THE WORK OF RUNNING WATER. 129 



depends on its distribution in time. A given amount of rainfall may 

 be distributed equally through the year, or it may fall during a wet 

 season only. The maximum inequality of distribution would occur 

 if all the rainfall of a given period were concentrated in a single shower. 

 With such concentration the volume of water flowing off over the 

 surface immediately after the down-pour would be greater than under 

 any other conditions of precipitation, and since velocity is increased 

 Avith volume, and erosive power with velocity, it follows that the ero- 

 sive power of a given amount of water would be greater under these 

 circumstances than under any other. Furthermore, a larger proportion 

 of the precipitation would run off over the surface under these cir- 

 cumstances than under any other, for less of it would sink beneath the 

 surface and less would be evaporated. If erosive power and rate of 

 erosion were equal terms, this would therefore be the condition for 

 greatest erosion; but erosive power and rate of erosion do not always 

 correspond. If the water falling in this way could get hold of all the 

 material it could carry, extreme concentration of precipitation would 

 be the condition favorable for most rapid erosion. But if the amount 

 of available material for transportation is slight, a large part of the 

 force of the water could not be utilized in erosion. It follows that if 

 there were a large amount of disintegrated material on the surface, 

 erosion would be greater the greater the concentration of precipitation. 

 If, on the other hand, there were but little disintegrated material on the 

 surface, frequent showers, with intervening periods when conditions 

 were favorable for weathering, that is, for preparing material for trans- 

 portation, might be more favorable for rapid erosion. While the total 

 energy of running water available for erosion under these conditions 

 would be less than before, there might in the long run be more material 

 for transport; for weathering in the presence of moisture, and all that 

 goes with it, might be more effective in preparing material for transpor- 

 tation, than weathering during the long periods of drought which 

 would occur if the precipitation were concentrated to its maximum. 

 Temperature favoring, the uniform distribution of moisture through the 

 year would allow the growth of vegetation, which, although favoring 

 some processes of weathering, retards erosion in general. While 

 therefore it is not possible to say what distribution of rainfall favors 

 most rapid erosion without knowing the nature of the surface on which 

 it is to fall, enough has been said to show that the problem is by no 

 means a simple one. Some of the most striking phases of topography 



