United States. Inland Waterways Commission.

Preliminary report of the Inland Waterways Commission. Message from the President transmitting a preliminary report online

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lower in the forest, and therefore the air is nearer its saturation point.
Hence, the relative humidity of the air in the forest is higher than
that outside, even if the absolute humidity of the air inside and out-
side were not appreciably different. The relative humidity of the for-
est air is between 9 and 12 per cent higher than that in the open, and
is highest in summer.

Character of the soil cover. — The soil cover in the forest, which is com-
posed of a mulch of fallen leaves and humus, reduces considerably the
amount of moisture evaporated from the ground. Experiments con-
ducted during five years (1869-1873) in Bavaria have demonstrated
that a layer of fallen leaves is capable of reducing evaporation from
the soil 25 per cent. Thus while evaporation from the soil in the for-
est, deprived of leaf litter, amounted to 47 per cent of that in the
open, the evaporation from the same soil covered with a fairly deep
layer of leaf litter was only 22 per cent of that in the open. In other
words, while the forest cover alone diminished the evaporation from
the ground by more than half (53 per cent), the forest cover together
with the leaf litter reduced it by 78 per cent, making it less than one
quarter (22 per cent) of that in the open. In mountainous forests the
evaporation is reduced by the combined influence of the forest cover
and such soil covering to from 9 to 13 per cent of the precipitation,
thus allowing from 87 to 91 per cent to remain in the soil."

The lower temperatures which exist under a forest cover, the
greater relative humidity of the air, the lesser circulation of the air,
and the presence of a surface mulch of fallen leaves and humus all
tend to reduce the direct evaporation from the forest soil. Obser-
vations extending for. ten years (1876-1885) in various parts of Ger-
many and Austria showed that the evaporation of water in the
open in the vicinity of the forests is equal to 20.9 inches, while in the
forest it is only 9.5 inches, or expressed in percentage of the evapora-
tion in the open 46 per cent. In other words, the presence of the for-
est saves more than half of the evaporation in the open. The Prus-
sian observations for the ten-year period between 1876 and 1885
showed that the per cent of water evaporated from the soil in a beech
forest is 40.4; in a spruce forest, 45.3; Scotch pine forest, 41.8;
and in young plantations, 90.3 of that evaporated in the open.


Besides the loss of water through direct evaporation from the soil, a
much greater amount is returned into the atmosphere by the transpira-
tion of the leaves, which in distinction from the physical evaporation
may be called physiological evaporation, since it is essential to the
physiological function of the trees. Until lately the general opinion,
based on the classical experiments conducted by Risler, from 1867 to
1872 at the agricultural experiment station of Rothamsted, England,
was that cereals and grasses consume more water for plant transpiration
than forest trees. According to these experiments and those of F. B.

« Die gesammte Lehre der Waldstreu, etc. Dr. Ernst Ebermayer, 1876, p. 185.



Hohnel (Austrian experiment station 1878), the average amount of
water consumed by hardwood forests appeared to be from 6 to 8
inches in depth of water over the ground area in each growing sea-
son, and that by coniferous forests from 4 to 6 inches per year. Cul-
tivated crops, such as cereals and grasses, on the other hand, were
shown to demand from 12 to 18 inches on the ground area; clover
transpired from 13.6 inches to 18 inches, and grass crops even more
than this, or from two to three times as much water as average hard-
wood forests and from three to five times as much as coniferous

From the recent experunents by Ebermayer, Wolln}^, Henry,
Ototzky, and others, however, the water table appears to be mvariably
lower under forest areas than under grass cover, and under grass
cover lower than in a bare cultivated field. In the forest only the
upper layer of the soil was found to be moister than in the open, the
lower layers being always drier than in the open. Thus, according
to Ebermayer' s experiments of 1884-1886, the per cent of soil hu-
midity at different depths in an uneven-age spruce forest and in
the adjacent field was as follows:





years old.

age forest

years old.

one hun-
dred and

years old.






15 to 20 centimeters



45 to 50 centimeters .








