142 



FOREST INFLUENCES. 



are dependent upon differences in climatic conditions. Thus tropical 

 rivers show one or two regular high stages of water according to 

 whether they have one or two rainy seasons; in regions of equinoctial 

 rains a spring and fall freshet is normal, while the rivers may be almost 

 dry in summer or winter; the frequent thunderstorms in the mountains 

 of Switzerland produce short but rapid floods during the summer, 

 Avhile the autumn is characterized by low water in the rivers. This 

 climatic difference in water flow it is important not to forget when dis- 

 cussiug the influences which may modify the discharge of Avaters. 



W. Ule, in Meteor. Zeitschr., 1800, discusses the relation of water 

 stages in a stream and outflow in relation to precipitation. He comes 

 to the result that a direct relation from one to the other is impossible 

 to find, because of the complication of other conditions, which disturb 

 this a priori seemingly direct relation. 



He finds that amount of precipitation and water stages or even water 

 stages and amount of water flow are not x)roportionaI, so that with the 

 same water stage different amounts may run off'. If this discrepancy 

 appears in the annual mean water flow, it is still more noticeable in the 

 monthly means. In the river Saale the mean water stage for March, 

 1886, indicated a flow of 378,000,000 cu. m., while the daily measure- 

 ments gave 508,000,000, or 34 per cent more. The mean water stage in 

 March was 2.13 m., in December 2.15 m. Yet the amount of flow in the 

 latter month was 23 per cent smaller than in the former. With the 

 stage twice as high it was found that three times the flow resulted. 

 From these moie detailed measurements it appears that changes in 

 the amount of river flow are not necessarily due to changes in amount 

 of precipitation. 



In 1886 the amount of river flow was 14 per cent less than in 1884, 

 although the precipitation was by 1 per cent less in 1884. 



The gre:itest influence on river flow is assigned to the distribution 

 through tlie year of the precipitation, at least in regions with persistent 

 frost periods, Avhere, as in the Saale catchment basin, the river flow in 

 Avintei' is tliree times as great as in summer, namely, 51 per cent of the 

 precipitation in winter against 17.3 per cent in summer. Hence the 

 annual mean river flow reflects more the Avinter precipitation than those 

 of summer, as the following figures show : 



The larger amount of flow in 1882-'83 seems to depend on the fact 

 that much precipitation fell in winter, and, at the same time, the greater 

 percentage of flow in that winter is accounted for from the daily meas 



