Report on the lands of the arid region of the United States, with a more detailed account of the lands of Utah

by John Wesley Powell; Willis Drummond; Clarence E. Dutton; Grove Karl Gilbert; A. H. Thompson

CLIMATIC THEORY.

It is generally supposed that the change is a phenomenon of climate, and this hypothesis includes harmoniously the increase of streams with the increase of lake surface. By some it is thought that the climate of the district is undergoing, or has undergone, a permanent change; and by others that the series of oscillations about a mean condition which characterizes every climate has in this case developed a moist phase of exceptional degree and duration. The latter view was my own before I became aware of the features of the ancient storm line, but it now appears to me untenable. That a variable surface of evaporation, which had for a long period recognized a limit to its expansion, should not merely exceed that limit, but should maintain an abnormal extent for more than a decade, is in a high degree improbable.

It is far more probable that one of those gradual climatic changes, of which geology has shown the magnitude and meteorology has illustrated the slowness, here finds a manifestation. The observed change is apparently abrupt, and even saltatory; but of this we cannot be certain, since it is impossible from a record of only thirty years to eliminate the limited oscillation. It is quite conceivable that were such elimination effected, the residual change would appear as a continuous and equable increase of the lake. However that may be, a certain degree of rapidity of change is necessarily involved, for the climatic change which is able in a decade to augment by a sixth part the mean area of evaporation cannot be of exceeding slowness. If we can ascertain how great a change would be demanded, it will be well to compare it with such changes as have been observed in other parts of the country, and see whether its magnitude is such as to interfere with its assumption.

The prevailing winds of Utah are westerly, and it may be said in a general way that the atmosphere of the drainage basin of Great Salt Lake is part of an air current moving from west to east. The basin having no outlet, the precipitation of rain and snow within its limits must be counter-balanced by the evaporation. The air current must on the average absorb the same quantity of moisture that it discharges. Part of the absorption is from land surfaces and part from water, the latter being the more rapid.

If, now, the equilibrium be disturbed by an augmented humidity of the inflowing air, two results ensue. On the one hand the precipitation is increased, and on the other, the absorbent power of the air being less, the rate of evaporation is diminished. In so dry a climate the precipitation is increased in greater ratio than the humidity, and the rate of evaporation is diminished in less ratio; while of the increased precipitation an increased percentage gathers in streams and finds its way to the lake. That reservoir, having its inflow augmented and its rate of evaporation decreased, gains in volume and grows in breadth until the evaporation from the added expanse is sufficient to restore the equilibrium. Giving attention to the fact that the lake receives a greater percentage of the total downfall than before, and to the fact that its rate of evaporation is at the same time diminished it is evident that the resultant augmentation of the lake surface is more than proportional to the augmentation of the precipitation.

We are therefore warranted in assuming that an increase of humidity sufficient to account for the observed increase of 17 per cent. in the size of the lake would modify the rainfall by less than 17 per cent. The actual change of rainfall cannot be estimated with any degree of precision, but from a review of such data as are at my command I am led to the opinion that an allowance of 10 per cent. would be as likely to exceed as to fall short, while an allowance of 7 per cent. would be at the verge of possibility.

The rainfall of some other portions of the continent has been recorded with such a degree of thoroughness and for such a period that a term of comparison is afforded. In his discussion of the precipitation of the United States, Mr. Schott has grouped the stations by climatic districts, and deduced the annual means for the several districts. Making use of his table on page 154 (Smithsonian Contributions, No. 222), and restricting my attention to the results derived from five or more stations, I select the following extreme cases of variation between the mean annual rainfalls of consecutive decades. District I comprises the sea coast from Maine to Virginia, and the record includes five or more stations from 1827 to 1867. From the decade 1831-’40 to the decade 1841-’50 the rainfall increased 6 per cent. District II comprises the state of New York and adjacent regions, and includes five or more stations from 1830 to 1866. From the decade 1847-’56 to the decade 1857-’66 the rainfall increased 9 per cent. District IV comprises the Ohio Valley and adjacent regions, and includes five or more stations from 1837 to 1866. From decade 1841-’50 to decade 1851-’60 the rainfall diminished 8 per cent.

The case, then, stands that the best comparable districts and epochs exhibit extreme fluctuations from decade to decade of from 6 to 9 per cent, while the rise of Great Salt Lake implies a fluctuation of about 10 per cent. But before deciding that the hypothetical fluctuation in Utah is extraordinary, consideration should be given to the fact that in the dry climate of that region a given change in humidity will produce a relatively great change in rainfall, while an identical change of rainfall, measured in inches, acquires an exaggerated importance when expressed as the percentage of a small total rainfall. Giving due weight to these considerations, I am led to conclude that the assumed increase of rainfall in Utah is not of incredible magnitude, and consequently that the hypothesis which ascribes the rise of the lake to a change of climate should be regarded as tenable. It by no means follows that it is proven, and so long as it depends on an assumption the truth of which is merely possible, but not established, it can claim no more than a provisional acceptance.

It is proper to add that, so far as I entertain the idea of a change of climate, I do so without referring the change to any local cause. It is frequently asserted that the cultivated lands of Utah “draw the rain”; or that the prayers of the religious community inhabiting the territory have brought water to their growing crops; or that the telegraph wires and iron rails which gird the country have in some way caused electricity to induce precipitation; but none of these agencies seem to be competent. The weather of the globe is a complex whole, each part of which reacts on every other, and each part of which depends on every other. The weather of Utah is an interdependent part of the whole, and cannot be referred to its causes until the entire subject is mastered. The simpler and more immediate meteoric reactions have been so far analyzed that their results are daily predicted; but the remote sources of our daily changes, as well as the causes of the greater cycles of change, are still beyond our reach. Although withdrawn from the domain of the unknowable, they remain within that of the unknown.