U iets 
422 
report will be found a collection of observations on 
evaporation in this country and Europe. They show 
that large rivers receive in the main channels seldom 
more than one quarter of the average amount that 
falls on their watersheds. The remaining three 
quarters is evaporated. On small watersheds the 
proportion of loss from evaporation is small. The 
average flow into Croton Lake is about fifty per cent 
of the average rainfall on the gathering-ground, the 
area of which is 339 square miles. © 
The amount of water flowing from Cochituate 
Lake watershed, of 18.75 square miles, is, on the aver- 
age, forty-five per cent of that which falls; but the 
proportion varies greatly from year to year. In 1866 
only twenty-five per cent of the rainfall flowed into 
the lake; while in 1857, with almost the same rainfall, 
seventy-four per cent of the precipitation entered the 
lake. The difference in the amount evaporated in 
1866 over that of 1857 was equivalent to a depth of 
thirty inches of water over the whole gathering- 
ground. Experiments made in Denmark and Eng- 
land show that the mean annual evaporation from 
soil and grass land is from twenty-six to thirty inches, 
or from fifty-six to sixty-seven per cent of the rain- 
fall. The tables of rainfall and flow of the Sudbury 
River and Cochituate Lake show, also, that the sum- 
mer evaporation amounts to eighty per cent of the 
rainfall on these basins; and that in March and April 
all the rainfall may flow off, together with a large 
amount of water from melting snow accumulated 
during the winter. The Sudbury River tables show 
that on this watershed the loss of water by evapora- 
tion between May and December is from three to 
four times the quantity lost in spring floods. 
Effect of woods on the flow of streams. — The facts 
given prove that evaporation from the ground is 
the most effective cause controlling the summer and 
autumn flow in our streams: therefore whatever 
tends to retard evaporation will increase the summer 
flow of springs and streams. The great promoters 
of evaporation are heat, dryness of atmosphere, and 
wind. Woods, especially when the trees are large, 
act in three ways to prevent evaporation from the 
ground: they keep the surface cool, the atmosphere 
moist, and the lower stratum of air so still that the 
powerful drying-action of the winds is felt compara- 
tively little. The amount of water which will be 
lost from any watershed by the removal of the woods 
must depend on the steepness of its slopes, the char- 
acter and depth of soil, and the nature of the under- 
lying rocks. 
The greater part of the basin of the west branch of 
the Croton River is wooded. Its area is about twenty 
square miles. It yielded for four years an average 
flow of sixty-three per cent of the rainfall. The 
mean precipitation was 50 inches. The yearly evap- 
oration was as follows: 15 inches, 11 inches, 23.6 
inches, 15 inches. The Sudbury River basin in Mas- 
sachusetts, containing seventy-eight square miles, is 
wooded over only from one-sixth to one-eighth of its 
area. From 1875 to 1879 inclusive, the mean annual 
rainfall was 47.7 inches, and the amounts of water 
SCIENCE. 
[Vou. III, No. 61: 
lost, principally by evaporation, were 25 inches, 26 
inches, 19 inches, 27 inches, 23 inches. ‘The Cochit- 
uate basin is only partially wooded. Its average 
rainfall, 50 inches, is about the same as the West 
Croton and the Sudbury. The average yearly loss 
from the watershed is 27 inches, while in certain cases 
the evaporation alone must have exceeded 35 inches. 
The West Croton appears to yield in its flow some 
eighteen per cent more of the rainfall than the other 
watersheds mentioned; but the dissimilarity in their 
topography and geology makes it impossible to say 
how large a part the woods ‘play in the differences of 
flow. Yet the result bears out the ordinary rule 
deduced from observations taken in Europe, that the 
average flowin streams draining wooded and swampy 
basins will be from sixty to eighty per cent of the — 
mean rainfall, while those draining watersheds of 
undulating pasture and woodland generally receive 
into the main channel only from fifty to seventy per 
cent of the mean rainfall. Should the rule prove 
applicable in this country, the average increase of 
evaporation by the removal of woods from a-district 
may amount to ten per cent of the annual rainfall. 
This loss will occur mainly in summer and autumn, 
so that the flow during this season will be diminished 
in far greater ratio. 
With almost equal rainfalls (22.5 and 20 inches), 
more than two and a half times as much water (8.3 
inches) flows from the wooded basin of the West Cro- 
ton as is discharged from the comparatively unwooded 
watershed of the Sudbury River (2.9 inches) between 
June land Noy. 1. During the remainder of the year 
the discharge from these two watersheds is almost 
the same. These results, being deduced from only 
four years of observation, would be modified by fur- 
ther measurement. The great difference in the flow 
from these two basins, since it is shown to occur 
between June and October, is undoubtedly due to 
a difference in the amount of water evaporated; and 
this is only partly accounted for by differences of to- 
pography and soil. The woods are surely playing 
an important part in maintaining the summer flow 
of the West Croton. Cutting them off might easily 
reduce the summer and autuinn flow twenty-five per 
cent. While it must be understood that the facts 
are too limited to base any final results upon them, 
yet they indicate the probability that the summer 
and autumn flow of streams may be reduced in vol- 
ume twenty-five per cent by cutting off the woods 
from their watersheds. If the summer and autumn 
flows in the upper Hudson, the Mohawk, and the 
Black Rivers, were lessened twenty-five per cent from 
their present average volume, the navigation of the 
Hudson and the canal would be doubtful; and cer- 
tainly the loss in hydraulic power for manufacturing- 
purposes would be very great. While the people of 
New-York City are wisely taking an active interest 
in the protection of the northern forest, they should 
not forget that the preservation of the woods on 
the Croton watershed is of great importance in 
maintaining the summer water-supply of New — 
York. isa 
