Publisher's Synopsis
Excerpt from Irrigation and Water Storage in the Arid Regions: Letter From the Secretary of War, Transmitting a Report of the Chief Signal Officer of the Army in Response to House Resolution Dated May 23, 1890, Relating to Irrigation and Water Storage in the Arid Regions
In this connection it seems also important to consider the absolute humidity conditions over the arid region, that is, the actual quantity of aqueous vapor contained in each cubic foot of air at or near the surface of the earth.
On charts Nos. 2 and 3 are shown typical curves with reference to the absolute humidity of the air, which is expressed in the number of grains of water in each cubic foot of air. In preparing these charts it has been necessary to ignore State lines, as the distribution of moisture and damp air depends so very largely not only on the contiguity to the ocean, but also on the direction of the prevailing winds and the character of the intervening country over which the wind must pass from the source of water supply. Consequently these typical curves have been drawn for the Pacific coast region, the interior of California, and for Nevada, Utah, Colorado, New Mexico, and Arizona. The important hearing which the prevalence of moisture or dry air has upon growing vegetation is well known, but further than this it has an equally important bearing upon the methods of irrigation, since the drier the air the greater the quantity of stored or ?owing water which will be evaporated thereby. As will be noted by reference to these charts, the greatest quantity of water is not found in the same month over all parts of the arid region.
In addition to the charts, it appears advisable to make some general statements as to the prevalence of moisture or dry air over the regions under consideration. It is impracticable to draw for general conclusions any hard and fast line as to what constitutes very dry or very wet air, as this question turns on the normal temperatures, which in turn depend materially on the lati tude, the elevation, and the locality with reference to great bodies of water.
In general terms, however, it has been decided for present purposes to consider as very dry that air which contains during the colder half of the year - from October to March, inclusive - less than 1 grain of aqueous vapor to each cubic foot of air over Montana, Idaho, and the Dakotas, and less than 2 grains of aqueous vapor to each cubic foot of air over Arizona and New Mexico, with intermediate conditions for the intervening country. During the six warmer months of the year - April to September, inclusive - the limiting figures have been placed at 2 grains for the more northerly sections named and 3 grains for the more southerly. Under these limitations it appears that there is no very dry air over the arid regions during October, but that over the whole region west of the one hundredth meridian, except in California, Oregon, and Washington, along the coast region of the Pacific Ocean, the air gradually grows drier until January, when there is less than 1 grain of aqueous vapor to a cubic foot of air over the Dakotas, Wyoming, Kansas, Nebraska, and the greater part of Colorado; while less than a grain and a half per cubic foot is then to be found over northwestern Texas, the greater part of New Mexico, northern Arizona, Utah, Nevada, Idaho, and the western parts of Washington and Oregon.
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