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 II. 
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 aérial 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 16 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 
