176 
INFILTRATION 
It is hardly practicable to suggest a numer¬ 
ical definition of adequate infiltration, but 
some limiting data may be offered. On some 
of the larger islands of Hawaii, where the 
discharge through the shore perimeter in 
some sections reaches values of 5, 10, or even 
20 million gallons daily per shore-line mile, 
with other conditions favorable, Ghyben- 
Herzberg conditions are conspicuous and 
stable. These conditions imply some intake 
area with annual rainfall of 100 inches or 
more. In many other areas of Hawaii, on 
leeward coasts or on smaller islands, where 
rainfall is mostly under 50 inches and 
ground-water discharge is 1, y 2 , or 1/10 
M.G.D. per shore-line mile, the Ghyben- 
Herzberg condition is either missing or non- 
demonstrable on the scale of practical water- 
supply operations. However, it should be 
remembered that under wartime or expedi¬ 
tionary conditions a vestigial fresh-water 
lens may be of great temporary value even 
though it may fail on continuous mechan¬ 
ized development. 
The size of the island is important here; 
on an island 2 miles in diameter the dis¬ 
charged ground-water fraction required to 
equal 1.0 M.G.D. per shore-line mile is 42 
inches over the area, whereas on one 10 
miles in diameter 42 inches over the area 
will give 5 M.G.D. per shore-line mile. 
From experience in Hawaii the latter would 
quite likely have the Ghyben-Herzberg con¬ 
dition; the former very likely would not, 
unless the permeability or cap rock were 
especially favorable. 
It may now be practicable to say that the 
required permeability assumed to be fairly 
homogeneous throughout the whole thick¬ 
ness is such as will require the water table 
to build up 2 to 10 feet or more and remain 
at nearly constant levels perennially. With 
lesser heads some degree of concentration of 
fresh water may be found but it is less likely 
PACIFIC SCIENCE, Vol. I, July, 1947 
to be stable against mechanized exploita¬ 
tion. 
FLUCTUATION 
The permissible annual fluctuation so far 
as we can estimate at present is somewhat 
less than one half of the mean water-table 
head, and in most cases less than one fourth 
of that head. No data are at hand, for any 
system of the magnitude of that of Hono¬ 
lulu, in which greater fractions of fluctua¬ 
tion are known. For systems with head of 
5 feet or less it seems indicated that fluctua¬ 
tion from 2.5 to 7.5 feet of head would be 
fatal to useful Ghyben-Herzberg stability. 
No data are available to determine what 
ratio of short-term fluctuations might be 
tolerable, but naturally the tolerable limiting 
amplitude would be lower than for the 
longer term ones, and in the nature of the 
case they are materially less. 
REGULARITY OF PERMEABILITY 
By regularity of permeability we mean 
homogeneity of distribution and sizes of 
interconnected openings. A formation con¬ 
sisting of well-sorted sand or gravel would 
throughout its mass have regular permeabil¬ 
ity. A formation consisting of moderately 
permeable material but broken by large, 
irregularly spaced fissures or caverns would 
have irregular permeability. The adverse 
effect of irregular permeability would lie in 
introducing large and changeable irregular¬ 
ity of pattern in the three-dimensional net¬ 
work of surfaces of equal pressure and hence 
of lines of flow, of velocities, and of salini¬ 
ties. It is evident that in formations perme¬ 
able enough to meet the Ghyben-Herzberg 
requirement, large irregularities will pro¬ 
mote intermixing and tend to effacement of 
the zone of balance, which without frictional 
retardation can only be stable between im¬ 
miscible liquids. In a sense, large openings 
of such length and direction as to lead across 
