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Page:Popular Science Monthly Volume 81.djvu/102

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96
THE POPULAR SCIENCE MONTHLY

place, it is probable that the water table on the flood-plain of the Santa Cruz River, nearer the surface than at present, was tapped by the roots of the larger plants, for instance, the mesquite, growing there. The depth to water in other desert regions, as for instance, southern Algeria, is very variable, but usually great. For example, at the daya of Tilrempt, on the northern edge of the Sahara, the water lies between 50 and 90 meters deep, while in the vicinity of Ouargla it is frequently no deeper than 1.5 to 2 meters. In the latter case, however, the water is highly charged with salts.

The length of time that the water in available amounts remains in the soils following storms is a variable one. In the vicinity of Tucson the soils of the river flood-plain, and of Tumamoc Hill, remain moist for a period exceeding six weeks, but the deeper levels are moist for a somewhat longer period. It has been stated that in the vicinity of Tucson, at a depth of about 20 cm., the soil is sufficiently moist to be of benefit to plants throughout the year. However, it should be said that if the activities of the perennials, or of the annuals, can be taken as indicators, the period of maximum activity, which should indicate the optimum water content of the soil, is not of long duration, perhaps not exceeding six weeks, which would include the rainy season.

A relatively small amount of work has been done on the temperature of desert soils. For a period of about five years there has been kept at the Desert Laboratory a continuous temperature record at two depths—15 cm. and 30 cm. But only a relatively few observations have been made at a depth of 2.5 cm. As a general result of the soil temperature studies it can be said that at the depth of 15 cm. the greatest diurnal range, which usually occurs in March and July, is 12° F. The extreme yearly range at this depth is 73° F. In January the temperature begins to rise, and rises gradually until the last of March, when the rate becomes accelerated, so that by the last of spring the soil approaches the temperature characteristic of summer. The highest temperatures occur in July just before the midsummer rains. When the rains come the temperature falls 5° or 10°. The minimum for the year is reached in December.

Soil temperatures at the 30 cm. level are very different from those just given above. In the first place, the daily range in temperature is usually not over 2°, and the maximum not above 4°. The minimum temperature at a depth of 30 cm. occurs in March. In the first part of April the soil begins to get warm and the temperature arises until the rains of midsummer. The fall in temperature of the soil occurs during seven months of the year and the rise in temperature of the soil at this depth occurs during five months of the year.

Unfortunately the temperatures for 2.5 cm. depth have not been taken throughout the year, but are available for spring months only.