HEAVY EXCESS OF RAIN

Press, Volume LXXXIII, Issue 25246, 26 July 1947, Page 5

HEAVY EXCESS OF RAIN

Record of Recent Years

SERIES OF WET SEASONS

Figures showing how widely the rainfall of the last few years has diverged from normal Canterbury standards are contained in a letter irom Mr B. W. Collins, of the Geological Survey Branch -oi the Department of Scientific and Industrial Research, who is engaged in a study of underground water conditions in Canterbury. Mr Collins’ figures are for the area between Timaru and the Rangitata river, but they give a good indication of what has happened generally all over the Canterbury Plains.

Mr Collins’ letter refers to an article printed on this pe.ge on July 12, on the new drainage scheme at Seadown. The gist of his letter is that seepage is the result rather of the heavy rainfall than of the use of irrigation on the Levels scheme, ihe portion oi his letter examining rainfall is printed below, and the portion concluding his argument will be printed next week. “As you state, many settlers in the district are convinced that irrigation on Levels Plain has been the cause of their trouble. You also refer, however, to the fact that during the last lew years the ground-water level has been nigh almost everywhere on the Canterbury Plains, after wet years ail over the province. Just how wet the last few years has been is perhaps not appreciated by many, and, in fact, a study of the rainfall figures themselves is necessary in order to gain a clear idea of the exceptionally heavy rainfalls of the last decade.

“Taking the figures for Timaru (as fairly representative of South Canterbury), we find that the three highest annual falls since recording began in 1897 have all occurred during the last nine years. The earliest official rainfall figures tor South Canterbury begin in 1890 (at Kapunatiki, near the mouth of the Rangitata river), and the only annual falls exceeding 30 inches since then are as follows:—1902, 31.03 in; 1936, 30.74 in; 1938, 40.10 in (the record); 1945, 34.25 in; 1946, 31.29 in. “The Timaru average is 22.96 inches, and so even from these few figures it is obvious that since 1936 we have had a particularly wet spell in South Canterbury. (The same applies to many other districts on the Plains, too, but here only the figures for South Canterbury are being considered.) “It must be remembered also that it is mainly the heavy rains which cause a rise in the underground water level, since they entirely saturate the ground, while lighter rains may all or nearly all be absorbed in the soil Since 1936 some very heavy monthly falls have been recorded, as, for example:—February, 1936, 8.39 in; December, 1937, 6.78 in; April, 1938, 6.17 in; June, 1938, 6.29 in; and December, 1938. 6.76 in; December, 1944, 6.64 in; February, 1945, 9.71 in; and September, 1946, “Blit it is not only the monthly and annual rainfalls which are of import-* ance in connexion with the groundwater level. The movement of water in the saturated rocks underground is very slow, and the effect of heavy rainfalls may last for some time—probably years —before all the excess water has drained away. Hence the average annual rainfalls for each 10yearly period since 1897 were calculated. . , ... “The wettest 10-year period was that ending on December 31, 1946, which averaged 25.46 in a year. In descending order the seven periods with the next highest averages (with the rainfall in parenthesis) were:—l94s, 24.74 in; 1938, 24.32 in; 1908. 24.20 in; 1911, 24.20 m; 1913, 24.14 in; 1941, 24.14 in; and 1944, 23 93m. “Thus five out of the eight highest 10-yearly averages have occurred during recent years, and we have to go back nearly 40 years to find a similar period of high rainfall—and even then it was not so high as it has been recently. “Averages for each five years since 1897 tell the same story—four out of the six highest belong to the years since 1934.

“An even more informative procedure, however, is to calculate for each year’s rainfall the difference from the average (22.96 in for Timaru), and add those in excess of this figure and subtract those which are less. In this way figures are obtained for each year which indicate the accumulated departure from the average rainfall. Space is not sufficient to give all the figures, but the following should be enough to indicate the trend. I have used the Kapunatiki figures fdr the period 1890-1897, as this locality receives approximately the same amount of rain as Timaru, and in this way we are able to cover a longer period. I have also assumed that by the end of 1889 an excess of 15 inches of rain had been accumulated—this is in order that the excesses and deficiencies may be about equally spread on either side

1946 .. 19.17 “When such figures for each year are plotted on a graph an irregular line is obtained, with peaks and troughs corresponding to the greatest excesses and deficits. The figures for each pronounced peak and trough have been included in the above list. The graph (or the figures) show well, for example, the effect of the dry years 19141916, when at the end of 1916 a deficiency of more than six inches had accumulated. By 1920 this had bee i converted into an excess of about seven inches. The drought of the early thirties is also apparent, with deficiencies between 1931 and 1935 ranging between 10 and 14 inches. The record fall of 1938 produced an excess of nearly 12 inches, and it was just at this time that Seadown first became so badly affected by seepage troubles. “In 1889 and 1913 there were similar accumulated excesses of rainfall, but I do not know what conditions were like at that time. I have heard, however, that at about the end of last century gravel pits at Seadown were full of water as they have been recently, and there was also talk of converting

the Longbeach Road Board into a drainage board, so similar high warertable conditions may have been experienced then. “Two po.nts may, however, be mentioned: (1) the 1913 excess accumulated over a much longer period than that of 1938, six years for 15 inches in th? former case, as against only three years for more than 25 inches between 1935 and 1938. The very rapid rise from a deficiency of six inches in -943 to an axcess of 19 inches at the end of last year is similar. Such excessive rainfalls in a short period do not allow the infiltrated rainfall time to drain away, and hence the inevitable result is a long-maintained rise in the watertable or level of ground water. (2) It is very likely, as you have before remarked in your columns, that artificial drains are not in such good condition at present as they used to be, and this state of affairs must also obviously tend to aggravate the effects of heavy rainfalls during recent years. “It is to be noted that at the end of 1946 a record excess of rain had been accumulated—the 19 inches being nearly 5J inches more than the previous highest figure (for 1913).”

A sale of 8700 tons of malting barley was made by Australian interests last month to Belgium at the record price of 17s 7d f.o.b. Melbourne and Port Adelaide. The shipment included 5500 tons of South Australian and 3200 tons of Victorian grain. Exports from Australia this year are estimated at about 2,000,000 bushels, including some sent to New Zealand. Early in the export season the Australian price was 13s 3d f.0.b., but later shipments have been made at increasing prices. It is possible that the necessity to import malting barley into New Zealand is in some way connected with the New Zealand price of 5s 6d on trucks.

of zero:— End of— Excess. Deficit. 1889 .. .. 15.00 — 1898 .. .. — 9.69 1905 .. .. 8.96 — 1907 .. .. — 2.21 1913 .. .. 12.74 — 1916 .. .. — 6.53 1920 .. .. 7.19 — 1931 .. .. — 13.19 1935 .. .. — 13.61 1938 .. .. 11.86 — 1943 .. .. — 6.56 1944 .. .. — 0.45 1945 .. .. 10.84 —