MOISTURE IN THE SOIL. 

 TUMAMOC HILL. 



The water relations of the plants of Tumamoc Hill are in part condi- 

 tioned b)'- the configuration of the hill, in part by the character of the soil 

 and the vegetal covering, and in part by its relation to the rest of the 

 Tucson mountains. 



Tumamoc Hill is an outlying spur of the Tucsons and is lower than 

 many peaks of this range, for which reason the rainfall on Tumamoc is prob- 

 ably less and the temperature probably higher than on the more elevated 

 parts of those mountains. Owing to its isolation, Tumamoc, although lower 

 than the rest of the range to which it belongs, does not profit from the run- 

 oft' from these mountains. 



Tumamoc is flat-topped and has a gently sloping northern side, where 

 there are two or more shoulders, and sharply descending eastern, southern, 

 and western faces. The soil is deepest where the slope is least, that is, 

 on the northern shoulders. Here, also, because of the slight gradient and 

 because of seepage and superficial run-off from higher portions of the Hill, 

 the water relations are most favorable. 



Besides these factors the character of the rains, which are seasonal and 

 often torrential, and the want of a heavy plant-covering operate to render 

 much of the water which falls of no avail to the plants on the hill. The 

 rainfall and its amount and character for a period of 15 years at the city of 

 Tucson are given month by month in the following table.* 



Table i.- — Rainfall at Tucson (in Inches). 



Thus the heaviest fall of rain occurs in midsummer and in midwinter, 

 with little or no rainfall between. The character of the rains of the two 

 seasons is unUke, that of summer is frequently torrential, while that of 

 winter comes with less force. Much of the rain in the summer season, 

 therefore, is lost as superficial run-off. Consequently, rain falling in equal 

 amounts in winter and in summer probably produces unequal effects in 

 moistening the soil and in providing the plants with water. 



The penetration of the soil by the rain is not a fixed amount, but depends 

 on a variety of factors, among which are the amount and duration of the 

 rain. The depth of penetration is usually from 2 to 5 times the registered 



*Coville and MacDougal: The Desert Botanical Laboratory, Publication No. 6, 

 Carnegie Institution of Washington, 1903, p. 26. 



