The water which falls from the sky on to the earth's surface is disposed of in a variety of ways. A certain proportion of it runs off rapidly from the surface down slopes and finds its way into creeks and rivers and eventually into the sea. Some of it is immediately or almost immediately evaporated, and returns to the atmos-phere whence it came. A third fraction soaks into the soil and, as we shall see, may haye quite a complex history. The relative proportions of the rain-water which are disposed of in these different ways vary according to circumstances. For instance, if the rain is in the form of sharp, heavy showers, then much of it will run off, whereas if it is in the form of gentle, steady rain, much will soak in. Again, if the soil is sandy and porous, a greater proportion will soak in than if the soil is clayey and close-textured. And naturally when the temperature is high there will be a greater tendency to evaporation than when it is low.
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