CITY WATER SUPPLY.

New Zealand Herald, Volume LXIII, Issue 19235, 26 January 1926, Page 11

CITY WATER SUPPLY.

QUESTION OF RAINFALL. DEDUCTIONS CRITICISED, TAUPO ENGINEERS' VIEWS. Some interesting deductions concerning the yearly rainfall on the Waitakere Ranges have been made by Messrs. .T. Rogers and W. A. Gray, engineers, who reported on the Taupo water supply scheme. On the question whether the city should not put a stop to further development in the Waitakeres and join in the Taupo scheme, the engineers seek to show that the Waitakeres offer no certain future supply for the increasing demand of the growing city. They first of all express their disagreement with the city engineer's present conclusions respecting rainfall and daily yield, but say their views coincide with the statement made by Mr. Bush in his report of September, 1921, as follow: 'Mt is obvious there must be a large discrepancy between rainfall on a gathering ground and its actual yield in stream flow, and this can be ascertained accurately only from a continuous record of tlio stream /low for some, years, including a period of drought, compared with the rainfall records for the satnc period, and this information is not available for any part oi' the ranges." Yet, notwithstanding this assertion .Mr. Bush rashly advised the City Council to undertake works involving an expenditure of .£300.000 on the Huia gravitation scheme and £BO,OOO on the Waitakere, reservoir enlargement.

"Furthermore," the engineers say, "Mr. Bush gives interesting rainfall statistics, and on his mature judgment' assumes 45in. as the minimum annual rainfall over Nihotupu, and after making an allowance of 60 per cent, for loss due to evaporation and absorption, bases his calculations at 27in. of rain a year. In this connection we are unable to agree with Mr. Bush, notwithstanding the fact that lie shows a rainfall in 1914 of 48.57 in. at Nihotupu and 28.46 in. at Auckland. This year was one of prolonged drought, the greater percentage of the rainfall taking placo in April, May, June and July, and except for intermittent showers the remaining eight months were comparatively dry. As a matter of fact there was a registered rainfall of '7in. extending over the last five months of the year, consequently after making due allowance of 50 per cent, increase in rainfall at Nihotupu and deducting 60 per cent, for evaporation" mid absorption the available rainfall is equal to 6.3 in. or 1.26 in. a month, whereas Mr. Bush's 'computation shows 2.25iu. a month.

"We are of opinion that the city engineer's calculations of the available rainfall, based on a yearly average, are fundamentally erroneous, and (he conclusion arrived at of 1680 gallons a day an acre would not nearly be approached throughout the year of drought ; and the drought of 1914, or a worse one, nnty occur again any time with a larger population to supply."