Yan Yean Beservoir. 



179 



is so far favorable for lessening evaporation, as to render, in 

 my humble opinion. Dr. Wilkie's estimated rate of evaporation 

 too great, 



For the steep slopes of the ordinary forest ranges com- 

 prised within the watershed of the Plenty above Yan Yean, 

 promote the rapid conduction of rain-water to the channels 

 of drainage, and therefore tend to diminish evaporation ; this 

 obvious diminution is, h'owever, to a considerable extent, 

 counterbalanced by the increased evaporation that occurs on 

 some extensive tracts of wet undrained land, from whose 

 surface the evaporation is, according to Charnock, nearly as 

 great as from a sheet of water. In this condition are the 

 swamps that extend from the base of Mount Disappointment 

 to the village of Whittlesea, — a swampy tract north of Sher- 

 win's range, the sides of several of the mountain ravines, and 

 even part of the table land on the main range. 



From the want of extended meteorological observations 

 taken in connexion with the Upper Plenty districts, or what 

 would have been much more satisfactory^ a complete series of 

 stream-guagings to determine the annual discharge, the avail- 

 able rainfall of the district can be only analogically eliminated 

 from the general data afforded by the most trustworthy 

 English observations on evaporation, corrected for the average 

 differences of temperature for the various months of the year, 

 in London and Melbourne, as given in the Statistical Register- 

 for Victoria. Moreover, as wind, and the hygrometrical 

 state of the atmosphere exercise a marked influence on 

 evaporation, independently of temperature ; and as their 

 action is more intense here than in England, some additional 

 corrections should be applied to the English data for this 

 increased action. Due consideration must also be given to 

 the favourable nature of part of the surface of the Upper 

 Plenty district, especially that portion draining direct into 

 the reservoir. Having made allowance for all these con- 

 tingencies I have arrived at the conclusion that the total 

 annual rainfall at the Upper Plenty may be taken as equiva- 

 lent to thirty-six inches, and that the amount thereof 

 evaporated may be assumed to be 30'8 inches, leaving the 

 amount available for supply 5*2 inches, over 44,000 acres. 

 But if cuts were made on the catch water principle the amoimt 

 available might be greatly augmented. 



The annual evapoi-ation from the surface of water in 

 England amounts to from thirty-four to forty -four inches. 

 Both Dr. Wilkic and Messrs. Acheson and Christie, have 

 assumed nine feet as the annual loss from evaporation on the 



