100 FOREST INFLUENCES 



underneath, it permits an evaporation of only 0.13 per cent of a free 

 water surface. About seven-eightlis of the evaporation from the forest 

 is cut off by the woods and litter together. 



The ratio of evaporation from a water surface to the precipitation 

 for the same months is surprisingly large. According to these obser- 

 vations 5 per cent more than the j)recipitation during the warm season 

 could be disposed of by evaporation from a water surface. Many sur- 

 faces permit a readier evaporation, and if we add to evaporation from 

 a meadow, for instance, the large transpiration from the grass, it would 

 appear that on the average more water must be evaporated than falls 

 during this season, which is very imi)robable. The German observa- 

 tions give a more probable value, that of one to two. Evaporation is 

 very readily affected by slight causes, and it appears probable that 

 Dr. Ebermayer's instruments, or their exposure, may give occasion to 

 an increase of evaporation as comjiared with the German stations. 

 The annual rainfall is about the same for the period for which results 

 are published from the two services — 34.3 inches (871 mm.) in the one 

 case, 34.5 (87C mm.) in the other. Tbe amount of water returned into 

 the air over a forest is a quantity worth knowing, even though roughly 

 approximated, especially if it can be compared with that from other 

 forms of vegetation and from bare soil. The forest air is furnished 

 with vapor by the forest tlirough transpiration from the trees, through 

 evaporation from the soil, and through evaporation from the trees. 

 The transpiration has already been estimated at Co inches. This (and 

 other special action of the forest) is practically confined to the warm 

 season, which may be considered as extending from May to September, 

 these months included. The evaporation from a free water surface in 

 the open at the German stations for this time is 8.39 inches (213 mm,). 

 The transpiration is, therefore, 6.5-^8.39= 0.77, or 77 per cent of this 

 evaporation. 



The evaporation from the soil under forest litter is, according to 

 Ebermayer, 13 per cent of that of bare soil in the open in the warm 

 season. Dr. Wollny has carried on several series of observations on 

 evaporation through a forest litter, and from these it appears that the 

 litter reduces the evaporation by one-fourth or one-third. This would 

 give three-fourths of 0.39 or two thirds of 0.39, which are 0.30 or 0,20, 

 numbers considerably larger than Dr. Ebermayer's. As the lattei's 

 observations were made on an observational scale while Dr. Wollny's 

 were only on an experimental one, we will take Ebermayer's result of 

 0.13 as the measure of evaporation in woods. 



Only 70 per cent as much rainfall reaches the rain gauge in the 

 woods as falls in the open fields, the rest is caught in the leaves or 

 branches and moistens these and the trunk. 



Very little runs down the trunk, as has been shown by the observa- 

 tions at Nancy.^ This 30 i^er cent of rainfall nuist be again evaporated, 



' This statement is based on observations made under «me tree, the kind and con- 

 ditions of which are not oven stated, and the conclusions have been refuted by tho 



