Construction of Earthen Dams. 245 



watering between the months of August and October. An assured 

 watering in December is needed in most cases in the high veld of 

 the Transvaal to ensure a successful crop of mealies. 



Having arrived at the approximate amount of water required for 

 any given cultivated area, it will be necessary to carefully study the 

 question of the adequacy of the water supply. This is dependent on 

 the area draining into our storage works, the annual rainfall over 

 that area, and the intensity of individual rains. Unfortunately, the 

 data collected by the different Government Departments are mostly of 

 such recent date that it is not advisable to apply them without 

 caution. Generally speaking, it would not be wise to estimate the 

 run-off from a small catchment area at more than 5 per cent, and 

 for large areas 2h to 3 per cent, will be found safe. 



The water stored will be subject to heavy losses from the reservoir 

 from evaporation, which amounts in many cases to from 15 to 20 per 

 cent. Again, when distributing the water from the reservoir to 

 the lands a large amount is lost in the conduits by evaporation and 

 percolation. This amount may reach from 35 to 45 per cent., and is 

 dependent on the distance the lands to be irrigated are situated from 

 the storage works. If the lands are immediately below or in the 

 near neighbourhood of the works, this loss will be very much less. 



Having estimated the probable loss under the above headings 

 and determined the depth to which the acreage to be irrigated will 

 require flooding, the least volume of water to be stored can be readily 

 calculated. The next step to be taken is the careful in- 

 vestigation of the proposed site for the reservoir, and it is here 

 that expert assistance may become necessary. The Union Govern- 

 ment offers this assistance wherever wanted, and printed application 

 foims can be obtained from the Irrigation Department, Union Build- 

 ings, Pretoria, when any bona fide farmer requires such assistance. 



It is not proposed to describe here the methods of preparing for or 

 constructing large and far-reaching storage works. The methods of 

 constructing now described, it is hoped, will be sufficient to assist the 

 ordinary farmer to a right appreciation of what should be done and 

 much of what should be avoided. 



Earthen Dams. 



(1) Selecting Site. 

 In selecting the site for storage, the following points must be 

 observed : — 



(a) CatcTiTnent area or source of supply. 



(b) The basin to store the loater. 



(c) Froximity and svitabiUty of materials inth which to 



construct. 



(d) The suitability of the foundations. 



(e) The waste tceir. 



(f) The proximity of the lands to be irrigated. 



(a) Catchment Area or Source of Supx)ly. — It is obvious that the 

 amount of ground that can be irrigated is entirely dependent on the 

 volume of the water supply, and that this again is governed by the 



