280 YEAEBOOK OF THE DEPARTMENT OF AGRICULTURE. 



clear what are the various factors entering into the problem, and to 

 state some of the more important facts that these observations in 

 southern California reveal. 



In the San Bernardino Mountains records of precipitation for several 

 years, at a large number of stations, show that differences in forest 

 cover are closely correlated with differences in rainfall. This correla- 

 tion is so close that it is possible to judge the mean annual precipitation 

 with a fair degree of accuracy from the appearance of the forest alone. 

 In these mountains forests cover the slopes wherever the mean annual 

 rainfall exceeds 20 to 24 inches; however, on southern and western 

 slopes forests of equal density represent a larger rainfall than on 

 northern and eastern slopes. 



Other things being equal, regions having the greatest rainfall bear 

 forests of the greatest density and luxuriancy of growth; but where 

 the mean annual rainfall falls below 18 to 20 inches, types of vegeta- 

 tion in which trees predominate are replaced by those in which shrubs 

 and herbage predominate. 



WHAT CAUSES RAINFALL. 



Because rainfall is most abundant where forests grow, many believe 

 that forests exert an important influence on the amount of precipita- 

 tion. A more reasonable inference, however, is that rainfall is the 

 great factor in controlling the distribution and density of forests. 



Precipitation occurs whenever the air is suddenly cooled below the 

 dew-point. The most effective cause of this is the expansion of air on 

 .ascending. This upward movement is caused very largely by cyclonic 

 storms. 



Whether forests have any appreciable effect in cooling the air to 

 below the dew-point is uncertain. From the known effect of forests 

 on the temperature and relative humidity of the air, it is reasonable 

 to infer that they may have some such effect, at least to a small degree, 

 and consequently that they have some influence in increasing precipi- 

 tation. The present evidence, however, derived from many series of 

 observations conducted in Europe and elsewhere, is so conflicting that 

 a definite answer to this question, having the stamp of scientific 

 accuracy, is not possible. 



WHAT BECOMES OF THE RAINFALL. 



That the excessive destruction of forests is followed by the drying 

 up of streams and springs and by a diminution in the minimum flow of 

 rivers is a well-established fact. The forest is the most effective agent 

 known in regulating the disposition of the precipitation after it reaches 

 the ground. 



Rainfall escapes from the ground upon which it falls in five ways — 

 through evaporation, transpiration, surface run-off, seepage run-off, 



