Irrigation, and Water Supply in Au.st. Colonies. 25 



a weir or oversow, of such a length as would be necessary 

 to carry off the maximum rainfall, and at a suitable level. 

 It is also necessary to see that a suitable channel to carry oft 

 the overflow, either exists, or can be constructed. Note 

 must also be made of the land which is likely to be 

 submerged, as in many cases this is an important factor in 

 the practicability of the scheme. It must also be seen how 

 the head works are to be constructed, and what provision 

 can be made for the distribution of the water ; and also 

 what facilities, or otherwise, thei'e are for the provision of a 

 complete system of irrigation channels. In addition to all 

 these items, it is necessary to consider whether the quantity 

 of land under command is sufficient to comjiensate for the 

 construction of the reservoir and channels, and whether the 

 land immediately irrigable would give a return for the cost 

 of the reservoir, and if n(jt, whether the prospects of 

 additional land coming under irrigation are sufficient to 

 compensate for any present deficiency, and the cost of 

 pre]:)aripg land not previously irrigated for the reception of 

 water must not be lost sight of; and in estimating the 

 probable cost of the work, the accessibility of the site, or 

 otherwise, the means of obtaining the supplies, and 

 method of housing the men employed, must be taken into 

 consideration. 



Having determined that on all the foregoing poiuts there 

 is a balance in favour of the scheme, the surveys can be 

 proceeded with. These will commence with contours at 

 every 10 feet in height, so as to make svn'e whether the 

 anticipations are so far correct as to render it worth while to 

 proceed. This will be a very small expense, and it 

 found satisfactory, the survey would be continued, and 

 intermediate contours to ascertain the exact contents of the 

 reservoir must be taken. For this purpose, contours every 

 5 feet will ordinarily be sufficient to prove indisputable that 

 the storage capacity is sufficient for the area under 

 command, and that the rainftill will suffice to fill it up to 

 the height proposed for the weir, and to keep up the 

 requisite supply. In case there has not been a fairly 

 accurate survey of tlie drainage area, it may be necessaiy to 

 survey this also ; but in cases where there is no doubt that 

 the area is more than sufficient, or the rainfall is so heavy 

 as to make certain of a sufficient su])ply, this may be 

 postponed, uidess the site for the weir is restricted in 

 length, in which case it should be done at once, in order to 



