28 CHAPTER III 



IRRIGATION. 



The artificial watering of land for agricultural purposes is 

 termed irrigation. It forms an essential feature in the profit- 

 able production of crops in areas of insufficient rainfall. The 

 undesirable characteristics of the rainfall in many areas of the 

 Union are such that the annual precipitation, although judged 

 sufficient for the requirements for cropping in many parts 

 of the world, must be supplemented by irrigation. 



The main considerations may be discussed under the 

 headings : (a) Climate; (h) character of the water supply; (c) 

 nature of the soil; (d) surface conditions and topography of 

 the land, and (e) kind of crop grown. 



(a) Climate. — This has been dealt with in Chapter II. 

 It cannot be over-emphasised that in parts of South Africa 

 where irrigation is practised the evaporation is extraordinarily 

 high — anything from 65 to 90 inches. This has a practical 

 bearing on the duty of water and on the construction of reser- 

 voirs. With more depth many of the farm dams in the coun- 

 try would be more effective, even if less water were stored, 

 than in the shallow dams so often found. 



(h) The Character of the Water Supply. — The water 

 of highest value for crops is the sewage from large towns. 

 Such water has an added value due to its definite fertilising 

 action. Irrigation with water of this nature is not confined 

 to arid parts, as the larger towns are usually found in humid 

 and sub-humid areas. Next in value to sewage water is the 

 water of streams carrying considerable quantities of suspended 

 matter and valuable salts in solution. The soils of the Nile 

 owe their fertility to water of this nature, and afford an inter- 

 esting illustration. When the sediment from turbid streams 

 settles on sandy soils, they are not only enriched by the plant- 

 food added, but their physical condition is greatly improved 

 by the addition of silt and organic matter. In Egypt the old 

 system of basin irrigation, which allowed the mud to settle out, 

 kept the fields fertile for thousands of years. Recently 

 introduced systems which have not permitted the utilisation of 

 the mud have effected a rapid deterioration in the productivity 

 of the Nile soils. 



According to Juritz, a sample of Orange River silt from 

 the Prieska District gave the following figures upon analysis : 



