dicate part of the advantages to be derived from adequate protection from fire.

Investigations in the Lake States also show that freshly fallen litter in a Norway-jack pine forest would absorb but 0.014 inch of water, whereas older litter from a similar forest type retained on the average 0.17 inch. This increase of 1,200 per cent in the waterholding capacity is due entirely to the disintegration of the litter. Such data indicate the unseen damage that is done by fires which, in destroying the old litter, reduce materially both the water absorbed and held.

EFFECT OF GRAZING

It is significant to note the effect of overgrazed pastured forest lands upon the dissipation of litter and upon the water-holding capacity. Observations indicate a greatly reduced absorptive effect in pastured areas, where sprout growth and woody shrubs are so closely browsed that the wind can enter the woods, thereby scattering the leaves. The soil is then exposed and becomes subject to severe washing. A large part of the litter is blown from the area, or scattered, or washed down the slope. In the Appalachian region it was found that only 0.02 inch of water was absorbed by the litter on a heavily grazed forest slope, whereas on similar adjoining land burned over two years previously, the water-holding capacity of the litter was 0.13 inch.

7

Ralph K. Day, of the Central States Forest Experiment Station, found that the litter cover was twice as deep on ungrazed as on pastured areas of beech forest. In the oak-hickory type the litter accumulation was 40 per cent greater on the ungrazed than on the grazed area. His observation was that the litter distribution was generally uniform in all cases where no grazing occurred, but was erratic wherever stock were permitted. Thus the heavier the degree of grazing the less uniform the litter, the less its depth, and the less valuable its influence upon the flood waters.

RESTRAINING INFLUENCE OF FORESTS UPON RUN-OFF

In addition to the water actually absorbed by the leaf litter and so held, there is a restraining influence which is all too often overlooked. Thus while a heavy litter cover might actually absorb and so hold against capillarity an inch or more of water, the litter by means of its arrangement and its porosity holds much more water, under certain exceptional conditions probably several times as much. Hardwood leaves curl and cup, and water is so held and restrained from flowing away. The space between the bark and the wood of twigs and branches on the forest floor is often filled with water. spaces between bundles of needles hold water for a considerable time. The litter becomes a veritable sponge and permits the soil to absorb

water.

The

Newly fallen litter material is not as effective in restraining the flow of water over or through it as that which is partially decomposed. Hardwood leaves of many species are somewhat slick when they first drop and water does not wet them. Fresh needles are

7 Robinson, H. F., U. S. Indian Irrigation Service, states that the Zuni Reservoir was silted because of overgrazing, which by removing the ground cover allowed the run-off to reach the stream very rapidly and in greatly accumulated quantities. Proc. Amer. Soc. Civil Engineers, pp. 1939-1946, August, 1928.

FIGURE 15. Above: Typical forest litter as found in the Appalachian hardwood forests. Below: The same type of ground cover four months after a light forest fire. Note the exposure of the surface soil to erosion and how completely the fire has destroyed the protecting litter mantle. Repeated surface fires not only destroy the surface cover and expose the soil to washing, but also by burning off the humus from the topsoil leave a surface less permeable to water and also less absorptive.

It

FIGURE 16. On the well kept forest floor is a complete mantle of forest litter of all kinds which adds humus to the topsoil as it decomposes. This litter not only holds water itself but also increases the water-holding capacity of the soil. also protects the soil from erosion. Fires destroy this protective mantle of litter and also burn out the humus accumulated beneath it. The result is an increase in the run-off which may well develop into severe erosion. Destruction processes of just this sort are here illustrated, first by conditions after a fire as shown in the upper plate, and then those six years after the area had been logged and burned as shown in the lower

slightly resinous and water flows off as from an oiled surface. However, as decomposition takes place and water can enter the fibers, surface tension and cohesion help to hold water. In Europe Huffel found that a forest with leaf litter, after a rainfall of from 2.4 to 2.8 inches, did not give off, even on the steepest slopes, a drop of water in the form of surface run-off. He pointed out that, if water does run off from such stands it comes from the precipitation which falls on an area deprived of its forest cover-for instance, a road.

That forest plantations influence run-off is recognized by many water companies and in New England particularly are many companies that have planted their watersheds because of the favorable influence of the forest upon the regimen of water and upon erosion. The experience of the York Water Co., York Pa., is of interest in this connection. The company has planted some 700 acres above the intake dam with about 680,000 evergreens. It has been found, according to General Manager E. P. Kable, that

The quality of the water in the stream has also materially improved, as there is very little erosion from the banks, and, as an example, when there was a great downpour of rain *, the water in this dam remained clear, whereas some of the neighboring streams became very turbid.

