130 



liave a surplus of stored water we require a catchment area of 

 more tlian three times the surface area of the reservoir. Let 

 the reservoir occupy one-third of the collecting ground, then it 

 will be necessary to have the excavation five feet in depth if it 

 be required to hold one year's rainfall, falling in one shower, 

 but that quantity would be evaporated in the course of the 

 year. In the central parts of this country the rainfall is not 

 unf requently of that paroxysmal kind which I have assumed for 

 the sake of argument in the foregoing illustration ; consequently 

 provision must be made for the adequate storage of the annual 

 rainfall as a whole. But the ratio of the amount of evapora- 

 tion to that of the rainfall is much greater in the northern 

 parts of this colony than at Adelaide — say 10 to 1 — so that a 

 sheet of water 10 feet in depth, if it received no accession 

 during the year, would just be evaporated in that time. The 

 saline matter of that original body of water remains — again 

 the area is filled, again it is evaporated, leaving behind its 

 salt ; tluis, the salt is ever on the increase, though it is re-dis- 

 solved, all or in part, at each successive influx of water. Let 

 it now be assumed that the surface area of the impounded 

 water is reduced to one-half by doubling its depth over half 

 the original area, we shall then secure a surplus supply of 

 one-half of the impounded waters, by the utilization of which 

 to the last drop the next impoundment of water wdll not gain 

 in saline strength. Therefore, where practicable; clean out 

 the saltpans to remove the saline accumulations of years, and 

 considerably deepen at the lower levels, is my advice to the 

 pastoral leaseholder. 



The local distribution of water derived from the rainfall of 

 each natural watershed district depends on two conditions. 

 These are the surface undulations and the permeability or im- 

 permeability of the rocks and soils. Springs and underground 

 reservoirs, which constitute a second kind of natural water 

 resources, are maintained by the rain as it is absorbed at the 

 surfaces of pervious rocks, but the direction of the underground 

 flow to its issue at the surface in the form of springs or to its 

 storage is in no way related to the main lines of surface 

 drainage, but is determined by the inclination of the strata. 



The Chief Water-bearing Strafa, in ivhich the Supph/ is of 

 Local Origin. — The Pliocene drifts furnish large supplies of 

 water, but of very uncertain character. The source of supply 

 in them is either the soakage of rain over their surface or from 

 the hills which bound the plain of drift. These drifts consist 

 of permeable and impervious beds which do not show a con- 

 centric dip as would cause water to rise as artesian springs. 

 The strata dip away from the hills which they flank, and 

 approach more and more to horizontality as the distance from 



