Sessional_Paper_1902 — Page 542

446

Summary of conclusions

to Taitam system.

6

The Taitam Reservoir never filled in 1901. Consequently, the additional reser- voirs would not have filled, and the water-supply for the water-year 1901-2, would not have been increased, by their construction, by a single gallon.

If, however, the new reservoirs had been filled during the water-year 1900-1 and kept full, up to the end of that water-year, then their contents would be a most valuable addition to the supply at the present season. But would they have been filled? I find from the Log-book that, although the rainfall for 1900-1901 was 75 inches, the Taitam Reservoir did not overflow, or at least, if it did, the quantity which escaped could not have been great. Unfor- tunately, there is no means of estimating, with even an approach to accuracy, the amount of water overflowing during any season. The only plan, there- fore, is to ascertain the probable quantity which may be collected from a given area, with a given rainfall, from the results obtained during years in which there was no overflow. Again, the rainfall recorded at the Observatory has hitherto been assumed as the basis of all calculations, as to water-supply. Now, observations have been made at Taitam and Pokfoolum for a number of years, and it appears that the rainfall at Taitam is not always equal to that at the Observatory, but often greater. One has, therefore, to fall back on the assumption, not wholly scientifically warranted, that the rainfall at Taitam and at the Observatory, though not equal, are at least proportional. In other words, when a deficiency is recorded. at the Observatory, there will be a proportional deficiency both at Taitam and Pok- foolum. Again, supposing that the rainfall observations, taken at the Taitam and Pokfoolum stations, are absolutely exact, it does not follow that they represent the rainfall, over the whole of the gathering-ground. The rainfall on the heights may be more or less than that on the relatively low ground, where the gauges are situa-` ted. Therefore all calculations based on rainfall observations, made at any one sta- tion, are, to say the least, open to doubt. The only facts that we can rely upon are that, during recent years, unusual droughts have occurred, and that during these droughts, a certain supply has been collected from given gathering-grounds.

23. I have come, therefore, to the preliminary conclusion that, as regards the Tai- with regard tani system, the construction of additional reservoirs, within and fed by the existing Taitam gathering-ground, will not appreciably augment the water-supply, simply because there was no overflow in the two successive water-years, 1899-1900 and 1900-1901. The only way in which additional storage could effect any improve- ment, would be to make it so large as to make it possible to carry forward a reserve- stock, from some previous year or years of superabundance.

Arrangement

of

years misleading.

24. The arrangement of rainfall statistics and water-supply by calendar years, is of statistics misleading. The principal quantity of rain falls in May, June, July, August vide Table I). In September, also, welcome rain may be expected, filling up reservoirs that may be already falling, and replenishing the streams that are inter- cepted, along the lines of conduit. September, therefore, may be included as a water- giving month. In the five months, May to September inclusive, water has to be stored, in order to maintain the supply, through the ensuing seven dry months, during which but little rain is to be expected. Most of the water, reaching the water-works, during the dry months, is rain that has been absorbed and stored by the soil, and which is percolating through it, to the various streams, which form the feeders of the reservoirs. Some also may come in the form of violent showers, when a large per- centage of the rain-water runs off from the surface.