GROUND WATER LEVEL

Press, Volume LXXXIII, Issue 25282, 6 September 1947, Page 6

GROUND WATER LEVEL

Tests Made Near Ashburton

EFFECT OF IRRIGATION “ INFINITESIMAL ” The graph published below shows the results of observations made by the Public Works Department on the ground-water level at McCormick’s Pit, on Racecourse road, half a mile below the lowest waste race, and a mile or so from Ashburton. The graph, and the following observations on the subject of seepage, were forwarded to the Ashburton-Lyndhurst Irrigation Development Association by MY J. O. Riddell, irrigation engineer to the Public Works Department, in response to a request from the association for any reliable data on seepage at Allenton and Ashburton.

“In response to your association’s inquiry as to whether any reliable data is available which would throw any light on the suggestion that recent ground-water troubles in Allenton or Ashburton are caused by seepage from irrigation, I have to advise that this department has kept very complete tecords of ground-water movements in the locality over the last two years,” said Mr Riddell. “Observations of ground-water levels at a number of points have been made at intervals of a fortnight or less during this period, and to enable you to study the results I am forwarding a graph showing the variations observed at one of these points, namely McCormick’s pit. “This particular point has been taken as representative of the others for the reason that when discussing the matter last year with a group of people affected by high ground-water in the area, they agreed that if in a normal dry season the water level in this pit dropped, the case against irrigation could not be proved. The graph forwarded not only shows the fluctuations of water level in the pit but also a rainfall record for the same period, the amounts of rainfall shown oeing sixday totals. In addition, at the top of the graph are two indicators showing in one case the periods during which irrigation water nas been admitted to the Ashburton-Lyndhurst scheme and in the other the periods during which water has been flowing in to the diversion race. The diversion race indicator also gives an idea of the amount of water flowing in that race at any one time, the fun line indicating maximum capacity. “A study of the graph establishes the following points quite clearly:— (1) Despite two irrigation seasons and almost two years’ operation of the diversion races, the ground-water level is over two feet lower than when irrigation and the records commenced. (2) The lowest level of ground-water occurred in September, 1946, after the diversion race had been running full for six months. (3> The second lowest level of ground-water occurred in May. 1947, two months after the diversion race had been refilled to full capacity after cleaning. (4) During the 1945-46 irrigation season, the ground-water fell Ift 6in. During four months of the 1946-47 season, ground-water was consistently lower than for any other four months of the records. A- sharp upward rise in April is explained by heavy rainfall during that month, ana is confirmed by the fact that the peak is not sustained for any appreciable length of time. (5) Every rise in ground-water is related to rainfall.

“It will be noted that comparatively heavy falls in summer months generally have little effect on ground-water levels. This effect is characteristic of ! all similar records taken in other parts j of the county, and is attributed to high evaporation. “It is hoped that this information will assist the members of your association in forming their own opinions as to the value of the complaint that their operations are causing distress to others, and will give them a general answer to the seepage argument. Similar observations are being made in other parts of the county with exactly similar results. “In all other parts of• Canterbury spring water has been and still is abnormally -high. Farmers in these areas have* no irrigation or diversion race on which to place the blame, and naturally accept the only real reason, i.e., the abnormally wet seasons recently experienced. / It is unfortunate that the commencement of the heaviest three yearly rainfall period yet recorded in Mid-Canterbury should practically coincide with the filling of the diversion race, with the result that many sufferers from high groundwater prefer to ignore the rainfall effect to the detriment of the diversion race. Any fair-minded observer will, however, with a little thought and a few calculations, soon conclude that any possible effect from the diversion race can only be infinitesimal compared with that of rainfall. “You also ask me to comment on the widely circulated statement that eight draglines have been put on to the diversion race for six months in order to clean out a depth of five feet to enable water to reach Highbank. This statement is entirely incorrect.* The facts are that for three months during the summer two draglines were employed between the intake and Klondyke homestead, removing sand deposits and material that had rilled off the batters. Three other draglines operated over the same period between Mount Somers and the North Ashburton, cleaning out the race to its original size and shape, the depth of material removed ranging from nothing to about one foot. “The maliciously incorrect statement you refer to can have been circulated only by persons who have no regard for the accuracy of the infor-mation-they pass on, or by others who have some particular reason of their own for starting rumours they know to be untrue.”