evaporated in the months Of July and August; while from ground 

 under a heavy mulch of leaves on the forest floor it was but 1,150 grams. 

 In the thick spruce woods of Maine one will often find snow on the 

 ground in June, whereas in the open it disappeared before the first of 

 May. Evaporation is profoundly affected by wind. Observations 

 of the United States Weather Bureau indicate that with a wind velocity 

 of 5 miles an hour, other conditions being equal, the rate of evaporation 

 is 2.2 times that of a calm; at 10 miles an hour, 3.8; at 20 miles, 5.7; 

 and so on. It will be readily seen that, by the check in the velocity 

 of the wind that a forest cover causes, the amount of water lost in this 

 way is greatly reduced. Not only is the force of the wind broken 

 within the woodland, but it is retarded for a considerable distance to 

 the leeward. In general, the retardation is felt over 20 feet of hori- 

 zontal distance for every foot in the height of the trees. Thus a stand 

 of trees 50 feet in height all around North Watuppa Pond would 

 materially reduce the evaporation caused by the wind over a water 

 surface of 1.4 square mile, or about one-half the total area of the pond. 



Transpiration. 



Vegetation in the process of growth uses up a large amount of water, 

 which is gathered from the soil by the roots and is then transpired to 

 the air through the leaves. Only a small portion of it remains in the 

 structure of the plant. From a lengthy series of experiments, Risler 

 came to the conclusion that a forest takes up less than one-half as 

 much water as an ordinary agricultural crop. We infer from this that 

 a soil covered with grass or other herbaceous growth loses more 

 moisture from this cause than one covered by a forest. 



Different species of trees take up varying amounts of water. For 

 deciduous species the average amount during one season is 470 pounds 

 of water for every pound of leaf matter; but in the case of coniferous 

 trees it is but 43 pounds, or one-tenth as much. In one or two other 

 respects a broad-leaf wood has slight advantages over the evergreen 

 one as a conserver of moisture ; but this matter of transpiration points 

 to the latter as the most efficient protector of water supplies. 



Evaporation and transpiration represent actual losses of water. 

 Just how great this loss is, will appear from the following table, taken 

 from the excellent report of the Reservoir Commission for 1902. We 

 are indebted to Mr. Safford's work for a great deal of careful informa- 

 tion used directly and indirectly in this report. This table shows the 

 precipitation on the watershed of North Watuppa Pond, compared 

 with the amount of water which found its way into the pond, for the 

 different months of the years 1899 to 1902. On the average, nearly 



