134 THE FUTURE OF ARID LANDS 
tion ditch to return the silt and surplus water to the main channel. 
The flow is controlled by sluices. This method 1s easily operated 
and maintained, but the percentage of actual infiltration area is 
relatively small. 
For recharge by basins, water is impounded in areas of some 
400 feet to 100 feet by earth dams or banks some 3 feet high, 
built along the contours. The basins are arranged so that the 
water can overflow from one to the other, silt being deposited 
mainly in the highest one. The level of the water is generally 
maintained at a depth of about 6 inches. Where silting occurs, the 
surface is repeatedly broken by scraping or harrowing, as on the 
spreading grounds of Los Angeles, where this is done after each 
10 to 14 days (3). 
Where in a limited unit area, as defined by geological or geo- 
graphical conditions, data are available or could be acquired for a 
complete study of the hydrogeological cycle, it is sometimes pos- 
sible to make full use of the total water resources of the area. In 
Tunisia, for example, Tixeront (27) has studied the water supply 
resources of three limestone masses on which the towns of Tunis 
and Bizerta depend. The masses are individually of limited but 
known extent, and they are cut off by faults and impermeable 
beds. The rainfall, evaporation, and outflow of streams are all 
well known, and the movements of the water table are checked by 
observation wells, boreholes, and other works. The annual volume 
of the additions of water to the ground water reservoirs as ob- 
served and calculated accord satisfactorily. The results obtained 
enabled the authorities to follow the fluctuations of the reservoirs 
in the aquifers studied, to regulate their exploitation, and to 
predict the importance to the township supplies of any new 
additions of water. 
In British African territories much interchange of information 
and coordination of effort has been achieved by Inter-Territorial 
Hydrological Conferences which it is proposed to hold periodi- 
cally. One such conference took place at Nairobi in 1gs0 and 
another in Southern Rhodesia in 1954. The papers presented at 
these conferences, as well as the publications of the Water and 
Meteorological Departments, clearly indicate the wide range of 
