UNDERGROUND WATER

Press, Volume LXXXV, Issue 25934, 14 October 1949, Page 6

UNDERGROUND WATER

EFFECT OF DRY YEARS LEVEL IN CANTERBURY LOW The dry weather in Canterbury in the last two and a half years has had a marked effect on underground water levels, according to the district geologist of the Geological Survey (Mr B. W. Collins). Whereas three years ago many areas were suffering from inadequate drainage and waterlogging of the ground, few complaints of this nature are now heard. Instead, those dependent on wells for water supply are concerned at the falling water levels and some wells have already gone dry. In 1945 and 1946 Christchurch had nearly one and a half times its normal rainfall—in two years, three Years’ rain, said Mr Collins. In 194 1 the fall was slightly below’ average, last year only three-quarters of the average, and so far this year even less —72 per cent, of the normal up to the end of September. At Lincoln, perhaps more representative of the Canterbury plains as a whole, there was a similar set of circumstances, but the rainfall this year was so far only twothirds of normal. As underground water in Canterbury is largely dependent on rainfall for its replenishment, the lack of rain has had a serious effect on ground water levels. As much of the summer rainfall is lost by evaporation or used by vegetation, it is the winter rainfall that is most important for the purpose of recharging the ground water reservoirs. For three years in succession Canterbury has had a deficient ’winter rainfall. The following are percentages of normal rainfall at Christchurch and Lincoln between April and September for the last three years:—Lincoln: 1947. 88 per cent.; 1948, 77; 1949. 61. Christchurch: 1947, 99; 1948, 76. 1949. 63. The last winter has been the driest for many years. May Go Still Lower At this time of the year the water table in Canterbury was generally at its highest level for the year—after winter rains—and in years of normal rainfall would gradually fall to a low level next March or April, said Mr Collins. Although at present so low the water-table must be expected to fall still lower unless there were exceptionally heavy rains during the summer. If the rainfall was only average or below average, the coming summer would be a difficult one for those dependent on well water supplies—especially for those with relai lively shallow wells. The fall in water-table and artesian j levels is indicated by the following observations made on behalf of the | Water Resources Division. New ZcaI land Geological Survey:— j Canterbury Agricultural College, i Lincoln. —Water level in a well nas j gradually declined more than five feet in the last two years with no appreci- ’ able rise in cither winter. The level is still about 18 inches higher than the I lowest recorded level (February. 1923). but is seven feet six inches below the I peak level reached in October. 1945. I This well has been measured at interI vals since 1911. but very few readings . were taken in the dry period from 1931 I to 1935 when it might nave been lower ■ than in 1923. ‘ R.N.Z.A.F. Station. Weedons.—Since i November. 1947. there has been a fall ; in level of 13 feet with only a few , small rnses. ' R.N.Z.A.F. Station. Wigram.—A fall 1 of five feet in the last two years has been recorded. At other observation wells from I Rangiora to Timaru, similar trends have been noted, but all figures to date ; are not available. Rainfall figures i show that longer periods of deficient 1 rainfall have occurred in Canterbury I in the past or especially during 1926 to I 1935 —but few recorded gYound water i levels go back so far. Heavy Evaporation I Not only has the rainfall been de- ■ ficient. but in the last two years evaporation has been higher than normal —this also tending to reduce the I amount of water percolating into the ' ground. -Evaporation, measured at ! Lincoln, was in 1945 92 per cent. <»f the I average of the 1945-48 period; 1946. 105 'per cent.: 1947, 95; 1948, 110; and to date this year. 119. , Most of the wells that have already gone dry in Canterbury have been sunk in recent years when water levels were higher than average, continued Mr Collins. Old wells, constructed to 1 greater depths, were still satisfactory. I “It is to be emphasised that there is I no indication at present that there is 1 any shortage of underground water in Canterbury. The fall in levels seems to be due to natural causes, not to . overdraft of the supply. It is obvious. ; however, that those installing wells should see that they are deep enougn i to operate satisfactorily during periods of deficient rainfall." he continued. "When rainfall returns to normal, or greater than normal, water levels will rise. Many areas of Canterbury arc unfortunate in that the natural fluctuations of the water-table are of considerable range. Rainfall also seems to follow a cycle of wet and dry periods. It is uncertain yet whether these •cycles’ are regular or irregular—no long-term predictions of any great value can be made,’’ he concluded.