Some Sources of Water Supply

SOME SOURCES OF WATER SUPPLY. BY DR. JOHN A. WIDTSOE, DIRECTOR OF EXPERIMENT STATION, STATE AGRICULTURAL COLLEGE, LOGAN, UTAH.

Water vapor, obtained from the ocean, through the heat of the sun, is moved as clouds from ocean to land, and from continent to continent, by the ordinary circulation of the atmosphere. A moving atmospheric mass or wind which encounters a long mountain range, tends to force its way up the sides of the high mountains until the crest is reached, when, if strength still re- mains, it rushes down the other mountain side. But, in rising to higher altitudes, air expands, and the expansion, in accordance with a well established law of physics, results in a corresponding cooling. At the temperature which prevails near the surface of the ocean, the water vapor carried by the air is enabled to re- main in its vaporous condition; but, as it rises along the mountain sides, the cooling is sufficient to condense it to rain or snow. Thus it happens that the great mountain ranges and peaks become great condensers of the moisture carried from the ocean into the interior of the continents. This water, falling upon the mountains, finds its way back again into the ocean, whence it is again evapor- ated, driven landward, and again condensed by some mountain range. Thus, throughout all the ages, water has gone through the phases of circulation: ocean water, water vapor, clouds, snow or rain, river water, and ocean water again. In a similar manner, water which is evaporated from wet soil, or which is transpired by the leaves of plants, rises into the air to form rain clouds, which, whenever they are cooled sufficiently, es- pecially by coming into contact with great mountain chains, are precipitated as rain or as snow.

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Some of the water vapor taken from the Pacific Ocean is driven by the winds eastward until it strikes the Sierra Nevada mountains, where it is condensed and falls as rain or snow.

The air, coming from these mountains to us, is largely robbed of its moisture, and produces little or no precipitation. So also, the water vapor coming from the east is condensed by the high

mountains to the east of us. Thus, we, living in the Great Basin,

are, in a measure, surrounded by mountains which rob us of much of the water that would otherwise reach us; and as a result, the

climatic conditions under which we live are those of an arid country. Still, the fact that we have mountains throughout the whole state, is one reason why our conditions are not so desert as they might be; for, owing to these great water condensers, rain clouds, that may venture into the Great Basin, are condensed and precipitated for our benefit; and likewise the evaporation of water from lakes, rivers, soil and plants, within the Basin, is precipitated and largely prevented from leaving this locality. Water which immediately runs off the mountain sides into the rivers, is of little value to the people. Of the greatest benefit is that portion of the rain or snowfall which descends gradually into the valleys, to furnish the necessary flow of water during the summer, when crops are growing. One of the great allies of the farmer in retaining and delay- ing the loss of the water that falls upon the mountains, is the forest growth, which, under natural conditions, covers the greater part of the mountain districts of the State of Utah. It is frequently asserted that forests tend to cause precipita- tion of rain or snow; and it is also believed that the growing of many trees in a neighborhood increases the annual rainfall there. Long continued and numerous investigations upon this subject have not been able to prove conclusively that forests tend to in- crease or decrease the rainfall. Another statement, commonly made, is that hail-storms are very frequent where forests are abundant. This also has been subjected to careful study and has been shown to be unfounded. The main relation of forests to water supply is in the conser- vation of the water that may fall within the forest. It is true that, in certain respects, forests cause a loss which does not occur

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in districts that are bare. For instance, all of the rain that falls above bare ground, reaches the soil, while a certain amount of the rain that falls over forests never reaches the soil, but is retained by the leaves and branches and trunks of the trees. In like man- ner, a portion of the snow that may fall over forests is retained by the leaves and branches of the trees; and to that extent, forests cause a loss of water to the soil below them. Nevertheless, the conservation, by the forests, of the water that actually reaches the soil, is so great as to offset many times the loss which is due to the interception of rain or snow by the leaves and branches of the trees. The greatest loss of water from soils, is due to evaporation; and the two chief forces in causing evaporation are sunshine and wind. Anything that will diminish the amount of direct sunshine on a piece of wet soil, or diminish the amount of wind, will, to that extent, diminish also the evaporation of water from that soil. In this respect, forests are of prime importance. Within the forest groves there is more or less continuous dense shade; and even young forests have a great power of breaking the force of moving atmospheric masses. One need only recall that, in Utah, the mountain sides facing the south are bare in the spring from one to three weeks before the snow is melted from the sides that face the north, to prove the evaporating effect of direct sunshine. Likewise, every boy who has been brought up on an irrigated farm knows that one or two days of heavy wind will draw the water out of the ground, bake the soil and wilt the crops, and make im- mediate irrigation necessary. If for no other reason than the protection which they afford the soil against direct sunshine and wind, the forests of our mountains should be jealously guarded. Experiments on this subject have shown that one-third as much water evaporates from a pan full of water standing in the forest as from one that stands upon bare ground away from any shade; and it has been found that from a small piece of bare soil, on which no plants grow, within the forest, only one-third as much water evaporates as from the similar piece of ground in the open. With regard to snow, this relation holds true, also. As has been stated, a small part of the snow is intercepted by the leaves

