How Watersheds Are Affected by Forest Conditions
Influence of Forest Cover on the Erosion of Soils—Advantages of Owning the Catchment Area—Effects of Forests on Wind Influence
THE importance of forest covering for watersheds is emphasized in the following paper, especially under certain conditions of soil and the configuration of the land. The paper is written by an expert in forestry and contains much information of interest on the subject:
Forests may be employed in several ways in promoting the protection of potable water supplies. The most important ways, however, in which forests may function are in the protection of catchment areas and in the protection of reservoirs and settling basins. The employment of such a means of protection in no way, however, obviates the necessity of filtration or minimizes the requirement for sanitation. A reservoir having its surrounding slopes protected by forests may enable water of a lower turbidity to be delivered, and a forest cover may also prolong the life and value of a reservoir. The forest covered catchment area may insure the receipt at the filter of water of lower bacterial count as well as of lower turbidity. There will be many cases where the revenue which may be secured from a well managed forest on a catchment area will be a material contribution toward meeting its cost.
Influence of Soils
It is not intended to imply that the protection afforded by a forest cover is of equal value on all sites. The solid burden of streams which have their sources within regions of sandy or gravelly soils is relatively low. Such regions are the coastal region of the southeastern states, northern New England, New York and the Great Lakes region. It is frequently the case, however, that streams flowing through such sandy regions are discolored by the leachings of humus and other plant matter. Some of the streams of the coastal plain, especially in the sandy, hilly sections, are of crystal clearness and have a pellucidity which is unaffected by heavy storms. These sandy soils have high water storage capacity and great steadiness of stream flow.
On the other hand, many of the clay soils, particularly those of the Piedmont of the southeastern and Gulf States, have a high content of clay of colloidal fineness. Colloidal material of this kind settles slowly even in still water and is the principal cause of the prevailingly muddy streams of the Piedmont region of the southeastern states. On account of this colloidal clay a most moderate rain often causes excessive erosion from naked soils and results in a great increase in turbidity. Such clayey soils likewise have a lower absorptive capacity than sands and loams and more easily lose their granulation, and it is the surface granulation of a soil which determines its initial capacity to absorb rainfall. Heavy clays also have a lower water storage capacity. The result of these conditions throughout the Piedmont of the southeastern states and in similar regions of clayey soil elsewhere is short periods of very high and muddy water following storms, and often long periods during which the flow of the streams may be low though seldom entirely clear. This situation is trying on a water supply system and the cry of “shortage” is a familiar one during the late summer and autumn periods. It is evident consequently that the influence of the character of the soil is paramount in determining the regularity of stream flow and the amount of turbidity.
Influence of Forest Cover on Erosion of Soil
The value of the forest cover then as a means of protecting a soil against erosion varies. On very sandy sites, whether the terrain is level or hilly, the protective functions of forest cover are of low value. Storm water is readily absorbed by such soils for the granulation of a sand is a constant feature. The forest cover, however, exerts a high protective influence upon clay and clayey soils, especially upon such as contain a considerable colloidal content and upon such as are subject upon denudation to excessive erosion during storms. The forest humus and the admixture of humus in the soil tends to maintain granulation of clayey soils and in this way to promote absorption of heavy rainfall. The many small channels in the soil, caused by the decay of roots, likewise promote absorption, especially in the lower soil and subsoil. The carpet of litter and leaves beneath the forest receives the impact of raindrops and such water as is not absorbed by the surface soil largely runs off over this litter in place of over naked soil. The more broken the surface, the steeper the gradient, and the longer the slope the greater is the protective value of the forest cover. Likewise the depth of the accumulation of leaves and litter is important and forests which are burned and the litter beneath which has been destroyed, or reduced in thickness by fire or by excessive grazing, are less effective in affording protection to the soil than such as are in normal condition. As to how far the forest cover may be influential in equalizing stream flow is still in most regions an open question, but there is little doubt that as a rule more storm water is absorbed by clayey lands when in forest cover than either by cleared land or by land in grass, and consequently on clayey sites there is a tendency for springs to fail when forest conditions no longer exist. On the whole, this greater regularity more than compensates for the moisture removed from the soil by the trees of the forest in their physiological processes.
“It is believed if towns, especially within and near the mountains, fully realized the investmental possibilities of forested watersheds purely from a timber producing standpoint that more of them might follow the example of the city of Asheville and a score of others and seek to own the catchment basins from which their supply is furnished.”
Forested Catchment Areas
When a corporation owns the basin from which it derives its supply of potable water it can largely prevent its pollution and can secure at the filter a water the purity of which is further safeguarded by filtration. The high cost of acquiring entire watersheds frequently deters a municipality from such a purchase. It is true that New York City owns its watershed; but it was costly. The city of Asheville, in North Carolina, has acquired the entire basins from which it at present draws its supply, but unfortunately the city failed to look far ahead and its provision for a supply is already regarded as inadequate for its early future needs. It is believed if towns, especially within and near the mountains, fully realized the investmental possibilities of forested watersheds purely from a timber producing standpoint that more of them might follow the example of the city of Asheville and a score of others and seek to own the catchment basins from which their supply is furnished.
