Irrigation and Wuier Supply in Aaxt. Colonies. 23 



selecting these situations, and experience in irrigation works 

 is almost indispensable, so that wlien sites for leservoirs are 

 selected, it may be possible to ascertain correctly the amount 

 of rainfall which will be intercepted by the reservoir. The 

 gauges should be of one pattern, and a uniform system of 

 observation should be adoj)ted in every case. Care should 

 be taken to note the largest rainfall in limited peiiods, 

 from one hour upwards, as this is of the greatest conse- 

 quence to enable propei- ]H'Ovision to be made for the 

 discharge of the surplus water when the reservoir becomes 

 full, as any deficiency in this respect is the principal source 

 of danger to a resei'voir. Notice should likewise be taken 

 of the longest periods of drought and rainfall, and the 

 minimum and maximum rainl'all for extended and continuous 

 pei'iods, so that the capacity of the reservoir may be 

 determined — the object being to store sufficient water for any 

 period of drought, together with sufficient to keep up a full 

 supply before or after the drought, during any time that the 

 rainfall alone is insufficient for the requirements. 



The next thing to be ob.served is the determination or 

 the amount of I'un off corresponding to any i^articular 

 rainfall in any district. This depends upon the nature ot 

 the soil, the slope of the country, the ratio of rainfall to the 

 time in which it falls, whether the land is under cultivation 

 or not, whether it is bare or wooded, and other local 

 peculiarities. Attempts have been made to reduce these to 

 formulfe in particular instances, but so far no one has 

 succeeded in finding any formulae or constants Avhich would 

 be aj)plicable to difierent conditions. It is a subject u])on 

 which very little is accurately known, and although the 

 o-auoino- of rivers and streams would, to a o;reat extent, 

 solve the question, the ordinary systems of ascertaining 

 the velocity and discharge give results that are by no means 

 reliable. To oive one instance, a river was gauo-ed in the 

 ordinary way with fioats, and at the same time a series of 

 observations were taken for every 10 ft. in bi'eadth and every 

 2 ft. in depth, by means of an electric current meter, and 

 the results of the two seiies of experiments difiered by 

 more than 25 per cent. The same sections of the river were 

 used in each case, so that one element of uncertainty^ was 

 wanting. Probably the latter was about as correct a i-esult 

 as could be obtained for a large river where it was impossible 

 to take the actual quantity of water that had passed in a 

 certain time, and measure it ; but there is always some 



