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By RAPHAEL ZON, Chief of Silvics, United States Forest Service.


Of all the direct influences of the forest the influence upon the supply of water in streams and upon the regularity of their flow is the most important in human economy. Yet so many are the factors which play related parts in this influence, so great is the difficulty of observing them with precision, and so wide the range of economic interests affected, that considerable divergence of opinion has arisen on the subject. This, however, if prompted by a sincere desire to reach the bottom of a complicated and vital problem, can only be productive of results of the highest scientific value.

There is, perhaps, no other problem facing the American people to-day which demands such care in the scientific accuracy of its data and conclusions as does the relation between forests and water. It is imperative, therefore, that no final conclusions be drawn in regard to this relation until ample, reliable, and critically revised evidence upon which to base them is available. A national policy which, though considering the direct value of forests as a source of timber, fails to take full account also of their influence upon erosion, the flow of streams, and climate, may easily endanger the well-being of the whole people.

This paper aims to bring together impartially all the well-established scientific facts in regard to the relation of forests to water supply. Such a critical statement of our present knowledge of this subject should be helpful in separating what is definitely known of this relation from that which still needs to be determined.



The influence of the forest on streamflow can be determined in two ways: (1) By actual measurements, continued for sufficient time, of the total discharge and of the high and low stages of rivers having drainage areas essentially similar in regard to precipitation, geologic formation, topography, and soil, but differing in the amount of forest cover, and (2) by determining, through the measurement of individual factors, the total amount of water available for streamflow.

The first method, since it deals directly with the measurement of water, may properly be called the hydrometric method; while the other, which deals chiefly with the physical and mechanical effects of the forest upon evaporation, run-off, etc., may be termed the physical method. The first method studies the final result of all the factors affecting streamflow as they are shown in the behavior of the stream. It is the most direct, and would be the most practical, provided all the conditions of the watersheds studied, except forest cover, were essentially similar. Since, however, such similarity between watersheds is seldom met with in nature, this method, which has frequently been employed by engineers, is particularly liable to error. Further, the existing hydrometrical data are incomplete, for the hydrographic method requires the nicest observations, extending over a considerable period of time. The defects of the method in this regard were well brought out at the International Milan Congress of Hydrographic Engineers of 1905, as shown in the summary of the papers presented there.

The ideal application of this method would consist in comparing the regimen of a stream flowing from a completely forested watershed with the regimen of the same stream after its forest cover has been removed. As a control area there ought to be another watershed, exactly similar in character, on which the forest cover remains unchanged while that on the one under experiment is being removed. To make such a comparison thorough and accurate, the topography, geological formation, and character of the soil of the two watersheds should be surveyed, and each watershed then equipped, at different elevations, with ordinary and self-registering rain gauges, as well as with instruments for measuring evaporation, wind velocity, level of ground water, and especially the flow of the stream during different seasons of the year. This would yield reliable information as to the influence of forest upon streamflow for regions having similar soil, climate, and character of forest. Such experiments, however, with the exception of one now being conducted at Emmenthal, Switzerland, by the Swiss experiment station, and one at Wagon Wheel Gap, Colo., by the Forest Service, in cooperation with the Weather Bureau, have never been made in any part of the world. The most common practice, especially with engineers, has been to compare the run-off from drainage areas in the same general region, of which one may have more forest cover than the other. If the comparisons were based on equally large numbers of stream gaugings and dependable meteorological records, and the drainage areas compared were similar in regard to topography and soil, the average results might assume some practical value. Unfortunately, however, reliable gaugings for a given stream too often can not be supplemented by reliable meteorological data. for the same region. Therefore, while there is no end to the material gathered in this way, it is often contradictory, at best showing only tendencies, and failing to determine with any accuracy the nature and extent of the influence which the forest has upon streamflow.

The second method analyzes separately the influence of the forest upon each of the different factors affecting streamflow, and the final effect of the forest upon streamflow is deduced from the combined effect on all the factors. While less direct, this method lends

itself more readily to experimentation, and has been largely employed by foresters.

Since a large amount of reliable data concerning the influence of the forest upon the amount of precipitation over forested and unforested drainage areas upon evaporation, surface run-off, percolation, etc., has been obtained by this method, it will be discussed first.



The influence of forests upon climate has been a subject of investigation for a long time, and is not settled yet. Now and then this influence has been exaggerated, thus leading to the other extreme of denying it entirely. In discussing the subject, therefore, one has to be very careful in selecting facts and in drawing conclusions from them.

The physical and physiological processes which accompany any plant growth must necessarily reduce the temperature of the air, at least during the vegetative period. First, because the leaves evaporate water. Second, because the heat of the sun is consumed in this evaporation, and the plant can not become heated to the same extent as, for instance, a rock or soil without any vegetative cover. Similarly, the ground under plants can not become greatly heated on account of shading. Third, the surface from which heat radiates at night is much greater when vegetation is on the ground than when the ground is bare. The cooling effect on the air by crops has been experimentally proven. For every pound of dry substance produced it has been found that corn evaporates 233 pounds of water and turnips 910 pounds.