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and branches of the trees, yet that portion which falls within the forest, lies there more evenly, and longer; and' the soil is pre- vented from freezing to great depths, so that it may begin early in the season to absorb the water from the melted snow. There is

an idea that snow lying in the open is easily drifted into the ravines where a thick layer is formed, which lasts longer into the summer season than would a thin layer. This fact is sometimes used as an argument against the advisability of protecting the forests of our mountains. This notion is based on a misapprehen- sion; for every thinking observer knows that winds "wear out the snow." As the snow is drifted from place to place, it is reduced to fine particles, which expose new surfaces, from which evapora- tion and melting begin anew; and before the ravine is filled, many times the weight of the snow deposited has been worn out by the wind and returned to the air as invisible water vapor. The amount of water that comes, during the summer, from such drifted snow- masses is small as compared with the water which has been stored in the soil upon the mountain sides, and which percolates into the streams the whole summer long. Thus we come to the question concerning the manner in which the water from the rains or melted snows of the mountains is held back until late summer, when it can be used most advan- tageously for purposes of irrigation. Water, poured upon the soil, soaks downward and forms a thin film of water around every soil particle as far down as the water can penetrate. After all the soil particles have been moistened, the addition of more water will tend to fill up the spaces between the particles, and this water, by its own weight, will gradually move downward until it reaches the bottom of the roil, when it will filter out and perhaps become a mountain spring, or may be car-

ried by subterranean channels into the rivers. If the surface of the soil is loose, the rain or melted snow that falls upon it, soaks into and sinks gradually to the lower layers. If evaporation it easily,

from the surface is prevented or diminished, the water stored by the soil will continue, during the spring and summer, to filter

down through the soil and feed the mountain springs. If the sur- face soil is heavily packed together, the rain or snow water may remain on the surface as puddles, and is then likely to be evapor-

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ated by the sunshine or the wind; or possibly, if the country is not level, it may run off the hillsides to the rivers, and be carried down the valleys early in the spring. In spite of the best that we can do, much of the rain and melting shows will run off in this manner, and the great problem before the citizens of an arid country is to reduce the quantity to a minimum. In an open place, not sheltered by forests, the heavy rains,which often occur in the mountains, strike the soil with great force and pack the upper layers of soil very solidly; and, al- so, if a large number of animals are allowed to tramp upon a piece of ground, the surface is packed until percolation of water into the soil is prevented. Under the forest cover, the case is

quite different; for the force of the raindrops is so broken by the trees that a gentle drizzle falls upon the forest floor. It is, of course, true, that a great number of animals tramping continually upon the forest floor, finally pack the soil as effectually as is done in the open. Leaves, falling from the trees year after year, form a soil

rich in vegetable matter with a very great power of absorbing water. It is porous and elastic, and not easily compacted by the

rains or by animals. It has been found that soil which contains considerable moisture is prevented from losing its water, if the top layer of soil is quite thoroughly loosened or cultivated. It is, of course, impossible or impracticable to cultivate all the top soil of our mountain ranges; and in fact it is unnecessary, for it has been found that the loose, porous vegetable mould which is formed on the forest floor has the same effect in retarding evaporation as — has an earth mulch, that is to say, this loose layer of top soil will allow moisture to pass into and through it with great rapidity,

but is inactive in drawing the water from below upward. It has been shown that one-eighth as much water is evaporated from soil in the forest, covered with loose vegetable mould, as from a bare soil exposed to the full action of the sun. Within the forest, two plots of ground have been compared,— one of which was bare, and — the other covered with vegetable mould and it was found that one third as much water evaporated from the covered as from the bare plot. This fact, when we consider the thousands of tons of water that reach the mountain soils in the form of rains or melted

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snow from year to year, is of the highest importance to the dwell- ers of this inter-mountain country. Plants, in their growth, evaporate, or transpire, a certain amount of water from their leaves and thus occasion a loss of soil moisture. Experiments have determined, first, that the transpira- tion of forest trees, comparing equal areas, is less, —not much more than one-half — than that from ordinary farm crops; and secondly, that the amount of water transpired by the leaves of trees is very small, compared with the total amount that soaks in- to the soil. This factor, therefore, though it means a certain loss of water, falls into insignificance when compared with the great gain due to the saving influence of the forests. Such facts, and many more that might be produced, lead to the firm conviction on the part of unbiased students that the pre- servation of our forests is an absolute necessity for the proper husbanding of the water supplies of the state. It should be a question, not of personal preference or desira- bility, but of state patriotism to maintain our forests; to graze the mountain hillsides in a careful, scientific manner; to take out trees only that are mature, and to replace, as far as possible, those trees with young ones, so that our forests, instead of diminishing in extent, will increase, and with that increase, our water supply will also The mountains we have, [no one can destroy increase. them, but over the forests, which are equally important, man has complete control. For years forest denudation has been go- ing on, chiefly from ignorance, seldom from malice. But recent years have brought to light the true relations of forests to water supply; and we, children of Utah and lovers of our homes, should use all our knowledge, all our influence, all our powers, in chang- ing the methods of the past, and in preserving for ourselves and our children, the water supplies upon which the present prosperity and future growth of Utah depend.