. 
Van Yean Water Works. 191 
the River Plenty, therefore, as measured by us, is only an 
index of the supply derivable from one of the sources, which 
has but a limited drainage area, and does not represent the 
quantity due from the surface drainage of its basin. Hence 
any calculations founded on this discharge as the only avail- 
able amount passing through the Plenty, and for supplying 
the reservoir, must be erroneous, inasmuch as it is onl 
storage water from the ranges, and not due to recent rainfalls. 
We are further of opinion, that in consequence of the steep 
character of the basin in question, facilitating a rapid delivery 
of its rainfalls into the Plenty, and the Plenty discharge being 
always dependent upon the amount and duration of the rain- 
falls, and hence constantly varying, that no single measurement 
of discharge, at any given time, can be depended upon for a 
useful result. 
Impressed with these views, we consider our actual 
measurement of discharge taken above and below the swamps, 
as only valuable in furnishing us by their difference, with the 
amount of loss from absorption and evaporation in the 
swamps. 
With a view therefore to estimate the total amount avyail- 
able from the River Plenty, for the supply of the reservoir, 
we propose to take the rainfall on the basin supplying that 
part of the Plenty, as a basis, and from it make deduction for 
surface absorption, and evaporation loss, by swamps, &c. 
The River Plenty, above the reservoir, drains a basin com- 
prising at least sixty square miles of superficial extent, 
including the southern half of Mount Disappointment and 
contiguous ranges, it rests, with the exception of the ranges, 
on the slate formation, and has a close impervious surface, 
incapable of more than surface absorption, and the whole 
presents, with the exception of a few square miles, a steep 
basin-like form, favorable to a rapid delivery of its rainfalls 
into the Plenty, which is materially assisted by the non- 
absorbent character of the surface. 
The mean rainfall upon this basin will therefore represent 
the total supply of water thereto, the mean annual rainfall for 
Melbourne, according to Archer’s Statistical Table, is 30-85 
inches, or thirty-one inches nearly; in the absence of experi- 
ments on the rainfall in this locality, we are compelled to accept 
this amountas the general rainfall, but it was perfectly evident, 
that over Mount Disappointment, and the surrounding ranges, 
there was a much greater amount of moisture derived, either 
from rainfall or atmospheric humidity, or both, due to its 
superior elevation ; this was abundantly proved by the altered, 
