It is, therefore, important to consider the relation of the average rainfall (i) to that of the period for which stream gaugings are available, and (2) to falls in dry periods in that area.