Feb. 1, 1902.] 
THE TEOPIOAL AGRICULTURIST. 
513 
THE EFFECT OF FORESTS ON THE CIR- 
CULATION OF WATER AT THE SUR- 
FACE OF CONTINENTS.* 
The whole of this subject ia exceedingly compli- 
cated, because it depends largely on a number of 
elements liable to vary widely within narrow limits 
of time and place. It is thus difficult to frame any 
rule of general application, and for the present the 
enquiry must be limited to defined localities in the 
hope that a large number of observations continued 
for a long series of years, and the progressive im- 
provement of scientific methods, may eventually 
permit of their being combined into one harmonious 
whole. 
The subject, " circulation of water at the surface 
of the soil," must be understood to include move- 
ments in the atmosphere, as well as in the soil and 
ou its surface. The water in the soil may be more 
or less stagnant if the subsoil strata are level and 
impermeable. 
It may now be considered a fact that large 
forests in the plains do indeed act like hill ranges 
as regards the precipitation of moisture. 
Numerous experiments carried out by the Eoole 
Forestiere of Nancy in the Foret de Haye, by M. 
Fautrat in the forest of Halatte, by M. de Pous 
in the forest of Troucais, also in Germany, Austria, 
Russia, and even in India, show clearly tha 
more water falls on forests than ou the open lands 
adjoining. The difference is not very great, but 
m iy be 12 to -iO per cent. 
The additional height due to the trees seldom 
exceeds 130 feet, and is often only half as much. 
The effect is nevertheless noticeable. Throughout 
the year, but especially during the moi-.ture seasons, 
the forests evolve a considerable amount of humidity 
into the atmosphere, and so render valuable assis- 
tance to the surrounding crops. If this were 
visible as fog, it would be seen that apart from 
wind, each forest gives rise to a moist and cool 
layer extending, as shown by ballooning experience 
as high as 4,500 feet. Resinous species liberate 
more water than broad-leaved ones. The tree 
crowns prevent a portion of the rain from reaching 
the ground at all. This quantity instead of being 
carried away by the streams, is re-evaporated and 
passed on further in the atmosphere, and so does 
doable duty. Sooner or later this mass of moist 
air meets a current at a different temperature, and 
the result may be that the same water falls a 
second time as rain in a different place. Hence 
a country possessing a fair share of forests can 
pursue agriculture under much more favourable 
conditions, and a country without forests, like the 
Deccau, Central Asia, and parts of America, is in 
a fair way to become a desert, Consequently the 
creation of forests in the plains is hardly less a 
measure of expediency than in the mountains, and the 
expediency is greater when the plains have naturally a 
light rainfall. Engineers, even of eminence, especially 
if intoiested in irrigation, will dispute this, but 
they do not know everything any more than 
foresters do, and the foresters' side is the side of 
safety. The question divides itself into two parts, 
plains forests and hill forests, In tiie former the 
benefits desired are largely atmoppheric, in the 
latter they a':e rather in the direction of protecting 
the soil itself and of regulating the flow, 
I. — Plains Fokests. 
What becomes of the atmospheric precipitations ? — 
(a) part is retained and evaporated from the 
trees, &c., 
(b) part is evaporated on or in the soil, 
[c'l part flows away along the surface and 
streams. 
* Derived principally from an article by M. E, 
Henry in the E'.Wi des Eaux et Forets. 
65 
(d) part soaks into the soil up to saturation 
^ point, 
(t) part is absorbed by plants for their growth 
and transpiration, 
(/) the remainder sinks to lower levels, where 
it either forms subterranean reservoirs or 
percolates till it again comes to the 
surface as springs. 
Calling E the total rainfall — 
R = a + i + c + d + e + f. 
On a level plain there is no surface flow, and the 
equation becomes — 
/= R — {a + h + d + e.) 
In the simplest case, that of a perfectly bare 
plain, a, c. e becomes zero, and the equation is 
t^n — ib + d.) 
Of ail these factors the only one that can bo 
measured with any-thing like accuracy, is the portion 
a evaporated oS the trees. Even this factor, accor- 
ding to the Mariabrunn observations, is liable to 
very considerable errors of determination, since no 
two rain-gauges will give the same readings evea 
under the same tree, It is necessary to employ a 
large number of rain-gauges, including some embrac- 
ing the trunks of the trees. Even employing 20 
rain-gauges, totalling 10 square feet of opening, the 
probable error is at least 1 per cent, of the fall. 
The measurements should be made either (I) for- 
each individual shower, (2) for a long series ; (3) by 
grouping the showers according to their intensities. 
It has been found in Europe that a broad-leaved 
forest prevents 1 to 3 tenths, and a conifer forest as 
much as 5-teLths, of the total precipitation from 
reaching the ground at all. But these figures hold 
good only for those localities where they were 
obtained. In countries where the rainfall is heavy 
and continuous, the forest soil in any case will ba 
about as thoroughly watered as a bare soil. 
All the other fractions, 6, c, e,/, composing the 
total fall, are still very undetermined in forests and 
other lands alike, and they vary so much with 
every possible local difference that they are hardly 
likely ever to be capable of satisfactory measure- 
ment. 
Reasoning from the known facts that, in spite of 
obstruction by the crowns, the forest soil is as well 
watered as the soil outside, and that the evaporation 
in a forest is much less as proved by the greater 
moisture of the surface soil ; it was supposed that 
forests contributed more than anything else to the 
maintenance of subterranean supplies (level plains 
are still referred to). The results obtained in 
Russia, therefore, came as a great surprise. Sound- 
ings taken during the growing season (1st June to 
1st September) inside and outside the forest of 
Chipoff (Government of VVoronej) showed that the 
water level below the forest was some 32 feet lower 
than outside. In the Black forest (Government of 
Kherson) the level was some 12 to 16 feet lower. 
Presumably these figures are extremes for the 
following reasons — (1) the measurements ware taken 
at the season when transpiration is greatest, (2) 
they were made in localities where the rainfall waa 
only 12 inches in the year, where there waa as a 
probable natural consequence of the dryness an 
almost complete lack of natural forest, and where 
consequently the forest would have to pump all it 
could in order to maintain itself. An increase of 
forest area would probably reduce the necessity for 
so much pumping by the roots. The experiment 
was repeated by M. Ototzky much farther north, 
under the 59th degree of latitude, in the Govern- 
ment of St. Petersburg, where the climate is 
cooler and moisture and the rainfall averages 20 to 
30 inches. The subterranean water is plentiful, yet 
again, the forest lowered the level, but this time 
only by 20 to 46 inches. In order to check the 
Russian results, an experiment has been started by 
