59 



other so far as the amount of evaporation is considered. This is 

 not true, however, in arid and sub-arid regions, because during 

 years of minimum rainfall the upper layers of the soil are often 

 so dry for months at a time that there is very little moisture to 

 evaporate, while on the other hand during years of maximum 

 precipitation the atmosphere is not sufficiently saturated to check 

 rapid evaporation. 



There is little or no difference between evaporation from a 

 water surface and from any other surface that is thoroughly wet, 

 when both are exposed to the same atmospheric conditions. The 

 evaporation from a water surface is, however, always the same 

 under the same conditions, but it is not the same from other sur- 

 faces, because they vary from completely wet to completely dry. 



In the forest the crowns of the trees remain wet but a short time 

 after precipitation. During this period, however, the evaporation 

 is undoubtedly very rapid, on account of the large surface and 

 from the fact that the crowns are exposed to the wind and sun. 

 But in a long series of investigations made at the Forest Experi- 

 ment Station at Nancy, France, and recently published, it was 

 found that a deciduous forest near that station held back from 

 the ground less than 8 per cent, of the total precipitation. Al- 

 though this is almost immediately returned to the atmosphere in 

 the form of evaporation, it is a comparatively small amount of 

 the annual rainfall. On the other hand, evaporation from the soil 

 in the open and in the forest continues often for long periods 

 after the precipitation ceases. After the crowns become dry, 

 evaporation is much retarded in the forest, because the forest floor 

 is protected from the wind and sun. To such an extent is this 

 true that the loss of moisture through evaporation is much less 

 than that lost from an equally saturated soil or from a water sur- 

 face in the open. Repeated European observations, extending 

 over long periods of time and shorter observations made in this 

 country, conclusively show that evaporation from water or other 

 wet surfaces on the floor of the forest is but one-third or one- 

 fourth that from similar surfaces in the open. From the inves- 

 tigations of the moisture content of isoils in the San Bernardino 

 Mountains, the results of which are as yet unpublished, it appears 

 that the first foot in depth of the mineral soil in the forest may 

 contain two or three times as much moisture as soil of the same 

 general character from similar situations in the open. 



During the summer it is impossible to determine by actual mea- 

 surement the loss of water from the soil either in the forest or in 

 the open, because conditions as to the moisture content constantly 

 vary. During the winter, however, the evaporation from a snow 

 surface can be measured with a fair degree of accuracy. Measure- 

 ments made in the San Bernardino Mountains show that evapora- 

 tion from snow surfaces may be four or five times as great as 

 from water surfaces under similar exposure, and also that the rate 



