Popular Science Monthly/Volume 7/June 1875/Forests and Rainfall

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THE question of the influence of forests on the hydrology of a region is one that has been warmly discussed. Some men of science, Becquerel for example, hold that forests increase the amount of water received by the soil; while others. Marshal Vaillant among them, assert that forests diminish the quantity. Some savans, such as M. Mathieu, sub-director of the Nancy School of Forestry, have endeavored, by way experiment, to get together such facts as might, if they did not set the question at rest, at least clear up some points and supply a portion of the experimental data needed for a full explanation at some future time. M. Mathieu undertook to "determine the amount of rain-water received by the soils of two neighboring districts, one of them covered with timber and the other arable land; and to find out whether, in consequence of the covering of trees which intercepts the rain-water, the soil of the woodland is as abundantly watered as that of the open." His conclusion is, that timbered soils receive as much, and more, rain-water than the open country.

These experiments are of great importance; the results obtained are noteworthy, and, taken in connection with Becquerel's observations, seem to be decisive of the question. Still, in order to meet an objection that might be raised against this mode of experimenting, viz., the difficulty of finding two districts near to one another and fairly comparable, we have made experiments from another point of view, which confirm those made by M. Mathieu.

No matter how you select two neighboring districts, it is not easy to prove that they are absolutely comparable to one another. The amount of rainfall may be seriously affected by the altitude, and particularly the relative altitude; by the situation of the district; by the relief and configuration of the land in the surrounding country; and by other unknown conditions which may in a greater or less degree change the direction or the velocity of the rain-current, or the point and degree of condensation of the watery vapor contained in it.

M. Dausse, in a memoir which appeared in the "Annales des Fonts et Chaussées," uses the following argument: "Rain is formed when a warm and humid wind comes in contact with strata of cold air; and since the air of forests is colder and more humid than that of the open, rain must fall there in greater abundance."

To gauge experimentally the influence of forests on the rainfall of a district, or, in other words, to ascertain the condensing power of forests, we have compared the results obtained in observations made: 1. Above the forest; and 2. At the same altitude, and at so small a distance from the forest, that any observable difference could be attributed only to the influence of the latter.

"We now made the following observations in the heart of the forest of Halatte, which embraces 5,000 hectares of land. At the height of about six metres (say 20 feet) above a group of oaks and hornbeams eight or nine metres high, we placed a pluviometer, a psychrometer, maximum and minimum thermometers, and an evaporometer, so as to ascertain at that point the amount of rainfall, the degree of saturation of the air, and the rate of temperature and of evaporation.

In the open air, at the distance of only 300 metres from the forest, and at the same height above the ground as in the former case, we placed similar instruments under the same conditions. With regard to the rainfall and the degree of saturation, we give a summary of the first six months' observations, as follows:

Month Amount of rainfall
In the Forest. In the Open.
February 1874...... 18.75 mm. 18.00 mm.
March "....... 15.00 " 11.75 "
April "....... 27.50 " 25.75 "
May "....... 39.25 " 33.50 "
June "....... 51.25 " 48.25 "
July "....... 40.75 " 37.75 "
Total......... 192.50 mm 177.00 mm
Difference in favor of forest 15.50.

Month Degree of saturation of the air in 100ths.
In the Forest. In the Open.
March 1874....... 71.1 70.0
April "....... 64.3 64.2
May "....... 64.1 60.4
June "....... 60.9 60.1
July "....... 54.6 53.8
Total......... 315.0 308.5
Mean......... 63.0 61.7
Difference in favor of forest 1.3.

If these observations, which are still made daily, continue to give the same results, it may then be affirmed that forests constitute vast condensing apparatus, and the conclusion will be inevitable that more rain falls in wooded land than on bare and cultivated soil.—Comptes Rendus.

  1. Translation of a communication to the French Academy of Sciences, by L. Fautiat and A. Sartiaux.