246 Journal of the Department of Agriculture. 



mean annual rainfall over the area draining into the selected site for 

 storage, the extent of this area, and its topography. The rainfall can 

 be arrived at from observations taken by the Meteorological Branch at 

 stations inside the catchment area, or, failing these, by approximation 

 from, snrronnding stations, and as near as possible to the selected site. 

 Local inspection and information obtained from farmers living 

 around will suffice to determine the extent of the catchment area, 

 and it is also clear that the water draining from hillj-, rocky country 

 will be more than that draining from gentle slopes well covered with 

 grass, or some other dense vegetation. The percentage to allow for 

 the run-off should not exceed 5 per cent, for small catchments, and 

 2| to 3 per cent, for larger ones. As an example, if we have a 

 medium catchment area, with a rainfall of 20 inches, from which 4 

 per cent, may be expected to run off, and if the losses by evaporation 

 in the reservoir be 20 per cent., and 30 per cent, of the remainder is 

 lost in the canals, it will require the storage of the water from 27 

 morgen to irrigate 1 morgen of ground with four 3-inch waterings. 



In many cases it will be found impossible to select a suitable site 

 for a storage dam in an adequate catchment area ; but outside the 

 main drainage course a suitable site may be available. li may then 

 be possible to construct diversion works in the main drainage course 

 and lead the water by means of a canal to the selected site. This, of 

 course, is a more expensive procedure than having Ih'^ selected site on 

 the main drainage course. 



(b) The Basin. — The catchment area being large enough to give 

 the required amount of water, it is necessary that the works will retain 

 this amount. As the cost of an embankment increases considerably 

 with its height, it is essential to endeavour to obtain our purpose 

 with a low wall. In selecting the basin it will be necessary that the 

 site of the wall be as narrow as possible, i.e. a narrow contraction in 

 a depression. Behind the wall where the water has to stand the 

 depression should preferably widen out and the slope of its bed should 

 be even and not steep, so that the edge of the water will be a good 

 distance away from the wall. 



It is, however, unwise to carry out these principles to the extreme 

 as a too shallow wide basin is subjected to excessive losses in 

 evaporation. 



In order to find out the volume of water a basin will store, a contour 

 survey of the basin is required. But as farmers are unable to carry 

 out such work themselves a rough idea of the volume of water can 

 be obtained by taking the area of the water at full supply level of 

 the dam (i.e. the level of the sill of the waste weir) and multiply 

 this by one -third of the depth of water above the outlet-pipe. 



All the water stored in the basin is not available for irrigation, 

 as during the period of storage a large amount is lost by evaporation 

 and percolation. This loss varies from 15-30 per cent. 



(c) Pro.TWiity and Suifabilit]/ of Material. — The soil to be used 

 in the embankment must be such as will consolidate and form, ns 

 far as possible, an impervious homogeneous mass. A sandy loam 

 containing clay is very suitable for this purpose. Black turf or any 

 other earth containing vegetable matter should not be used. The 

 ntarer the place where such suitable soil can be found to the seat of 

 the wall, the lesser the expense in constructing the embankment will 

 be. Avoid excavating below the dam. 



