244 Journal of the Department op Agriculture. 



CONSTRUCTION OF EARTHEN DAMS. 



By I. J. P. Kleyn, C.E., Department of Irrigation. 



General. 



StccEssFUL farming in South Africa is very much dependent on the 

 water supply available on any selected farm, no matter what class of 

 farming is practised. The natural water supply of this country, 

 however, is erratic. Rivers having a perennial flow are few and far 

 betv/cen and are t)nly found in those regions where the rainfall is 

 sufficient for agricultural requirements appropriate to the local rainy 

 season. With a few exceptions the flow in these rivers decreases to 

 such an extent in the dry season that there is barely enough flow in 

 them for agricultural operations, and frequently what little water 

 there is flows for long distances literally underground below the sandy 

 bed of the channel. The best conditions are found in a narrow strip 

 along the coast of the Cape Province, Natal, and the portion of the 

 Transvaal east of the Drakensberg. The summer rainfall over the 

 greater part of the Transvaal and the northern and eastern portion of 

 the Free State is usually sufficient for summer crops, such as mealies, 

 millet, etc. For spring crops, fruit and permanent crops, irrigation 

 is necessary. Unfortunately, when spring or summer crops require 

 water badly, the rainfall throughout the country cannot be relied 

 upon ; this period is from August to the end of October, and irrigation 

 must be resorted to. As most of our rivers are intermittent streams, 

 i.e. rivers that only flow during the rainy season or after a heavy 

 rain, it is essential that provision be made to conserve the flow, or 

 some of it, during the wet season, in order to carry out operations 

 when it is dry. 



It will thus be seen that the most important factor in irrigation 

 in South Africa is conservation, and that successful agricultural 

 operations are almost entirely dependent on adequate storage works. 



Before deciding on the extent of the works necessary for any class 

 of farming, one should know what is the amount of water required 

 to bring any particular crop or fruit to maturity. In a country where 

 the climate and rainfall vary so considerably, it is impossible to lay 

 down a hard and fast rule. In the Karroo aod the north- 

 west of the Cape Province the annual rainfall is often not 

 more than 8 inches per annum, and the country is subjected 

 to very dry and hot winds, while in the Transvaal the 

 annual rainfall is 28 inches on the high veld, and hot and dry winds 

 are unknown ; it is therefore clear that the amount of water required 

 to bring a crop to maturity in the latter area will be totally in- 

 adequate for the same purpose in the former area. As a general 

 rule it can be taken that the amount of water required to mature a 

 crop by means of irrigation and rainfall combined varies from 18 

 inches to 36 inches. 



No matter in which part of South Africa agricultural operations 

 are carried out, provision must be made to give the crop at least one 



