The source of all running water, whether surface or underground, is atmospheric precipitation. All springs and streams are merely rain (or snow) water collected and fed from reservoirs. The rain-water which falls upon the land is disposed of in three ways: one part is returned to the atmosphere by evaporation; another part flows over the surface to the nearest watercourse. The remainder sinks into the soil to a greater or less depth, and though some of it is returned to the surface in springs, yet a great part must reach the sea by subterranean channels. The surface flow, together with the supply from springs, constitutes the "run-off".

The relative proportions of these three parts of the total precipitation vary much in accordance with the climate and with the topography of the land surface. In a moist climate with heavy rainfall the run-off may amount to one-half of the precipitation, and the loss by evaporation is at a minimum. In arid regions, where evaporation is very great, the run-off is from one-fifth to zero. Climatic factors being equal, run-off increases with' the steepness of the slopes and is thus relatively less in large drainage basins than in small ones.