The middle-age forest always produced the greatest desiccation
of the lower layers of the soil. This was especially noticeable in
summer and fall. In such a forest the soil humidity at a depth of
47 cm. averaged for the summer 15.12 per cent, while in the field at
the same depth it was 19.89 per cent. The greater dryness of the
soil under the forest is claimed, therefore, to be caused by the trans-
piration of water by the forest. Since trees produce more organic
substance per year per unit of area than other plants, their expendi-
ture of water for the formation of cells must therefore also be greater.

Whether or not this greater transpiration of water by the forest is
compensated by the smaller direct evaporation from the soil and
greater condensation of vapor over forested watersheds is still an
open question. It is, however, generally admitted even by those who
ascribe to the forest a depressing influence upon the water table that
in mountainous regions with a strongly dissected topography, heavy
impermeable soils, abundant snowfalls, short springs, and intermittent
summer showers, the amount of water that penetrates into the ground
deprived of a forest cover is very small — so small, in fact, as to more
than offset the loss of water through transpiration by the forest. In
such cases the forest actually increases the amount of water available
for tree growth and feeding of streams. If to this be added that sur-
face run-off carries away the soil and in this way reduces the volume
of water that can be retained by the remaining soil, the dangers wliich


are threatened by the removal of the forest become very evident.
The experiments thus far carried on were chiefly in the forests of the
plains, where the hydrogpraphic influence of the forest is at its mini-
mum, and further investigations are necessary to make these results
conclusive. Further measurements of the cUscharge of streams from
forested and unforested watersheds are necessary to determine
whether or not forests actually increase the total amount of water
available for stream flow. Mr. Vermeule,'* the consulting engineer
of the New Jersey geological survey, who made a study of the water
supply resources of the State, while unwilling to admit any increase
in the discharge of streams from forested watersheds, recognized
an intimate relationship between yield of water and temperature,
namely, that a difference of 1° in mean temperature will mean 5 per
cent difference in the annual discharge from a given drainage area.
If this observation is correct, then in the light of the results obtained
by carefid observations on the temperature outside and inside the
forest, the latter, by lowering the temperature, by 1° or 2°, increases
the total yield of water from forested watersheds by 5 or 10 per cent.
Mr. George W. Rafter,^ a civil engineer, who more than anyone
else has studied the water supply of streams and the factors affecting
it, attributes to the forest a most marked mfluence on the increase of
the total discharge of a given drainage area. From his studies of the
rivers of New York he came to the conclusion that in the State of
New York and in the neighboring region a forested watershed is
capable of yielding annually from 4 to 6 inches more water than a
deforested one.


That part of the total precipitation which is left over and above
the amount evaporated and transpired by the leaves is the part that
is available for stream flow. This part, therefore, plays the most
important role in feeding the streams, and the effect which forest
cover has upon the behavior of this residue is of the greatest sig-
nificance. While the effect of the forest cover upon the total increase
of the water yield is still problematic, the effect which it has upon
the behavior of the water that reaches the ground is undisputed.

The part of the precipitation which is left over and above the
amount evaporated and transpired by the forest is disposed of fi'om
the surface in two ways: By surface run-off and subsurface drainage.
By surface run-off is meant that part of the water which flows
directly from the surface-and reaches the nearest water course without
absorption. By subsurface drainage is meant that portion of the
water precipitated on the earth which succeeds in sinking deeper into
the soil and passes for gi-eatly varying distances under the surface
before reappearing again at the lower levels of the same or possibly
some other drainage area. The water that flows dnectly from the
surface is flood water which, unless controlled, may cause great
destruction. At a time of heavy rains, or sudden melting of snow,

« Wood Lands and Water Flow in New Jersey. By C. C. Vermeule, Proceedings
of the American Forestry Association for 1894-1895. p. 130.

6 Hydrology of the State of New York. By George W. Rafter, Bull. 85, New York
State Museum.


such water rushes down the mountain slopes, swells the streams,
overflows the low-lymg country, and deposits the sediment eroded
from the mountains on the agricultural land below and in the channels
of the streams.

