DISTRIBUTION OF WATER. 141 



I also recall here the water balance drawn for the upper Elbe Elver 

 watershed on p. 128, from which it appears that the river flow repre- 

 sents about 25 per ceut of the precipitation. 



The amount of river flow, to be sure, does not permit a calculation of 

 the amounts of water originally available (after they have fallen and 

 been diminished by the factors of dissipation) for local use in both sub- 

 terranean and surface runs, since the river flow exliibits both kinds of 

 drainage, but not at the same time. Nor do the wholesale methods 

 sometimes employed to determine the relation of river flow to precipita- 

 tion promise a solution of either that question itself nor of the ques- 

 tion, how far surface conditions of the soil have a bearing upon drain- 

 age. 



A detailed study of smaller and confined catchment basins alone 

 like those referred to hereafter (see p. 154) will give results that may 

 eventually lead to i)racticable methods of calculations. 



DISTRIBUTION OF TERRESTRIAL WATERS. 



The distribution of the available water supply is almost as important 

 and often a more important factor in the economy of the water than 

 tlie quantity of available supply itself, and the manner in which this 

 distribution takes place influences considerably the ultimate availa- 

 bility of the supply for human use. 



In discussing the distribution of the water supply it is desirable to 

 folloAv the natural division of the waters into superficial and under- 

 ground drainage. The surface runoff gives to brooks and rivers all 

 their rapid variations of stage; the underground drainage gives them 

 their permanent regime. 



ANALYSIS OF DETERMINING INFLUENCES. 



The luoportionate division of these two classes of run-off, then, is of 

 the highest possible importance; we will, therefore, analyze the condi- 

 tions which determine their relative proportion in order to find how 

 the forest may influence the same. It is evident that the first condi- 

 tion is to be found in the amount and character of the precipitation. 

 A violent rainstoim will furnish more superficial run off than Avhen, 

 the rain falling slowly, time is given for the soil to absorb it. 



Eainy and rain-i^oor or arid climates, short and insignificant rains, 

 short but violent, long and mild, or long, plentiful rains, also periodical, 

 seasonal rains and irregular raintalls, all these constitute differences 

 ill the nature and time of occurrence of the rainfall, which must neces- 

 sarily affect the relative amounts of the run off'. The effect is still 

 further complicated where the jirecipitation is i)artly snow, when not 

 only the inass of accumulated su])])ly but also the progress of melting 

 determine the result of the run-ofl". 



We find, therefore, based upon this one factor, namely, the nature 

 and time of occurrence of iirecix^itation, differences in the run -off' which 



