VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 13] 
water supplies is intimately bound up with the question of grazing 
control, without which overgrazing and its attendant evils would 
occur and the final state of the territory would be worse than the 
first. 
Use of Surface Waters 
In arid lands generally the question often arises of the possibil- 
ity of making use of the storm water flows of the ‘“‘dry”’ sandy 
river and stream beds which normally run to waste, or ultimately 
spread out over flats from which they are evaporated with little 
benefit to ground water sources below. Temporary or even per- 
manent local supplies are sometimes found in them, and in some 
territories, as in Tanganyika, it has been found that in the dry 
season the water in sandy river beds tends to be concentrated in 
lenticular, but not necessarily connected, sand reservoirs, and that 
the individual sand lenses are slowly draining downstream (4). 
Sub-surface dams are frequently proposed with a view to hold- 
ing up the sub-surface flow in sandy river beds. They sometimes 
meet with marked success, but the common obstacle to greater 
development of such supplies is the difficulty of finding sections 
of stream channels that are sufficiently impervious along both 
floor and sides, together with the difficulty of finding low-cost 
materials and methods of construction in isolated places. 
There is usually great scope for the construction of dams and 
tanks of various kinds to take the runoff from natural rock and 
other catchments and from artificial catchments, but the high 
evaporation losses, which may amount to as much as 8 feet in a 
dry season, have usually acted as a deterrent to such schemes, 
since they tend to make it impracticable or unduly costly to pre- 
serve water until late in the dry season. The recent investigations 
in Australia, Kenya, and elsewhere on the reduction of evapora- 
tion losses by the use of cetyl alcohol, and related compounds, 
which form a thin film on a water surface and thereby substan- 
tially reduce evaporation, raise new hopes for the conservation of 
water supplies in arid lands. Experiments to date have proved 
highly successful, and large scale trials are now in hand. The 
chemicals used are harmless to animals and plants. 
