414 A TREATISE ON METAMORPHISM. 



about 50 per cent of the total precipitation." In the humid region of the 

 United States comprising the Great Lakes region and a broad belt extending 

 through to central Texas the rainfall varies from 50 to 100 cm. The 

 run-off in this region varies from 12 cm. to nearly 50 cm., or from 25 to 

 47 per cent, the smaller percentages of run-off corresponding to the lesser 

 rainfalls. In the semiarid portion of the Cordilleran region, where the 

 rainfall is 25 to 50 cm., the run- off varies from 5 to 12 cm., or is from 

 20 to 25 per cent of the precipitation. In the arid regions, where the 

 precipitation is 25 cm. or less, the run-off varies from to 5 cm., or from 

 to 20 per cent. Thus, according to Newell, the water flowing in the 

 streams varies from 50 per cent or more of the precipitation in the most 

 humid regions of the .United States to 1 or 2 per cent in the most arid 

 regions.'' 



All these cases fall under the general rule given by Powell, "that the 

 coefficient of run-off increases with the rainfall;"" and it maybe stated 

 generally "that in humid countries the percentage of run-off to rainfall 

 is larger than in the arid regions; and, of several localities, that one having 

 the greatest mean annual rainfall will, cseteris paribus, have a run-off 

 disproportionally large to the others.'" 1 However, as pointed out by 

 Powell, it does not follow from this that the annual run-off of any one 

 basin is directly proportional to the measured annual rainfall, for there are 

 many factors entering into the run-off besides the amount of the precipi- 

 tation. Some of these factors are the distribution of precipitation during 

 the year, the temperature and other climatic conditions, the topography, 

 the chemical and physical character of the soil, cultivation, vegetation, etc. 

 Of these secondary factors that of topography is very important. In 

 proportion as the declivities are steep the run-off is likely to be large; in 

 proportion as the slopes are gentle the run-off is likely to be small. It 

 follows from this that the relative run-off is less in large drainage basins 

 than in small ones, because the large drainage basins are apt to have a 

 greater percentage of level land. 6 



a Newell, F. H., Results of stream measurements: Fourteenth Ann. Rept. IT. S. Geol. Survey, 

 pt. 2, 1894, pp. 149-154, pis. v and vi. 



& Newell, F. H., Irrigation in the United States, Crowell & Co., New York, 1902, p. 27. 



c Powell, J. W., Second annual report of the irrigation survey: Eleventh Ann. Rept. U. S. Geol. 

 Survey, pt. 2, 1891, p. 27. 



d Powell, cit., p. 27. 



« Ne well, F.H., Water supply for irrigation: Thirteenth Ann. Rept. U. S. Geol. Survey, pt, 3, 1893, 

 pp. 13-14. 



