Southeastern Washington and Adjacent Idaho.^ 45 



growing seasons of 1913-14. One was located on a southwest 

 slope about 65 feet from the crest of a hill somewhat less than the 

 average height, and another at an equal distance from the crest 

 of the same hill and on an average northeast slope. The average 

 daily evaporating power of the air at the two stations respectively 

 is shown in Fig n. 



An inspection of these graphs shows a remarkable similarity in 

 the general course of either pair throughout the season. How- 

 ever, the difference in the amount of evaporation on north and 

 south slopes is just as striking. During 1913 the average daily 

 evaporation on the northeast slope was 16 c.c., which is only 64 

 per cent, of that on the southwest slope (25 c.c.). The season of 

 1914 was considerably drier and gave higher evaporation rates on 

 both slopes (23.9 c.c. on northeast, and 33 c. c. on southwest) with 

 a ratio on the two slopes of 72 : 100. 



The evaporating power of the air on the southwest slope and at 

 a height of 50 cm. is also shown in the figure. It may be seen 

 that the evaporating power of the air at this height is increased 

 20 to 26 per cent, over that at 17 to 23 cm. This factor of in- 

 creased desiccation plays an important role in determining the 

 low stature of many of the prairie plants. Because of the cool 

 nights, about 80 per cent, of these high water losses occur during 

 the day. When we consider that the daily evaporation in July 

 and August often reaches 40 'to 55 c.c., and at a time when the 

 available soil moisture is depleted to a depth of two feet, we can 

 see the necessity for extensive root-systems in the substratum 

 as well as aerial structures for enduring drought. 



The greatest factor in causing the differences in evaporation on 

 the two slopes is the same one that has so profoundly influenced 

 the topography, namely, the wind. During the season of 1913 

 (April 1 6 to September 3) the wind was from a southerly direc- 

 tion 68 per cent, of the time and a total of 13,605 miles passed 

 over the southwest slope at a height of 50 cm., while only 56 per 

 cent, as many miles were recorded by an anemometer similarly 

 placed on a northeast slope. In general, these conditions were 

 duplicated in the season of 1914 with the ratio of 100: 49. Since 

 the factors of temperature, humidity, and wind movement are 



. 45 



