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

METHOD AND SCOPE OF INVESTIGATION.

Where agriculture is dependent upon irrigation, the extent of land that can be put to agricultural use is determined by the relation of the quantity of available water to the quantity of available land. There is a certain amount of water needed by a unit of land, and wherever the land susceptible of cultivation requires more water than is obtainable, only a portion of the land can be utilized. But there is also a limit to the amount of water that can be profitably employed on a unit of land, and where the supply of water is in excess of the quantity required by such lands as are properly disposed to receive and use it, only a portion of the water can be utilized. In order to ascertain, therefore, the extent of agricultural land in a given district, it is necessary to make a measurement of land, or a measurement of water, or perhaps both, and it is necessary to know the amount of water demanded by a unit area of the land under consideration.

The proper quota of water for irrigation depends on climate and soil and subsoil, as well as on the nature of the crop, and varies indefinitely under diverse conditions. As a rule, the best soils require least water; those which demand most are light sands on one hand and adhesive clays on the other. Where the subsoil is open and dry, more water is needed than where it is moist or impervious. Wherever there is an impervious substratum, the subsoil accumulates moisture and the demand for water diminishes from year to year. These and other considerations so complicate the subject that it is difficult to generalize, and I have found it more practicable to use in my investigations certain limiting quantities than to attempt in every case a diagnosis of the local conditions. By comparing the volumes of certain streams in Utah, that are now used in irrigation to their full capacity, with the quantities of land that they serve, I have found that one hundred acres of dry bench land (i. e., land with a deep, dry, open subsoil) will not yield a full crop of grain with less than one cubic foot of water per second, and this under the most favorable climate of the Territory. Where the climate is drier, a greater quantity is required. Where there is a moist subsoil, a less may suffice.

In the drier districts, where the streams are small, they are usually employed upon the dry benches, because these are most convenient to their sources; and it is very rarely the case that their utility is increased by the presence of a moist subsoil. But it is also in the drier districts that the extent of agricultural land is ascertained by the measurement of streams; and hence there is little danger of error if we use in all cases the criterion that applies to dry bench land. In the discussion of the lands of northern Utah, I have therefore assigned to each cubic foot per second of perennial flow the reclamation of one hundred acres of land, with the belief that the consequent estimates would never underrate, though they might sometimes exaggerate, the agricultural resources of the districts examined.

In the measurement of streams the following method was employed: A place was sought where the channel was straight for a distance equal to several times the width of the stream, and where for some distance there was little change in the dimensions of the cross section. Measurement was then made of the width (in feet), of the mean depth (in feet), and of the maximum surface current (in feet per second). The mean current was assumed to be four-fifths of the maximum current; and four-fifths of the product of the three measured elements was taken to give the flow in cubic feet per second. This method of measurement is confessedly crude, and is liable to considerable error, but with the time at my disposal no better was practicable, and its shortcomings are less to be regretted on account of the variability of the streams themselves.

All of the streams of Utah that flow from mountain slopes are subject to great fluctuations. They derive a large share of their water from the melting of snow, and not only does the melting vary as to its rapidity and season, but the quantity of snow to be melted varies greatly from year to year. A single measurement standing alone is quite inadequate to determine the capacity of a stream for irrigation, and as it was rarely practicable to visit a stream more than once, an endeavor was made to supplement the single determination by collating the judgments of residents as to the relative flow of the several creeks and rivers at other seasons and in other years. In districts where the water is nearly all used and its division and distribution are supervised by “watermasters”, those functionaries are able to afford information of a tolerably definite character, but in other districts it was necessary to make great allowance for errors of judgment. Certainly, that element of my estimates which is based on inquiries cannot claim so small a probability of error as the element based on measurements.

Streams that are formed in high mountains reach their highest stage in June, and their lowest in September or October. Streams from low mountains attain their maxima in April or May, and reach their low stages by August or September. In the low valleys the irrigation of wheat and other small grains begins about the first of June, and continues until the latter part of July. The irrigation of corn and potatoes begins in the early part of July, and continues until the middle of August. In the middle of July all of the land calls for water, and if the supply is sufficient at that time, it is sure to meet all demands at other times. It will be convenient to call that time the critical season. In the higher agricultural valleys corn and potatoes are not grown, but the irrigation of small grains and hay is carried on from the middle of June to the middle or latter part of August. Through all this time the volume of the streams is diminishing, and if they fail at all it is at the end of the season. The critical season for the higher valleys is about the middle of August.

In order to estimate properly the agricultural capability of a stream, it is necessary to ascertain its volume at its critical season. In the investigations of the past summer, this was accomplished by direct measurement in but a limited district. For the remainder of my field of operations I was compelled to depend on the estimates of others as to the relation between the volumes of streams at the time of measurement and at the critical season.

As will appear in the sequel, the uncertainty attaching to these determinations of volumes affects the grand total in but small degree. The utility of the large streams is not limited by their volumes so much as by the available land suitable for overflow, a quantity susceptible of more accurate determination, and the extent of land irrigable by the large streams is many times greater than that irrigable by the small.

No streams are used throughout the year, and few can be fully utilized during the spring flood. Wherever it is practicable to store up the surplus water until the time of need, the irrigable area is correspondingly increased. Enough has been accomplished in a few localities to demonstrate the feasibility of reclaiming thousands of acres by the aid of reservoirs, and eventually this will be done; but except in a small way it is not a work of the immediate future. For many years to come capital will find greater remuneration in taking possession of the large rivers.

In estimating the agricultural resources, it was, of course, necessary to take account of all future increase, and wherever storage by reservoirs seemed practicable a rough estimate was made of the extent of land that could be thus reclaimed.

There are a few restricted areas in Utah that yield remunerative crops to the farmer without the artificial application of water. Their productiveness is doubled or trebled by the use of water, and so far as they are susceptible of irrigation they need not be distinguished from the irrigable lands. When the greater rivers shall have been diverted to the work of irrigation, nearly all such areas will be supplied with water, but a few will not. The endeavor has been to include the latter as well as the former in the estimate of the agricultural land.

The term “agricultural land” is construed to include that which is used or may be used for the production of hay as well as that cultivated by the plow. Most irrigable lands may be utilized in either way, but there are some tracts which, on account of the severity of the climate or the impurity of the water, are adapted to the growth of grass only.

I have sought in the foregoing remarks to set forth as briefly as possible the methods and scope of my investigations, and to indicate the degree of accuracy to be anticipated in the resulting estimates. To these estimates we will now proceed.