62 ATTNTJAL EEPORTS OF DEPARTMEITT OF AGEIOTTLTTJEB. 



expedition naturally was unable to accomplish, all that it had set out to 

 do by reason of failure in transportation arrangements. It, however, 

 determined the fact that the snowfall of the winter of 1914-15 in the 

 mouQtains of eastern Arizona was extraordinarily heavy. 



Fortunately for agricultural and other interests in Arizona that 

 depend upon the water supply, the precipitation of rain, as well as 

 snow, during the winter, was abundant. On April 14, 1915, water 

 began to run over the spUlway of the Roosevelb Dam, storage capacity 

 in the reservoir being reached in that month, a fact easily foreshad- 

 owed by the reliable reports of heavy snow in the mountains that were 

 made at the close of January, 1915. 



Much work remains to be done in the mountain regions of extreme 

 eastern Arizona. Indeed, the work thus far accomplished can only be 

 considered as preliminary to a more general campaign. 



EVAPORATION. 



Data on evaporation constitute a term in the chmatological factors 

 of any given region that is of very great practical value. However, 

 the amount of evaporation from the free surface of a body of water 

 of greater or less extent, as a reservoir, an irrigating ditch, a lake, or 

 the hke, and the evaporation from the surface of soils of various 

 compositions and conditions of vegetal covering, or the absence 

 thereof, also the evaporation from such objects as forests, fields of 

 growing grain, etc., are so different under the same general meteoro- 

 logical or climatological conditions that thus far no satisfactory 

 means of measuring evaporation under the several conditions men- 

 tioned have been found. These conditions, perhaps more than any 

 other, have been a barrier to the serious undertaking of definite and 

 long-continued series of observations of evaporation according to 

 some one particular method of measurement, which at the best would 

 probably not entirely meet the requirements of any one of the lines of 

 study in agriculture, engineering, forestry, and the like. 



Notwithstanding the foregoing, the Weather Bureau has adopted 

 a standard type of apparatus and inaugurated measurements of 

 evaporation thereby. Moreover, the results of various determinations 

 of evaporation that have been made from time to time, either in the 

 Weather Bureau or in cooperation with other Federal services, or 

 that might possibly be available from independent measurements, 

 are now being collated and prepared for pubUcation in appropriate 

 form. The several types of data thus available, however, are not 

 strictly comparable, principally because there has been lack of uni- 

 formity in the methods and apparatus used. In many cases no 

 attempt was made to separate rainfall from evaporation, so that the 

 records are for the most part fragmentary, discontinuous, and more 

 or less imsatisf actory. 



While numerous attempts have been made to correlate evaporation 

 with the meteorological conditions prevailing while it occurs, yet no 

 dependable formula is known. Temperature of the water surface is 

 an important factor, and this datum is generally wanting in many of 

 the older records. It thus appears that the direct determination of 

 the rate of evaporation in various portions of the country by actual 

 measurements under standard conditions at stations weU distributed 

 is certain to ultimately yield data of very great value. 



A model Weather Bureau evaporation station is now maintained 

 at Massachusetts and Nebraska Avenues, Washington, D. C, in co- 



