132 THE FUTURE OF ARID LANDS 
Small-scale out-of-door tests of cetyl alcohol by C.S.I.R.O. in 
Australia have now been in progress in Victoria for about eighteen 
months, and the results show an average of 50% reduction in 
evaporation. A large-scale test was conducted in the summer of 
1954 on a town reservoir at Woomerland, Victoria. Two acres of 
water were treated, and, although the results were complicated by 
seepage, it seems likely that evaporation was reduced by 30%. 
Additional larger-scale tests are in progress. 
Recharge of Aquifers 
Finally, there is the question of the recharge of aquifers which 
normally takes place under natural conditions but can also be 
effected artificially. Of any given rainfall the residue, after runoff, 
evaporation, and transpiration by plants have been accounted for, 
percolates downward through the soil and ultimately augments 
the ground water. In arid regions the proportion of rainfall that 
reaches the ground water is small, say 4 or 5%, and in extreme 
cases, as in parts of the Kalahari, recharge 1s considered to be nil. 
In the drier parts of Tanganyika, for example, the precipitation is 
considered to be accounted for as follows: runoff 4 to 6%, evapo- 
transpiration minimum 72%, average 85%, maximum go%, and 
percolation the remainder, say about 10% (4). But the losses by 
runoff and evapotranspiration can to some extent be controlled by 
manipulation of the vegetation and surface conditions with a 
view to increasing the recharge of ground water. Grass sometimes 
reduces runoff as compared with natural vegetation, and some- 
times evapotranspiration can be reduced if the natural vegetation 
is replaced by grass or cultivated crops. In Tanganyika a consid- 
erable rise of shallow ground water has been observed in some 
areas as a result of the clearing of the bush for cultivation. Soil 
erosion not infrequently develops at a later stage, and then runoff 
is increased to such an extent that very little percolation takes 
place and the shallow ground water reserves are destroyed. 
An important natural recharge takes place by influent streams 
in arid areas, and this can be increased by the building of dams 
and weirs which increase the opportunity for infiltration along 
the stream channel; this is sometimes employed for improving the 
supplies to wells and boreholes along stream courses. 
