The maxima and minima observed rainfalls are given, month by month for comparison, so as to show the range of variation which may be expected.
From this it appears that May, June, July, and August are wet months, during which rain may be collected and stored for use during the succeeding eight months, which are relatively dry. In September, also, rain may be expected, and again in April. If this takes place, the dry season, during which the flow has to be supplemented from store, will be materially shortened.
Now the main points to be determined are
(a) The least rainfall that is likely to occur in any twelve consecutive months, commencing with 1st May.
(b) Each rain-year must stand for itself, for it is impracticable to make reservoirs so capacious as to be capable of carrying forward from year to year.
(c) The Proportion of rainfall that flows off, and is available for use.
(d) The amount of storage accommodation necessary to develop to the utmost, the most unfavourable conditions of rainfall.
The probable minimum rainfall can only be ascertained from an examination of a record extending over a number of years. The ratio of available flow to rainfall can be only ascertained in like manner by experiments extending over many years.
Fortunately during the period over which experimental data exist, two droughts have occurred of more than ordinary severity, that of 1890-91 and 1895-96. They differ in character, but both are in-