Page:Transactions NZ Institute Volume 5.djvu/139

Rh followed some months of comparatively dry weather, though it commenced one showing a total of 7.399 in.

Floods may also suddenly arise after a drought, from the fact of some kinds of soil being so thoroughly baked as to have a surface almost as impervious to rain as rock itself, and thus it delivers the water almost as it falls; but such floods will be of short duration, and only until the surface has had time to soften a little and allow the usual amount of soakage.

I have referred to the rainfall at Dunedin, because unfortunately we have no records of its amount for the Taieri basin, and if we were wanting to go minutely into the matter, as we shall see immediately, the rain gauge in this city would be apt to mislead rather than enable us to arrive at correct results. Those who have paid any attention to meteorology must be aware of the great diversity of rainfall in different districts, even within a few miles of each other, and consequently of the necessity for separate observations in various localities, if any practical result of value is to be evolved. We are safe enough, however, in taking the evidence of the Dunedin observations to prove generally a season of unexampled moisture within the Taieri basin; but it will be seen by a study of the figures representing the flow of the Taieri during the flood of 1868, that for particular occasions they are of little value. Thus, for the flood of January 28th, a fall of 1.648 in. was shewn, yet that did not raise the Taieri river so much as a fall registered as 1.37 in. did a few days afterwards. This, however, does not completely prove the position that the Dunedin register is different from what the Taieri one would be, for it is still possible that the whole area of the gathering ground was not saturated to its fullest extent on the 28th January, so that a greater degree of saturation, as on 4th February, would greatly aid the rain of the latter date in producing a flood greater than the one a few days previous. Better proof, however, is found in referring to the delivery by the river at Outram, which, as recorded by Mr. Thomson, was at the rate of 4,653,068 cubic feet per minute; which would show a fall at the rate of 1.67 in., instead of 1.37, even with the whole water run off to the river just as it fell. Even this, however, is not quite satisfactory, for it is quite possible, and indeed likely, that in both localities there was a space of time—perhaps extending to hours—in which the rainfall was of greater intensity than even the higher of these sums represents. As it is impossible, however, to have the records for each few hours, we must generalize from the most frequent, viz., those for each twenty-four hours.

I shall now endeavour to ascertain the length of time which may be considered as the duration of the flood, or how many hours elapsed between the moment when the banks overflowed, and the outlet to the sea was too small to allow the whole waters which were issuing upon the plain, and the Waipori and Waihola lakes, to pass off.