The surface run-off is responsible for all the sudden variations in
the water stages of brooks and rivers; the subsurface drainage
contributes to their steady and permanent flow. It is important,
therefore, that as much as possible of that surface water shall enter
the ground. The chief and most effective function of the forest is in
increasing the subterranean drainage at the expense of the surface

The ratio of the surface run-off to subterranean drainage is deter-
mined by several factors. Chief among them are: (1) The amount
and character of precipitation; (2) the angle of the slope; (3) the
character of the soil cover; (4) the amount and character of the soil,
subsoil, and underlying rock.

1 . Amount and character of precipitation. — The heavier and more
violent the showers, the greater, all other conditions being equal, is
the surface run-off. The forest by its foliage and branches breaks
the force of the rainfall so that the water reaches the soil without
violence, and at the same time prolongs the duration of the ramfall.
After a storm, water continues to drip from the leaves and twigs for
one or two hours. The water in the forest, therefore, falls more
quietly and for a longer time, and is thus allowed to be absorbed by
the soil.

The rapid melting of the snow in the spring, especially when the
ground is frozen or is saturated with water, favors surface run-off
and lessens seepage. By protecting the snow from evaporation by
the wind during the winter, the forest prevents its wasting away, and
by protecting it during the spring fi"om radiation protracts its melting
for tliree weeks or more. This, together with the less fi'ozen ground
and the greater water-holding capacity of the forest soil, favors sub-
terranean drainage and lessens surface run-off. In this connection
the conclusions reached by Mr. L. G. Carpenter, of the agricultural
experiment station of Colorado," who has made a study of the relation
of forest, melting of snow, and water supply of hrigation streams, are
of interest. They are as follows:

(a) . The mountain streams in the early irrigation season are largely
supplied by melting snow.

(6) . There is a marked diurnal fluctuation, greater with high water
than with low, due to the daily variation in the rate of melting.

(c) . The stream at high water may be one-half greater than at low
water on the same day.

(d). Cloudy weather in the mountains, protecting the snow from
the radiation of the sun, causes the fluctuation to disappear and the
flow to decrease.

(e). This decrease is so great that the cloudiness associated with
continued^ rain usually more than counterbalances the gain from the

(/) . The loss of snow by evaporation is considerable, especially when
exposed to winds.

« Forests and Snow. By L. G. Carpenter, Colorado Agricultural Experiment
Station. Bull. 55. 1901, p. 14.


(^) . Snow remains in the timber and in protected spots much longer
than where exposed.

(h) . This is due not so much to drifting as to shelter from the radia-
tion afforded by the forest cover.

(i) . Hence, the greater amount of forest cover the less violent the
daily fluctuation, the more uniform the flow throughout the day and
throughout the season, and the later the stream maintains its flow.

(/). The loss of the forest cover means more violent fluctuation
during the day, greater difficulty in regulating the headgates and
keeping a uniform flow in ditches, and hence an additional difficulty
in the economic distribution of water. Also the water runs off
sooner, hence the streams drop earlier in the summer and on accoimt
of the lessening of the springs, the smaller is the winter flow.

(Jc) . The preservation of the forest is an absolute necessity for the
interest of irrigated aOTiculture.

2. The angle of the slope. — The steeper the slope the greater, all other
conditions being equal, is the surface run-off. The surface rim-off
is greater from naked soil than from soil covered with grass, and
from grass-covered soil greater than from soils bearing a forest cover.
The naked soil, under the influence of the erosive action of the sur-
face run-off, becomes constantly steeper and this in turn still further
increases the surface run-off. By protecting the soil from washing
and by checking the surface run-off, the forest tends to counteract
the erosion and thus prevent the topography from becoming rugged.
Forest-covered summits and ridges possess therefore a more rounded
and broader form than deforested ones. A forest cover on steep
slopes and summits tends to moderate the topography and thus re-
duces the surface run-off from steep slopes and favorably affects the
permanent flow of streams.