Under good cultivation an acre may produce about 7 tons of dry substance. If the evaporation of water be only 500 times more than the amount of dry substance produced, then an acre will evaporate during the vegetative period about 3,500 tons of water. This example shows the extent to which ordinary crops can contribute to the moisture content of the air and the cooling which accompanies this evaporation. Forests, being the most highly developed form of vegetable life, exert this influence in the greatest degree.

The first systematic observations upon the effect of forests on climate were made in Bavaria. The climatic influences first observed were those which are least changeable from year to year and which are fairly uniform over large areas, namely, the temperature and humidity of the air, evaporation, temperature of the soil, and percolation of the water into the ground.


Table 1 gives the results of early observations upon the temperature of the air outside and inside the forest carried on in central Italy, eastern France,1 in the mountains of Alsace, Bavaria,2 and eastern Prussia. The stations inside and outside the forest were

1 Woeikov, A. I.

2 Ebermayer, E.

lin, 1873.

Muttrich, A.

The climates of the world (in Russian). St. Petersburg, 1884. Die physikalischen Einwirkung des Waldes auf Luft und Boden. Jahreshericht uber due Beobacht ungsergebnisse der im Konigreich Preussen und in den Reichslanden eingerichteten forstlich-meteorologischen Stationen, 1875, 1876.


scarcely a mile apart. The differences between the temperature of the air outside and inside the forest are shown by plus and minus. Plus indicates that the temperature in the forest is higher; minus, lower than in the field.

TABLE 1.-Temperature of air in the forest compared with that in the open.

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This table brings out clearly the moderating influence of the forest upon temperature of the air. In the forest the maximum temperature is always lower, and the minimum temperature higher, than outside. More recent observations extending over long periods in France, Germany, Austria, Switzerland, and other countries have confirmed the earlier results.

The yearly mean temperature at equal elevations and in the same locality has invariably been found to be less inside than outside a forest. In a level country this difference is about 0.9° F. It increases, however, with altitude, and at an elevation of about 3,000 feet is 1.8° F.

The monthly mean temperature is less in the forest than in the open for each month of the year, but the difference is greatest during the summer months, when it may reach 3.6° F., while in winter it does not often exceed 0.1° F.

The daily mean temperature shows the same difference, but to a greater degree. During the hottest days the air inside the forest was more than 5° F. cooler than that outside, while for the coldest days of the year the difference was only 1.8° F.

The temperature of the air within the forest is, therefore, not only lower, but also subject to less fluctuation than in the open.

It is in tropical and subtropical regions that the influence of the forest upon the temperature of the air is probably greatest. In northern British India, in latitude 24-27° N., the mean annual temperature for the year varies but little with latitude. In winter the rainfall is very scant, and in summer very abundant. Before the rainy season the valleys of northern India have a period of extreme hot weather and drought. On the whole, the temperature and the dryness are moderated by proximity to the sea.

The country along the Ganges, and in general to the west of the lower extension of Brahmaputra, is almost treeless, while in Assam, along the middle extension of Brahmaputra, there are large forests. A comparison of temperatures at these places, made by Woeikov,1 and given in Table 2, shows clearly the influence of the forest upon the temperature and humidity of the air. Of the places named, Brahmaputra lies at 24° N., the others between 25° and 27° Ń. The arrangement is from west to east.

TABLE 2.—Difference in temperature and humidity between treeless and forested regions, British India.

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The figures in the table indicate that forests play a much greater part in moderating the temperature in the hot and dry months of April and May than does proximity to the sea. The same thing holds for the relative humidity, especially in Sibsagar, which is the center of the forested region. The most striking influence of the forest is the lowering of the absolute temperature maxima. Proximity to the sea has but little effect upon this, but as soon as the forest region is reached, the absolute maximum temperature is at once decreased by about 15° F. Especially striking is the difference in the average temperature in May between Benares and Goalpara. The distance between the two places is only 462 miles, the latitude about the same, and the intervening country level. Both are at considerable distances from the sea, yet the difference in the average temperatures for May is 13° F., or about 1° for every 35 miles. This difference in temperature Woeikov explains by the presence of forest in Goalpara.

Woeikov further cites observations in the basin of the Amazon River, which possesses the largest forest area in the world. The middle and upper extensions of the Amazon River are about 621 miles from the Atlantic Ocean and are separated from the Pacific Ocean by very high mountains. At such great distances from the ocean, and so close to the equator, one would naturally expect to find very high temperatures and great dryness. Yet there the average temperature of the warmest month and the absolute temperature maxima are not greater than at the sea, and even not as high as the temperatures often experienced in middle latitudes. This is shown in Table 3.

1 Woeikov, A. I. The climates of the world (in Russian). St. Petersburg, 1884, p. 321. 36135°-S. Doc. 469, 62-214

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