Contents ix 



PAGES 



parts of Asia In Algeria In Egypt In Cape Colony 

 In Madagascar In the Hawaiian Islands In Java 

 In South America In the Argentine Republic In 

 Western United States Amount of land irrigated 72-89 



The Climatic Conditions Under which Irrigation Has Been 

 Practiced. Amount of rainfall where irrigation has been 

 practiced Distribution of rain with reference to the 

 growing season 89, 90 



CHAPTER II 



THE CONDITIONS WHICH MAKE IRRIGATION IMPERATIVE, 

 DESIRABLE, OR UNNECESSARY (pages 91-116) 



Objects of Irrigation. To establish right moisture relations 

 To increase fertility To change texture of soil To 

 build up low areas For sewage disposal 91-94 



The Least Amount of Water which Can Produce a Paying 

 Crop. Importance of the subject Amount of water 

 needed for wheat Slow rate of evaporation from dry 

 soil Average yield of wheat as related to rainfall 

 Dry farming 95-101 



Like Amounts of Rainfall not Equally Productive. Differ- 

 ences in yield and in rainfall Causes of differences . . 101-106 



Frequency and Length of Periods of Drought. Abundant 

 watering at short intervals needful Type of rain dis- 

 tribution Ineffective rains Length of rainfall periods 

 in Wisconsin Yield of crops compared with rainfall 

 Length of too long periods of no rain Yields due to 

 rainfall and to irrigation compared 106 110 



Conditions which Modify the Effectiveness of Rainfall. In- 

 fluence of soil texture Amount of moisture in soil 

 when growth is checked Loss of water by percolation 

 Rapid percolation chief cause of poor yields Supple- 

 mentary irrigation helpful on light lands Topographic 

 conditions influencing effectiveness Sub -irrigation may 

 supplement rainfall 110 116 