3. The character of the soil cover. — The character of the soil cover has
a decided influence upon the percolation of water in the soil ; and of
all forms of soil cover, that or the forest is by far the most effective
in increasing subsm'face run-off and lessening surface run-oft". The
cro^vns of the trees prevent the compacting of the soil under the
forest by breakmg the violence of the rainfall. The presence of a
surface mulch of leaves and twigs protects the soil and allows it to
retain its granular structure. This increases its absorptive power,
which is often still fiu-ther enlianced by a layer of moss. Penetrated
by a network of roots and covered by branches and stumps, the
ground offers many obstructions to the surface run-off and causes
the water to sink into the ground, which tendency is further in-
duced by the presence of deep channels in the ground left by the
decay of large roots. In a forest the siuface litter is capable of ab-
sorbing more than 2 inches of rain water during twenty-four hours.
Even after it is fully saturated it will give off only very slowly any
additional deposits of water. Experiments carried on for four years
at Haidenhaus, Switzerland, upon the percolation of water inside and
outside a forest and reported by Professor Biihler at the Interna-
tional Congress of Experiment Stations in 1901, have sho\%Ti that
outside the forest the amount of water percolated through the soil
(clay) formed 32 per centof water precipitated, while inside of a beech
forest on the same kind of soil it was 50 per cent, and in a spruce
forest 45 per cent. Thus under the crowTis of trees in the forest
more water sinks into the ground than in the open, in spite of the


fact that in the beech forest 73 per cent of the total precipitation
reached the ground and in the spruce forest 53 per cent. If the
amount of water that penetrated the ground in the open be taken
as 100, then in the beech forest it would be 156, and in the spruce
forest 142.

Thus the forest produces a soil cover which increases the percolation
of water into the ground. This function of the forest is destroyed or
reduced by fires, excessive grazing, or overcutting, which change the
natural conditions of the forest soil cover. It is not sufficient there-
fore to have merely a forest, but it must be protected from fire and
overgrazing and properly managed if its function in reducing surface
run-off and increasing subsurface drainage is to be exercised. It is
justly claimed that the flow of surface water can be completely ar-
rested on forest slopes if these are clothed with a healthy forest growth
so long as the leaf canopy is maintained.

4. The amountand character of soil, subsoil, andunderlyingrock. — The
amount of water wliich actually gets into the soil is greatly determined
by the character and the amount of soil. A barren rock can not retain
water, which runs off as fast as it falls. It is the soil that forms a reser-
voir for retaining the water, and the more there is of it the greater volume
of water may be retained by it. The character of the soil, and there-
fore the kind of rock from which it is derived, affects the percolation
of the water mto the ground. Thus clay soils will favor less percola-
tion than sandy soil, but no matter what the character of the soil may
be, a thin soil cannot retain much water. A forest cover increases
both the amount of soil and its absorbing capacity. The forest in-
creases the amount of soil in two ways: (1) from above, by addition of
leaves and twigs, which on their decay become a constituent part of
the soil; and (2) from below, by inducing disintegration and decomposi-
tion of the underlying rock. By constantly increasing the depth of
the soil, the forest at the same time guards it from washing away and
thus retains it where it was formed. The addition of organic matter
to the soil increases its water-holding capacity; at the same time the
roots enter the narrow fissures of the rock and by their mechanical and
chemical action widen them and thus produce many openings into
which the water may freely sink.

Thus it is certain that, of all forms of soil cover, mountain forests
most favor the absorption of rainfall by the soil at the expense of sur-
face run-off. This they do by checking the rapidity and force of rain-
fall ; by preventing the washing away of the soil ; by increasing its
amount and improving its absorptive properties, and as a result increas-
ing its storage capacity.


The facts which have thus been established relative to the influence
of forests upon stream flow may be briefly summarized as follows :

1. By lowering the temperature, forests induce condensation of
vapor, and by increasing to some slight extent the aqueous precipita-
tion (dew, mist, rain, snow), make available larger amoimts of water
for stream flow.

