28 DATE VARIETIES AND DATE CULTURE IN TUNIS. 



has about the same total precipitation as Tougourt, in Algeria, con- 

 siderably more than Ouargla, Algeria, and considerably less than 

 Biskra, Algeria, and Phoenix, Ariz., the last two localities having 

 nearly the same total. The Colorado Kiver Valley and the Salton 

 Basin in the United States receive considerably less rain than any of 

 these North African localities. Regarding the distribution by sea- 

 sons, we find that nearly one-half of the total precipitation falls in 

 spring in the Jerid oases, a larger proportion than at any of the other 

 localities given in the table. The autumnal precipitation is relatively 

 smaller at Tozer and at Nefta than at any of the other localities 

 excepting Volcano Springs, Cal. At Phoenix and Yuma, Ariz., the 

 rainfall in summer forms a much higher proportion of the total for 

 the year than at any of the other points included in the table. 



IRRIGATION AND DRAINAGE. 



As the water supply and irrigation conditions of the oases of the 

 Nefzaoua, Gabes, and Gafsa have been briefly discussed under the 

 heading " Geography," only the Jerid region will be treated in this 

 place. 



WATER SUPPLY. 



The oases of the Jerid, unlike those of the Oued Rirh, in Algeria,* 

 are watered by means of springs instead of wells. This is explained 

 by the fact that the Jerid oases are situated at the very base of the 

 terrace that marks the northern frontier of the desert, while the Oued 

 Rirh group lies well into the Sahara. Attempts to obtain artesian 

 water in the Jerid region have so far been unsuccessful. 



The springs of the Jerid are all situated at about the same eleva- 

 tion — 135 to 150 feet above sea level. They originate in strata of 

 cretaceous formation at the base of the line of bluffs. As the springs 

 are very numerous and in many cases very large, maintaining a prac- 

 tically constant flow throughout the year, it seems unlikely that the 

 scanty rainfall of the region itself can account for the abundance of 

 the water supply.^ The general temperature of the water as it issues 

 from the ground is about 86° F., and varies little from season to 

 season. 



a See Bui. 53, Bureau of Plant Industry, U. S. Dept. of Agriculture, pp. 44-46, 

 where the irrigation of the date palm in the Algerian Sahara is described ; also, 

 Bui. 80, Bureau of Plant Industry, pp. 36-38, where this subject is discussed. 



6 Rollaud, Hydrologie du Sahara Algerien, pp. 182, 183, holds that the water 

 which feeds the springs of the Jerid must constitute an artesian sheet, derived 

 largely from the eastern spurs of the Aures Mountains and flowing deep under 

 the elevated plain which abuts on the shotts of southern Tunis. On the other 

 hand, Doumet-Adanson ( Sur le regime des eaux qui alimentent les oasis du Sud 

 92 



