DEPARTMENT OF BOTANICAL RESEARCH. 69 



close to the sea, a second pair of instruments was installed in the Car- 

 mel Valley, 7 miles from the ocean. During June, July, and August 

 the maximum on the coast was 4.1° higher on the south-facing than 

 on the north-facmg slope, with a smaller difference in the minima. 

 In the Carmel Valley the maximum was 5.8° higher on the south 

 than on the north slope. These figures indicate that the results 

 secured at Tucson are at least not of universal validity, if, indeed, 

 they are true of steeper slopes in that region. 



A comparison of the soil-temperatures on the coast and in the in- 

 terior at Carmel shows that the maximum during the clear days of 

 June was 11.5° higher at the Carmel Valley station and the minimum 

 11.1° higher, while during the foggy weeks of July the corresponding 

 figures were 17.7° and 14.6°. In both periods the maxima were from 

 6° to 8° higher on the north slope in the interior than they were on 

 the south slope on the coast, which is a striking index of the difference 

 between the environmental conditions inside and outside of the 

 coastal fog belt. 



.4 Method for Measurement of Evaporation from Soils in Place, by 



Forrest Shreve. 



In order to determine the amount of water lost by different types 

 of soil and by the same soil at different times, under the natural con- 

 ditions of surface, wetting, and penetration, a method has been de- 

 veloped and used in connection with field work at Tucson. This is 

 an adaptation of the polymeter method for measuring the transpira- 

 tion of plants in their natural setting. It consists, essentially, in 

 placing a large bell-jar over a spot which is free from plants, placing 

 under the jar a polymeter calibrated for the range of humidities to be 

 encountered, and reading the initial and final humidities and tem- 

 peratures over an accurately measured period of time, which should 

 be varied with the speed of the rise in humidity. From the initial 

 and final vapor pressures the amount of water may be calculated 

 which has resulted from evaporation during the period of the observa- 

 tion. Reductions may be made easily into terms of water lost per 

 square meter per hour. 



At the relatively low humidities under which the method has been 

 employed (15 to 25 per cent) it is found better to observe the time 

 required for a rise of 10 per cent in humidity, although it is possible 

 to observe the humidity attained at the end of a definite period of 

 time. The principal limitations of the method are the necessity of 

 shading the bell-jar in order to prevent too rapid a rise of temperature, 

 and the inevitable stoppage of any air movement that may be acting 

 at the time to increase evaporation. As all readings are taken under 

 identical conditions in these respects, they serve to compare the 

 evaporation in different soils and at different times in a much more 



