Art. XLIII.—Note on the Fluctuation of Water-level in a Christchurch Artesian Well.
[Read before the Philosophical Institute of Canterbury, 2nd August, 1916; received by Editors, 30th December, 1916; issued separately, 10th December, 1917.]
Many writers, notably Hutton, Speight, and Hilgendorf, have discussed the Christchurch artesian system, and have shown that the water-level rises every evening, that it rises with ram, and that the Waimakariri River has no apparent influence. Hutton also described a “Sunday rise.” These results were founded on intermittent observations made at hourly or less frequent intervals. To obtain more definite knowledge the writer, in conjunction with Dr. Hilgendorf, set up an instrument to make continuous records of the fluctuations in water-level. An isolated well at Papanui, not in use, was selected, though unfortunately its depth is not yet ascertained. This note deals with some of the results obtained through the
records of twenty months. The records give a fairly accurate tracing of the movements of the water-level, but owing to a lag in rise and fall of about ⅓ in. the curve is slightly flattened. The error in timing sometimes amounts to three hours. The records show three distinct classes of fluctuation—a daily fall and rise, a rise with rain, and erratic fluctuations without recognized causes.
The Daily Fluctuation.—Except when rain interferes, the well traces a daily curve of fairly constant form with a maximum height between 6 a.m
and 9 a.m., and the lowest point at about 4 p.m. Fig. 1 shows three typical weeks with the daily curves well marked. The daily curve is gradually modified day by day, and the amount of fall varies between ¼ in. and 1½ in. At times “flat days,” of which an example is given, appear in a series of well-marked curves. No marked difference between Sundays and weekdays has been detected. The modifications of the daily curve do not appear
to be seasonable, though the boldest curves usually appear in spring and autumn. Displacement of the time at which the daily fall commences is apparently associated with rain. While the causes of the daily tide are still obscure, the writer believes it to be a natural phenomenon, and not due to human interference with the water system.
The Rise after Rain.—As was expected, rain causes a rise of level, the well responding with remarkable rapidity, and on certain observed occasions a rise has commenced within an hour of the rain commencing. With a heavy rain the rate of rise may amount to 1 in. in three hours and a half. Shortly after the rain stops the level falls rapidly for a period—about four or five times as long as the rising period. In this fall the greater part, sometimes the whole, of the gain disappears. Fig. 2 is a record of a heavy-rain
rise in which the whole gain was lost. Fig. 3 is a comparison of the monthly average heights of the well with the monthly rainfalls at Christchurch, and shows how they fluctuate in sympathy.
The years 1914 and 1915 were very dry, and 1916 is about normal in rainfall. The similarity of the 1916 portion of this well graph to that of the Lincoln well obtained in 1910 by Hilgendorf* is remarkable.
It was noted that the well rises with very light rains, and on one particular occasion responded to a rain which, terminating a long dry period, wet garden soil to a depth of only ½ in.
Other Fluctuations.—There are occasionally other rises without apparent cause: none could be attributed to the influence of the Waimakariri River. Heavy traffic on the tram-line passing about 30 ft. distant causes a distinct wave-like oscillation in the well.
The writer's thanks are due to Dr. F. W. Hilgendorf for assistance and advice, and to Mr. H. F. Skey for rainfall data.
[Footnote] * F W. Hilgedorf, Fluctuations of the Level of the Water in some Artesian Wells in the Christchurch Area, Trans. N.Z. Inst., vol 44, 1912, p. 143.