22 FORREST SHREVE 



spot, would be less than the figures given, according to the rocki- 

 ness, but would be proportional to them. This indicates that in 

 the soil of Tumamoc Hill at its minimum annual water content 

 there is between one-fourth and one-fifth as much water as there 

 is at its maximum annual content. 



The average annual rainfall of the Desert Laboratory is 14.60 

 in., or 37.1 cm., an amount which places on the surface of each 

 soil unit 371 k. of water per annum. At its minimum water 

 content the soil contains, therefore, somewhat less than one- 

 fourteenth of its available annual supply. This fraction is les- 

 sened in the case of rocky units of soil, and is increased in the 

 case of a year in which the nm-off is great. 



RATIO OF EVAPORATION TO SOIL MOISTURE 



The curve of weekly rates of evaporation given in figures 2 

 and 3 is based on figures secured from porous cup atmometers 

 which are run continuously on the roof of the Desert Laboratory. 

 The results of these readings are collated and reduced to stand- 

 ard by Dr. B. E. Livingston, to whom I am indebted for the 

 figures used here. The readings of evaporation were taken on 

 the same day of the week that the soil samples were secured, and 

 the reading plotted to each date is the total evaporation for the 

 preceding seven days. 



The weeklj^ loss registered on August 23, 1910, was 318 cc. 

 For three weeks thereafter the rate rose until it reached 1084 cc. 

 on October 12, the maximum for the arid after-summer. The 

 rate then fell throughout the winter to the annual minimum of 

 173 cc. on the week ending January 16. The rate remained low 

 thereafter until March 6, with a reading of 242 cc, and then 

 rapidly rose to a loss of 836 cc, on April 18. From the latter 

 date until July 4 there was a sustained high rate of evaporation, 

 attaining its maximum of 990 cc. on May 15. 



The total evaporation from August 3, 1910, to July 31, 1911, 

 was 31,447 cc. In order to convert this amoimt into terms of 

 loss from a free water surface it is necessary to multiply it by 

 0.76, a factor which has been experimentally determined by com- 

 parison of losses from the atmometer and from a petri dish of 



