YaM 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 only 

 storage water from the ranges, and not due to recent rainfalls. 

 Wc are further of opinion, that in consequence of the steep 

 character of the basin in question, facihtating 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 avail- 

 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 amount as the general rainfall, but it was perfectly evident, 

 that over Mount Disappointment, and the surrounding ran2:es, 

 there was a much greater amount of moisture derived, either 

 from rainfall or atmospheric humidity, or both, due to its 

 superior elevation ; this w^as abundantly proved by the altered 



