24 • FORREST SHREVE 



The annual march of the ratio of soil moisture to evaporation 

 is drawn (as a dotted line) together with the curve of evaporation 

 on figures 2 and 3. The arbitrary figures in which the ratio is 

 expressed were secured by dividing the weekly evaporation rate 

 in cubic centimeters by the figures for the percentage of soil 

 moisture at 15 cm., as determined on the same day. The ratio 

 curve will be seen to follow the evaporation curve in a general 

 manner. The high points in the curve indicate a high rate of 

 evaporation with a low soil moisture, and the low points indicate 

 the converse. The ratio curve, as plotted, approaches most 

 nearly to the evaporation curve when the evaporation is high 

 and the soil moisture is low, as on May 8 and 15. The two curves 

 depart slightly further when the evaporation is high and the 

 soil moisture is relative!}" high also, as on August 9 and April 24. 

 During December, January and February the ratio curve runs 

 about half-way between the evaporation curve and the base line, 

 whereas in September and in May and June it runs well above 

 the middle of this interval. In other words the ratio is relative!}^ 

 high in the winter months, as contrasted with the evaporation 

 rate. 



The dotted curve shows that week by week the evaporation 

 rate is dominant in controlling the ratio itself, while betw^een 

 the seasons of the year the soil moisture is potent in determining 

 the values of the ratio. This result is due in part to the small- 

 ness and narrow range of the figures which represent the soil 

 moistures and the largeness of the figures which indicate the 

 evaporation rate; in part it is an expression of the conservative- 

 changes of the soil moisture in contrast to the wide fluctuations 

 of evaporation rate. 



The highest value for the ratio curve is 110 for May 15, the 

 lowest is 11.5 for January 16. Since the ratio itself is an expres- 

 sion of the conditions that chiefly control the maintenance of the 

 absorption-transpiration balance in plants, the maximal and min- 

 imal values of the ratio indicate that these conditions are ten 

 times as severe in the most arid part of the year as they are in 

 the least arid. 



A determination of the soil moisture and evaporation ratio 