** *

Recent and as yet incomplete detailed investigations by the California Forest Experiment Station indicate that the surface run-off from forest soils from which the litter has been removed, is from ten to thirty times greater than from soils with a complete and undisturbed mantle of forest litter. The reason for this is simple. The litter prevents the beating drops of rain from so rearranging the particles that they clog up the pores in the soil and cement the channel openings. In other words, with a litter mulch the rain does not disturb the surface soil by its beating action, the water is kept clean at all times as the litter and raw humus strain out any pore-clogging material, and the water reaches the soil by percolation, rather than directly, and in such a fashion that the water can be absorbed at a much greater rate than from a bare soil.

The surface soil of a natural forest is usually covered with leaves and twigs, which protect it from erosion. It suffers little so long as this natural protection remains undisturbed. The raindrops do not usually strike the soil direct and thus destroy the granules, as they tend to do in cultivated fields. When this covering which nature provided is removed or destroyed erosion takes place."

The results of the California studies above referred to indicate that the litter is perhaps the most important element of the forest in determining the distribution of rain into superficial run-off and into seepage. Still further do they show that the function of the forest litter to absorb water is insignificant in comparison with its function to maintain the percolation capacity of the soils. This operates to keep the water reaching the soil through seepage channels clear, even during the most intense beating storms, whereas the superficial rain-off on areas devoid of litter soon become muddy by picking up small particles in suspension, which were filtered out at the surface as the muddy water percolated into the soil.

To determine the effect of the percolation of both clear and muddy water through soil columns of otherwise uniform nature, a laboratory experiment was carried out. In this clear water was passed

Mosier, J. G., and A. F. Gustafson, Soil Physics and Management. 1917.

p. 359.

through four tubes of soil for 10 days to establish the relative characteristics of the material used. Thereupon muddy water was applied to two of the tubes, the water passing through the other two remaining clear. The percolation rate of the first two tubes dropped from a rate of 1,100 c. c. per hour to 500 c. c. in four hours, and then continuously in a parabolic curve to a percolation of 90 c. c. per hour. After another 10 days, muddy water was applied to the last two tubes of soil through which the clear water had passed at a uniform rate, and the behavior of these soils was the same as that of the first two. The results are practically identical. By this it appears evident that the sealing effect of silt filtered from percolating muddy water is sufficient to account for the increased superficial flow from denuded. soils.

This experiment, highly significant in itself, only proves experimentally what has been known in southern California for many years, that muddy water may not be spread upon the gravelly débris at the mouth of the canyons if the water is to be stored in the underground artesian basin. Where muddy water has thus been distributed over the surface gravel beds, the interstices between the soil particles are effectually sealed, resulting virtually in covering the soil with an impervious layer, preventing the absorption of water into the lower soil levels. This sealing effect of muddy water on bare soils accounts in large measure for the great differences that exist between run-off from a covered soil and from a barren or denuded one. This fact has long been overlooked or minimized by those who have discounted the influence of forest and of forest cover upon run-off and upon erosion.

Studies in China of the superficial run-off from forested and denuded areas, made by Lowdermilk," show the average run-off to be forty-seven times greater from the deforested than from pine-forested areas, the extremes varying between twenty-six and one hundred and thirty-two times. In spruce-larch forests in the same general region, the run-off from denuded plots ranges from fifty-two to one thousand five hundred and five times more than from the forested plots, the quantities varying with the amount, distribution, and character of the rainfall. Another part of this same study indicated that under the conditions of the experiment the forest vegetation operated to increase the absorptive capacity of the soil over that of barren soil from three to five times (averaging 3.5 times), even in prolonged rains such as 14 inches in 40 hours.

Investigations by H. S. Gilman, of San Dimas, Calif., in 1926 afford a striking illustration of the value of litter. In 1919 about 50 per cent of the area in the San Dimas drainage basin burned over, destroying completely the heavy litter and chaparral cover. By 1926 the chaparral cover had, in a considerable degree, returned, but the heavy layer of litter characteristic of the watershed prior to the fire had not yet been formed. In April, 1926, the measured flood flow in the flood-storage reservoir was 1,100 acre-feet, of which threequarters came from the old burned-over part of the drainage. Thus as 275 acre-feet came from the 50 per cent that was unburned, the flood discharge from the entire area in this flood should have been but

Lowdermilk, W. C., Factors Influencing the Superficial Run-off of Rain. Proc. Third Pan-Pacific Sci. Congress. Tokyo. 1928.