16 DIMINISHED FLOW OF ROCK RIVER. 



conditions, as well as the protection afforded by the leafy canopy 

 against the rays of the sun, materially retard evaporation. It should 

 be remembered, also, that the forest protects the soil from the drying- 

 action of the winds, which in open areas constantly absorb the mois- 

 ture from the surface. As evaporation increases very rapidly with an 

 increase in the velocity of the wind, the opposition offered by the for- 

 est is a means of protection of the first importance. 



A very instructive study of soil evaporation within and without the 

 forest has been carried on at the experiment station connected with 

 the national forest school of France. The results of thirty-three years 

 of observation, recently published.'' are thus summarized: 



It appears, therefore, that during the months of November, March, and April, for 

 which complete data are available, the instrument situated on open ground has 

 always lost about twice as much water as the one situated within the forest. * * * 

 During the summer the difference is much more striking, owing to the presence of 

 foliage, and varies considerably, being very decided as the temperature rises. The 

 proportionate amounts evaporated outside and within the forest are as 3 to 1 in May, 

 5 to 1 in June. July, and August. 4 to 1 in September, and 3 to 1 in October. J > 



In winter and early spring- the forest is useful in preserving the 

 snow cover, which furnishes a considerable part of the water supply 

 of the ensuing season. This preservative influence is less in deciduous 

 forests than in such as are composed of evergreen species; neverthe- 

 less, the trunks and branches of the trees and the dried foliage retained 

 by oaks and several other species during a part of the winter protect 

 the snow to a certain degree from the sun and wind. The leaf litter 

 on the forest floor is also of some value, because snow that falls on it 

 does not melt as readily as on bare soil. Scientific investigations have 

 shown a decided difference in the preservation of snow on forested as 

 compared with unforested areas, resulting in a more uniform and sus- 

 tained flow of the streams where forests are present. ' ; 



To appreciate the influence of a forest cover in all its aspects, the 

 amount of moisture required for the growth of the forest itself should 

 be taken into consideration; and the fact should also be noted that the 

 crown cover of a forest intercepts some of the precipitation, which, 

 therefore, never reaches the soil. The loss occasioned by the inter- 

 vention of the crown cover, however, is not excessive, and may be 

 offset in part by the effect that the saturated foliage probably has in 

 retarding evaporation from the soil. Some idea of the loss due to the 

 presence of the crown cover may be obtained from the results of 

 studies carried on in a deciduous forest situated on a plateau to the 



« Observations de Meteorologie Forestiere. M. Raoul de Drouin de Bouville. 



&The months of December, January, and February were not included in the sum- 

 mary, because the results for these months were unreliable on account of frequent 

 frosts. 



c 'Seealso Forests and Snow, by L. G, Carpenter. Colorado Agricultural Experi- 

 ment Station Bulletin 55, 1901. 



