FOREST AREAS AND SOIE WATER 
143 
has found the same decrease of evaporation with succession of 
plant associations in Washington and Idaho. In Minnesota and 
Nebraska, Weaver and Thiel found the same relation to exist. 
Gleason and Gates have studied the influence of the various 
types of vegetation in Central Illinois. They have presented 
facts to show that here, as in other regions, the differences in the 
amount of evaporation in various associations are due chiefly to 
the nature of the vegetation, and that the more primitive types 
have the higher rates of evaporation, while those most nearly like 
the climax types have the lowest rates. 
In comparing the daily rate of evaporation at the top of the 
slope with that at the middle, the averages are found to be; 
Top 
Middee 
Timber 
11.24 c. c. 
7.12 c. c. 
Brush...... 
15.83 c. c. 
12.87 c. c. 
Open 
20.25 c. c. 
18.69 c. c. 
Thus it will be observed that in each case the average daily 
rate is less at the middle than at the top of the slope, and ref- 
erence to table B shows again that this is uniformly true for 
each day. 
Again, comparing the rates for the north and south slopes as 
shown in table C the following averages are obtained : 
North 
South 
Timber 
5.58 c. c. 
6.79 c. c. 
Brush 
9.64 c. c. 
12.26 c. c. 
However, while the averages in this case are lowest for the 
north slopes it is not true that the readings for each individual 
day show the same relation. For example, on July 15, the evapo- 
ration was 1.44 c.c. higher on the north than on the south timbered 
slope. The same is true in several other instances. It is impor- 
tant to note in this connection that, in the cases where the evapo- 
ration of the north slope exceeded that of the south slope, the 
direction of the wind was northwest, northeast or westerly. 
The lowest minimum evaporation for the season occurred on 
July 13 on the north timbered slope, being only 1.44 c.c. The 
lowest maximum (11.52 c.c.) occurred on the same slope on 
11 Wieaver, J. 1)., A study of the vegetation of Southeastern Washington and 
adjacent Idaho: Reprint from Univ. Studies, Univ. Neb,, XVII, No. 1, January, 1917. 
12 Weaver, J. E.., and Albert F. Thiel, Ecological studies in the tension zone be- 
tween prairie and woodland: Bot. Sur. Neb., New Series, No. 1, April, 1917. 
13 Gleason, H. A., and F. C. Gates, A comparison of the rates of evaporation in 
certain associations in Central Illinois: Bot, Gaz., 53, 478-491, 1912. 
