For our Water Supply. 253 



where all are equally accessible, how some should be emptied 

 by cattle, while others, apparently, lose very little water. 



And this objection cannot apply to my observations with 

 reference to the Deep Creek, as the land is enclosed for 

 cultivation. 



The water of these ponds is lost, therefore, either by 

 evaporation, or absorption. Either admission would be alike 

 fatal to the prospects of the Yan Yean Eeservoir. 



If so much Avater can be absorbed through the slate strata 

 which form the bed of the Deep Creek, what reasonable 

 grounds have we to expect that the same amount of absorption 

 will not take place through the slate strata that form the bed 

 of the reservoir ? 



_ It may be noticed that some settlers have great confidence 

 m the Yan Yean scheme from observing that small artificial 

 water-holes are often permanent in the summer months. 



If my reasoning is correct with regard to the effects of 

 evaporation in this country, we may assume that the evapora- 

 tion from the surface of water is nine feet, and that one-ninth 

 of the ram may be relied on as the watershed. 



The extent of drainage area necessary to give a permanent 

 supply of water to any pond can, therefore, be easilv 

 determined. 



With a rainfall of thirty-six inches, the ratio of the drainage 

 area to the surface of the pond must be greater than eighteen 

 to one, in order to secure a permanent supply. 



The ratio of the Plenty basin to the surface of the reservoir 

 IS about twenty-seven to one, but more than one-third of the 

 watershed is not available for the reservoir, a large amount 

 being lost in the swamps, and it being necessary to leave a 

 certain proportion to maintain the flow in the river. 



Thus the ratio is practically reduced to eighteen to one^ 

 and there is, therefore, no more than sufficient to cover the 

 evaporation. 



Keservoirs in England seldom exceed fifty acres, and they 

 are generally much smaller, hence the loss from evaporation 

 is very trifling, and the area of surface drained very lar^e in 

 proportion. ^ 



The reservoir which supplies New York is 400 acres, with 

 a depth of forty feet, and an unlimited command of water, 

 the loss from evaporation is, consequently, not equal to one- 

 third of that which will be sustained at Yan Yean. 



Had the Yan Yean Eeservoir not exceeded 400 acres there 

 would have been a saving of water equivalent to supply 

 188,500, at forty gallons per head, per day, with a depth of 

 fifteen feet eight inches, instead of four feet four inches. 



