VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 13s 
In arid regions the losses of rain water as a result of runoff and 
evaporation in pans are enormous, and the question of the possi- 
bility of conserving these supplies for dry season use by means of 
storage or recharge frequently arises. If the methods of artificial 
recharge now commonly applied in humid and_ semi-humid 
countries could be employed in arid regions they would be of 
untold benefit, but so far there has been only limited scope for 
such application. In the first place, water intended for artificial 
recharge is generally pretreated so that it will not clog the 
pores of the recharge basins, and this would ordinarily not be 
practicable in arid countries. Secondly, the form and capacity of 
the recharge aquifer and its suitability for retaining and yielding 
up the water can generally be determined, whereas this would also 
be difficult with the limited resources and data of the arid regions. 
Again, in humid countries, it is often found that under certain 
conditions of natural vegetation and soil a rapid absorption of 
water spread over a surface can be effected. In general, therefore, 
recharge in arid areas is possible only if the water can be applied 
in its untreated form, if a suitable absorption surface can be found, 
and if a suitable aquifer is readily available. 
So far, little if any application of these methods has been prac- 
ticable in the more arid parts of Africa, but some successful at- 
tempts in this direction have been made in southern California, 
where the exhaustion of aquifers used for the irrigation of crops 
has been prevented by the storing of flood waters of mountain 
streams in the ground for later use. This was done mainly by 
surface flooding or by infiltrating water from ditches or basins into 
the fans of gravel, sand, and silt which extend from the mouths of 
canyons to the lower cultivated areas, as has been described by 
Lane (16) and by Michelson and Muckel (20). 
In the flood method, water is allowed to pass over the ground 
as a thin sheet controlled by ditches and embankments. Experi- 
ments and field observations have shown that the highest percola- 
tion rates are obtained where the natural vegetation and soil are 
least disturbed. 
Where ditches are used, they are flat bottomed, 3 feet to 12 feet 
wide and g inches deep. They are constructed in a variety of ways 
so as to disperse the water over a large area, usually with a recep- 