2. By their foliage, soil covering, protection from wand, low tem-
peratures, greater relative humidity, etc., forests reduce the direct
evaporation from the ground to about one-fourth of that in the open.


Thus a large proportion of the total precipitation is allowed to become
available for stream flow.

3. Forests stimulate the absorption of water by the soil at the ex-
pense of surface run-off. By preventing surface run-off and retaining
the water in the soil, they tend to moderate freshets and floods and
provide for the steady flow of streams.

4. Forests retard the melting of snow and thus provide for the
gradual feeding of mountain streams.

5. By preventing sui-face run-off, forests protect the surface soil
from erosion and thus reduce the amount of sediment carried by

6. By stimulating the absorption of water by the soil, forests act as
a filter in purifying the water supply.

Such in brief are the scientific facts relative to the influence of
forests upon stream flow. TMiile this influence exists wherever forest
exists, there is considerable variation in the degree in accordance with
the climatic conditions. The forest exerts a lower influence in regions
of high humidity, gentle topography, light summer rains, and abun-
dant snow than it does when the topography is more broken, the
evaporative factor high, and the rainfall concentrated and intermit-
tent. Its influence increases with the increase in the aridity of cli-
mate and irregularity of rainfall. Its efficacy, however, is at a max-
imum in a region of heavy intermittent rainfall where the humus is
most eflicacious in promoting absorption and the protective cover
in lessening: erosion.



By W. W. Ashe
Forest Assistant, V. S. Forest Service


The physical influences of forests upon streams affect their volume,
the regularity of their flow, and the clearness of their water, and are
so of great importance through increasing the usefulness of the water-
ways for navigation, for irrigation, and for power. Their extent
varies Math the conditions of the watersheds, topography, climate,
and soil. They are greater where the surface is broken, the soils
either close-textured or deficient in cohesion, the evaporation ex-
cessive, and the rainfall heavy, than in regions of gentle topography,
permeable soils, high humidity, light summer rains, and abundant
snowfall. Forest influences reach a maximum where long droughts
and heavy rains alternate, and where the soils are slowly permeable
or unconsolidated. Under such conditions the humus is most effi-
cient in promoting absorption, and the protective cover of the forest is
more beneficial in reducing erosion. On the other hand, forest in-
fluences reach a minimum in level regions of high humidity and evenly
distributed rainfall, if the soil is cohesive yet loose enough for water
to enter freely.

The conclusions in regard to the physical relation of the forests
of the United States to the rivers may be briefly summarized as
follows :

1. The influences of the forests upon stream flow and turbidity,
while by no means negligible elsewhere, are paramount on the rivers
heading in the Southern Appalachians and the eastern slope of the
Rocky Mountains, and on the streams of the Southwest flowing
through forested or partly forested watersheds.

2. When the natural relations between forest and stream are dis-
turbed in a region where forest influences are high, there is a great
increase in the number and height of the floods; there are longer
periods of low water; and the damage from floods and from silt
and sand which is eroded and deposited as sandbars, goes forward
at an increasing rate. This is shown by the wide destruction from
floods of the Southern Appalachian streams during the past decade,
and by the destruction wrought by the recent uncontrollable tor-
rents of the Southwest.

In accordance with the varying influences of the forest upon stream
flow, the most important rivers of the United States msij be divided



into six groups, each group situated within a region having similar
physiograpliic and cHmatic characteristics, where the forests or the
watersheds exert Hke influences upon the flow of the streams.
These six groups are:

1. Rivers of the Northeastern States and the Great Lake region;

2. Rivers of the ]\iiddle Atlantic coast region;

3. Rivers of the Appalachians;

4. Rivers flowing from the eastern and southern slopes of the
Rocky Mountains;

5. Sacramento and San Joaquin rivers;

6. Columbia River.


This system embraces the rivers of New England, .the Hudson
River, and the rivers in the region of the Great Lakes.

The most important streams of New England are the Kennebec,

Online LibraryUnited States. Inland Waterways CommissionPreliminary report of the Inland Waterways Commission. Message from the President transmitting a preliminary report → online text (page 58 of 83)