68 CARNEGIE INSTITUTION OF WASHINGTON. 



behavior which has been found in the Santa Catalina Mountains, 

 which present a uniform tj'pe of gneiss from base to summit; and also 

 to determine the indirect effect which the total height of mountains 

 exerts upon the gradient of vegetation presented by their slopes. It 

 has been found, in general, that the vegetation at a given elevation near 

 the summit of a low range is much more desert than it is at the same 

 elevation on the slopes of a higher range. The instrumentation carried 

 out on the Santa Catalina Mountains, when interpreted in terms of the 

 topograi)hic configuration of the range, indicates that the rainfall is 

 heavier at 5,000 feet, for example, on the slopes of a 9,000-foot moun- 

 tain than it is at 5,000 feet on the summit of a lower range. Further- 

 more, the absence of cold-air drainage on the summits of low mountains, 

 as compared with the slopes of higher ones, causes marked differences 

 in the temperature conditions which obtain at the same altitudes in 

 mountains of different elevation. 



These explorations are yielding a broader basis for our knowledge of 

 the correlation of vegetation and climate and are gradually affording 

 the materials for a classification of vegetational regions more satis- 

 factory than that which is now in use, while they also suggest definite 

 lines of instrumentation to be taken up in connection with further work. 



The Annual March of the Ratio of Evaporation to Soil Moisture, 



by Forrest Shreve. 



Data have been elaborated and published which comprise the weekly 

 percentages of soil moisture at three depths, the concurrent weekly 

 evaporation, and the ratio of the evaporation to the moisture of the 

 soil at the lowest of the three depths sampled. The soil investigated 

 was the heavy clay of Tumamoc Hill. The w^eekly values of the ratio 

 obtained are a concise expression of the complex of soil and atmospheric 

 conditions by which the water relations of desert plants are controlled. 

 Their value as a criterion of climatic conditions lies in the fact that they 

 express both the soil and atmospheric conditions, and much more 

 precisely than would a ratio of rainfall to evaporation. The highest 

 figures for the ratio — which indicate the least favorable conditions for 

 vegetation — are secured in the arid periods which precede and follow 

 the summer rainy season, while the lowest values are obtained in the 

 winter months. The annual maximum value obtained was 110, the 

 minimum 11.5, indicating an annual amplitude of moisture conditions 

 under which the winter is approximately ten times as favorable as the 

 arid portions of the summer. The annual fluctuation of moisture con- 

 ditions at the Desert Laboratory exhibits extremes, in other words, 

 which are as great as those found between the desert valleys and the 

 forested mountain summits of southern Arizona. (See Ann. Rept. Dept. 

 Bot. Research, p. 59, 1912.) 



