Irrigation. 153 



IRRIGATION— ITS RELATION TO CROP YIELDS. 

 SOIL, AND "BRAK" (ALKALI). 



By H. W. Turpin, B.A., M.Sc, Ph.D., Lecturer on Crops, Groot- 

 foutein School of Agriculture and Experiment Station, Middel- 

 burg, Cape. 



By irrigation is understood the artificial application of water in order 

 to ensure steady and profitable yields where the natural precipitation 

 is deficient in total quantity or is unfavourably distributed. 



The \eed fqr Irrigation. — If we study the rainfall of the world, 

 it will be noted that about 25 per cent, of the earth's surface has an 

 average precipitation of less than 10 inches, while some 30 per cent, 

 receives from 10 to 20 inches. In other words, about 6-lOths of the 

 earth's surface has less than 20 inches of rainfall. In the classification 

 of areas according to the precipitation, those receiving less than 

 10 inches are said to be " arid," while the parts getting 10 to 20 

 inches are spoken of as " semi-arid." It is in the arid and semi-arid 

 regions that irrigation is primarily required, although there are some 

 humid areas whose yields may be improved by the application of 

 water on account of the unfavourable distribution of the rain. 



Water Requirements of Crops. — The question may be asked why 

 it is that an area having an average annual precipitation of, say. 

 10 to 20 inches requires the artificial application of water in order 

 to produce annually a profitable crop. The answer is simply this : 

 On an average a crop, say, of mealies requires about 600 lb. of water 

 to be transpired (passed out through the leaves) for every pound of 

 dry matter produced. Now, of the rain that falls on any particular 

 piece of land, a certain amount is lost by evaporation, some by '' run 

 off," some by percolation through the soil, and what remains in, or 

 is retained by, the soil alone is useful to the crop in the production 

 of dry matter. We may safely assume, for experiments have proved 

 it, that under certain conditions 50 per cent, to 60 per cent, of the 

 rainfall is lost in one or the other of the ways mentioned above, so 

 that out of a 15-inch rainfall we can only reckon on about 6 inches 

 or less on which to mature a crop. (The principles of dry farming 

 make for the retention in the soil of the greatest quantity of two 

 seasons' rainfall for the use of one crop. In other words, the soil 

 l)ecomes the reservoir of the dry farmer, just as dams are the reservoirs 

 of the irrigation farmer.) The equivalent of 6 inches in terms of 

 tons of water per acre is roughly 680 tons, which is a quantity of 

 water just about sufficient to give approximately one ton of dry matter 

 per acre. In the case of the mealie crop, this will mean about 5 bag> 

 of grain and 12 cwt. of stover per acre, which can scarcely be regarded 

 as a paying proposition, seeing that it will cost around £2 per acre 

 to produce this crop. This illustration shows why, in arid and semi- 

 arid regions, it is necessary to supplement the natural precipitation 

 with artificially applied water. It has been estimated that, when all 