Cutting of Timber Need Not Pollute Watershed
The point may be raised that if a town owns such a watershed the timber cannot be used; or if used the watershed is open to the same forms of contamination as if in the hands of a number of private owners. This is not necessarily the case, for the town can so regulate the manner and time of the removal of the timber as to interfere but little with the regimen of the stream; can manage so as to protect its clearness, and safeguard its purity. No broad statement can be made as to the investmental value of such forest land or conditions under which purchases would he advisable. But it can be said that the values of such properties, if acquired at current market prices and if the potentialities as forest investments are carefully considered, are still low and that at least such cities or towns as are near suitable watersheds which are largely forested could well consider such a course. It is further believed that the time will come, and it is not so far off, when such forested watersheds, even though used as sources of municipal water supply, can likewise be employed under restrictions for recreational purposes.
Protection of Reservoirs
In addition to the sanitation and increased value of a water of lower turbidity, the loss in storage capacity in reservoirs through silting up is a condition which sometimes demands consideration. Where silt can be cheaply removed from a reservoir this subject is not important. When silt, however, cannot be removed it may at times be desirable, especially in cases when the larger amount of the silt which is being deposited comes from a limited, rugged or poorly farmed area, to consider the purchase of this land and the planting of it to trees; or in case the land is already owned by the corporation then the advisability of its planting can be taken up. On one hand, there will be the cost of acquiring such land and the establishment of forests on those portions which demand such protection. This cost will be offset by the lengthened life of the reservoir, due to the lowered turbidity of the affluent and reduced sedimentation and by the income from the sale of timber grown on such land.
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How Watersheds Are Affected by Forests
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Also at times it may be profitable to plant to trees unused, open lands, especially slopes immediately around reservoirs, even when erosion from these lands is not a serious menace. This should be done for tbe purpose of making each part of the investment produce some income or be of some service. It will be necessary in such a case to consider a choice of species for planting, selecting those kinds which are best suited to the soil and moisture conditions. as well as the rate of growth of the kind which is employed and the possible returns which may be secured from the sale of this timber.
Effects of Forests on Wind Influence
A further consideration which may warrant attention at times is the protection of reservoirs against wind influence. The finer material which occasions most of the turbidity settles in reservoirs very slowly. Sedimentation usually proceeds with a certain expected uniformity, a high percentage of turbidity being eliminated from water of a high turbidity and a smaller percentage from that of a lower turbidity, owing to the slower settling of the finer particles which cause the low turbidity. The greater proportion of clarification in water of high turbidity is suggested by Mason as being due to the well-known tendency of larger falling particles to drag down with them very fine particles, and even matter in solution. The subsiding, heavier silt drags down not only much of the finest clay, but bacteria as well. It is important, therefore, that the finer sedimentation be as thorough and uninterrupted as possible, since after the settling of the heavier particles the finer material which is left, having failed to be carried down by the coarse material, remains in suspension a great while and is with difficulty eliminated from the water by further sedimentation. Only a certain proportion of it is removed by filtration, and it follows that the lower the turbidity of the water as it goes to the filter beds the more pellucid the effluent.
Effects of Wave Action Produced by Wind
At different times, but usually during the winter and spring, on account of wave action produced by wind, sedimentation in the reservoirs not only takes place very slowly and irregularly or not at all, but additional turbidity is acquired by the water from the scouring of the sides or, at shallow places, the bottoms of the reservoirs, a portion of the thin coating of silt and clay that has previously been deposited being again taken up in suspension. With this increase in turbidity a portion of the recently deposited bacteria are also redistributed through the water.
During the winter and spring, moreover, when the temperature of the water is about the same throughout, constant wind action, though light, from any one direction, will easily cause a complete overturning of the water in the reservoir.
The period of wind agitation of water in small lakes and reservoirs, as has been pointed out by Birge, is largely limited to the winter and early spring months, after the water has become homothermous. During the summer and early fall the temperature of the surface water becomes much higher than that of the water at the bottom of the reservoir, and the surface water is consequently much lighter, there being a warm, superficial stratum of water, the colder bottom stratum, and a thin stratum between them in which the temperature rapidly falls, called by Birge the thermocline.
In order to lessen such wind action the planting of wind breaks of trees around the edges of reservoirs is recommended; as well as the separation of long reservoirs greatly exposed to wind action into two or more segments and the planting of trees along the separating embankments.
(Excerpts from paper read before the annual convention of the North Carolina Section of the American Water Works Association.)