9 o THE POPULAR SCIENCE MONTHLY 



place, it is probable that the water table on the flood-plain of the Santa 

 Cruz Kiver, 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 day a of Til- 

 rempt, 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 tem- 

 perature 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 tempera- 

 ture 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. 



