1 May, 1902.] QUEENSLAND AGRICULTURAL JOURNAL. 393 
Forestry. 
THE EFFECT OF FORESTS ON THE CIRCULATION OF WATER 
AT THE SURFACE OF CONTINENTS.* 
The whole of this subject is exceedingly complicated, because it depends 
targely on a number of elements liable to vary widely within narrow limits of 
lime and place. Jt is thus difficult to frame any rule. of general application, 
and for the present the inquiry 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 improvement 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 movements in the atmosphere as well as in the soil and 
on its surface. The water in the soil may be more or less stagnant if the sub- 
soil 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 Ecole 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, alsoin Germany, Austria, Russia, and even in India, show 
clearly that more water falls on forests than on the open lands adjoining. The 
difference is not very great, but may be 12 to 20 per cent. 
The additional height due to the trees seldom exceeds 180 feet, and is often 
only half as much. The effect is, nevertheless, noticeable. Throughout the 
year, but especially during the moisture seasons, the forests evolve a consider- 
able amount of humidity into the atmosphere, and so render valuable assistance 
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 
ieee 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 
tain 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 double 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, 
andacountry without forests, like the Deccan, 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 lightrainfall. Engineers, 
even of eminence, especially if interested 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 the former the benefits desired are largely atmos- 
pheric ; in the latter they are rather in the direction of protecting the soil itself 
and of regulating the flow. 
I.—Puains Forests. 
_ What becomes of the atmospheric precipitations >— 
(a) Part is retained and evaporated from the trees, &c., 
(4) Part is evaporated on or in the soil, 
(c) Part,flows away along the surface and streams, 
<a 
* Derived principally from an article by M. E. Henry in the Revue des Eaux et Forets. 
