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103. Dry Season of 1895-1896.-Considering the period from 1st September, 1895, to 30th April, 1896, inclusive, during which the rainfall amounted to 18.23 inches, and tabulating the results we get:-
Pokfoolum,
Taitam,
Valley.
Drawn from catchment area.
Loss in reservoir.
Yield of catchment area.
Percentage of rainfall.
gallons.
gallons.
gallons.
135,073,000
35,482,000
99,591,000
57
275,804,000
124,000,000
151,804,000
54
F
From October to April inclusive the yield of an additional 80 acres of catchment area intercepted by the catchwater to the east of Taitam was discharged into the reservoir. This amounted to 11,000,000 'gallons.
The water intercepted by the conduits during this period amounted to 84,502,000 gallons. 104. Considering the foregoing facts and after careful study of the rainfall statistics contained in the reports of the Director of the Government Observatory from which Appendices A, B and C have been compiled, I am of opinion that, in designing a project for the improved water supply of the City of Victoria, the following data may reasonably be taken as a basis for calculating the yield of the catchment areas and streams to be dealt with.
Rainfall from May to August inclusive 44 inches, of which 50 per cent. is available for distribution from the Pokfoolum and Taitam valleys.
Rainfall from September to April inclusive 20 inches, of which 33 per cent. is available for distribu- tion from the Pokfoolum and Taitam valleys. It will be observed that, with the exception of the dry seasons of 1890-91 and 1895-6 this is considerably less than the total rainfall given in Appendix A. The reason for this is that heavy rains frequently occur during September and October when the reservoirs are probably full, and a large proportion of the flow must run to waste, and further not unfrequently a considerable rainfall occurs in April, by which time if calculations were based on the total rainfall for the period only, the supply of water would probably run short before such rainfall was experienced.
105. The percentage of rainfall available from those valleys intercepted by open catch waters at a considerable elevation will probably be somewhat less than that adopted for the Pokfoolum and Taitam catchment areas.
In respect of these areas, I have adopted the following.
Rainfall from May to August inclusive 44 inches, of which 33 per cent. is available for distribution. Rainfall from September to April inclusive 20 inches, of which 30 per cent. is available for distribution.
106. On the above basis we get the following results in respect of the present works:-
Catchment
Valley.
area.
Yield May to August.
Yield Sep- tember to April.
Total.
acres.
gallons.
gallons.
gallons.
Pokfoolum,
416
207,000,000
62,000,000
269,000,000
Taitam,
680
339,000,000
102,000,000
441,000,000
Intercepted by Catchwater,...
220
Streams intercepted by conduits,
72,000,000 30,000,000 102,000,000
74,000,000
96,000,000 170,000,000
692,000,000
290,000,000 982,000,000
These give an average daily supply of, say, 2,700,000 gallons.
107. From the foregoing statement it will be seen that, during the period from September to April, viz., 242 days, the available "run off" from the several catchment areas is 290,000,000 gallons, whereas the supply obtainable based on the annual rainfall is 2,700,000 × 242, viz., 653,400,000 gallons.
108. In order therefore to equalise the daily amount of water available for distribution, reservoirs of a minimum storage capacity of 653,400,000 -290,000,000=363,400,000 gallons are required.
The storage capacity of the Pokfoolum reservoir is 68,000,000 gallons, and that of Taitam 350,000,000 gallons.
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