THE EVAPORATION OF STORED WATER

Wanganui Chronicle, 1 November 1950, Page 4

THE EVAPORATION OF STORED WATER

effort is being made in the United States to gauge the effect of evaporation upon stored water. This is a problem which will have to be given attention in New Zealand as the hydro-elec-trical system moves, as it is now doing, from using flowing water 1o using stored water. When flowing water runs to the sea unused the problem of the loss of water through evaporation does not present itself, but when stored water is depended upon to operate the power turbines every head of water becomes an asset. Th storage of water in Lake Taupo in recent years has been unsatisfactory, for, 'nothwithstanding the rainfall over the year being reasonably constant the quantity in the lake has not risen to its former level. Why is this?

When the lake falls the shores of the lake dry out and consolidate and with this operation they fall to a lower level. A period of inundation would lift them once again for the ground acts like a sponge and swells as the water content increases. The lake shore is, therefore, likely to be lower after a period of drying out and the basin may be to some extent steeper at the lip. But this does not account for any decline at the first dam in the Waikato River. Why is the lake not holding as much water as formerly? Is it due to the loss through evaporisation in the whole of the catchment area? This process is not to be wholly attributed to the heat of the sun operating upon a wider landface for it is more than likely that the high winds of the Taupo area have much to do with the absorption of a considerable amount of water.

If the wind movement is responsible for the taking off of a large quantity of water then the whole of the Taupo basin will have to be regarded from a different angle. The planting of trees will have to be engaged with a view to breaking the wind. It is accepted that a belt of trees will afford protection on level ground for about a quarter of a mile from the direct operation of the wind. Tn country with varying elevations the planting of trees would have to be made in relation to the contours of the country but always with a. view to providing protection against the wind. This opens up an entirely new outlook. It seems to indicate that a crisscrossing of the basin with belts of trees with meadows in between would be more desirable from a water conservation standpoint, than solid tree plantations.

A study of the Taupo basin is apparently required with a view to discovering to what extent the action of both sun and wind can be counter-acted. A preliminary survey by aerial photograph would probably proved to be helpful.