MOISTURE IN THE SOIL. 17 



reach deeper than this, for a period much exceeding six weeks following 

 storms. This defines the limit of life of most annuals, both those of winter 

 and of summer, and probably also the season of absorption of perennials 

 with shallowly placed roots. Plants having roots which reach to greater 

 depths than 15 cm. can obtain some moisture at all seasons. In order to 

 survive, seedlings must send their roots below 15 cm. within six weeks 

 following the close of a stormy period. 



THE BAJADA. 



The water relations of the bajada are very diverse, because of differences 

 in soils, in topography, and in its relations to other physiographic areas. 

 The nearly impervious caliche hardpan which underlies the bajada ever}-- 

 where prevents at once the deep penetration of the rains and the oppor- 

 tunity of tapping subterranean water. Save where the bajada constitutes 

 a drainage slope from a higher area, and well-defined channels are not 

 formed, the only water available to the plants growing on it is what falls 

 upon it directly. The water table of the bajada varies considerably in its 

 position with relation to the surface of the ground. That in the vicinity 

 of the University of Arizona, about a mile east of the Santa Cruz river, is 

 approximately 25 meters deep. In other portions, where the general level 

 of the bajada is higher, the perennial water is considerably lower than this 

 figure. 



The depth of the penetration of the rains is apparently entirely condi- 

 tioned on the thickness of the upper soils and on the presence of cracks or 

 rifts in the caliche hardpan which are filled with adobe. In favorable 

 places on the mesa, where there has been a considerable accumulation of 

 adobe soil, enough water is present to allow the growth of small specimens 

 of such perennials as are most abundant on the flood-plain, such as Prosopis 

 velutina, Acacia greggii, and Ephedra tr if urea. 



Where the bajada nears higher areas, as Tumamoc Hill, the most favorable 

 water relations are to be found. This may be attributed partly to the 

 larger rainfall in such areas, but mainly to seepage and to superficial run-off 

 from the higher ground . The adobe soil from the bajada north of Tumamoc 

 Hill has an admixture of fragments of caliche and of rock so that its moist- 

 ure-retaining capacity is about 20.1 per cent of its dry weight. For this 

 reason it is air-dry, to a depth of 20 cm., most of the year, and the perennials 

 which are to be found on it, mainly Covillea tridentata and certain cacti, 

 must get their water during dry seasons from the rotten caliche stratum 

 and from the more deeply placed cracks in the underlying caliche itself. 



During the period in which observations were made on the water content 

 of the soils of the bajada, October 3, 1907, to April 11, 1908, it was learned 

 that there was much less water in the soil on the bajada than in the soils 

 on the Hill. Soil samples were taken at two depths, 10 cm. and 20 cm. 

 From the beginning of the study until February 11, the water in the soil 

 at either depth was less than lo per cent of its dry weight. On February 



