282 YEARBOOK OF THE DEPARTMENT OF AGRICCTLTURE. 



on the other hand during years of maximum precipitation the atmos- 

 phere 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 surfaces, 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 Experiment 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. Although this is almost immediately 

 returned to the atmosphere in the form of evaporation, it is a com- 

 paratively 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 for- 

 est 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 surface 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 investigation of the moisture content of soils 

 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 unpublished, it appears that the first foot in depth of the mineral soil 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 measure- 

 ment the loss of water from the soil either in the forest or in the open, 

 because conditions as to moisture content constantly vary. During the 

 winter, however, the evaporation from a snow surface can be measured 

 with a fair degree of accuracy. Measurements made in the San 

 Bernardino Mountains show that evaporation from snow surfaces may 

 be four or five times as great as from water surfaces under similar 

 exposure, and also that the rate of snow evaporation is profoundl}^ 

 influenced by the wind. In our Western mountains, where the snows 

 are exposed to dry winds, the loss through evaporation is a large per- 

 centage of the total snowfall. In the San Bernardino Mountains, 

 snowfalls a foot in depth are sometimes evaporated in two or three 



